New titanosaurian caudal remains provide insights on the sauropod diversity of the Hațeg Island (Romania) during the Late Cretaceous

ABSTRACT The Upper Cretaceous sauropod record from Romania is abundant, but mainly composed by isolated and poorly preserved specimens. They were found in Maastrichtian units cropping out in both the western-southwestern margin of the Transylvanian Basin, and in the Hațeg and Rusca Montană basins. Two small-sized taxa, Magyorasaurus dacus and Paludititan nalatzensis, were considered as valid; however, the presence of other forms is not ruled out. An important sample of sauropod caudal vertebrae is described in detailed to provide new data about the diversity of the sauropods of the Hațeg Island during the Maastrichtian. Distinct morphologies are recognized for the caudal vertebrae, including four morphologies for the anterior-most elements; at least three for the anterior and middle ones, and at least two for the posterior caudals, including sauropods with opisthocoelous condition. The possibility of four different tail morphologies is suggested, which might belong to four different taxa, including Paludititan nalatzensis, Magyarosaurus dacus (the material previously referred to it seems to be composed by more than one taxon) and two other species. A new character for morphological datasets describing the position of the intraprezygapophyseal lamina in the middle and posterior caudals is proposed, which may be characteristic of some European taxa.


Introduction
Europe corresponded to a vast, complex, and dynamic island archipelago during the Late Cretaceous, characterized by a set of islands, which constrained the evolution and ecology of several groups of vertebrates, in particular sauropod dinosaurs (e.g.Benton et al. 2010;Csiki-Sava et al. 2015).The Upper Cretaceous fossil record from Europe is rich in sauropod occurrences, spanning from the Cenomanian to the Maastrichtian, which have been remarkably increased in the last three decades with new discoveries and the establishment of new taxa (e.g.Le Loeuff 1993, 2005;Sanz et al. 1999;Csiki et al. 2010a;Díez Díaz et al. 2016, 2021;García et al. 2010;Ortega et al. 2015;Ősi et al. 2017;Mocho et al. 2019a;Vila et al. 2022).In addition, the redescription of some previously established taxa (e.g.Díez Díaz et al. 2013Díaz et al. , 2018) ) and old specimens (e.g.Martín Jiménez et al. 2017;Vila et al. 2022), as well as the increasing number of new phylogenetic approaches incorporating European Late Cretaceous titanosaurian taxa (e.g.González Riga et al. 2016;Gorscak andO'Connor 2016, 2019;Díez Díaz et al. 2021;Vila et al. 2022), are inproving our knowledge about the evolutionary history of the titanosaurs from both Eastern (specially the Hațeg Island) and Southwestern Europe (i.e. the Ibero-Armorican realm).
In recent years, the number of studies about the European titanosaurs has increased, mainly based on the titanosaurian remains found in Southeastern Europe (France and Spain) (e.g.García et al. 2010;Díez Díaz et al. 2016;Vila et al. 2022).By contrast, our understanding of the titanosaurs from the Upper Cretaceous of Romania is still poor, being necessary to provide a full systematic reassessment of the historical material, as well as the description of new specimens.An important sample of sauropod material was recovered by members of the Babeş-Bolyai University (Cluj-Napoca).This sample includes the holotype of Paludititan nalatzensis (Csiki et al. 2010a) but also numerous unpublished teeth and postcranial remains, including axial and appendicular elements, from several locations.Here, we perform the detailed description of a set of caudal vertebrae of that sample, coming from different Maastrichtian continental fossil sites cropping out in the Hațeg and Transylvanian basins.The present work constitutes an opportunity to provide relevant data about the diversity of the sauropods of the Hațeg Island during the Maastrichtian.The comparative anatomical analysis performed here will be especially focused on the comparison of the new remains with the titanosaurian fossil record of other Romanian Upper Cretaceous sites, as well as with that from the Ibero-Armorican domain.This work is also an opportunity to analyze some morphological characters of the caudal vertebrae in European titanosaurs, some of them used in morphological data sets for cladistic analyses.

Titanosaurian fossil record from the Upper Cretaceous of Romania
The intensive work performed by F. Nopcsa during the transition of the 19 th and 20 th centuries in the Upper Cretaceous of Romania (especially focused on the Hațeg Basin) resulted in the find of an important volume of vertebrate fossil remains and, consequently, the first detailed data about this fauna (e.g.Grigorescu 2010).One of the first references on the presence of sauropod remains in the Hațeg Basin was provided by Nopcsa (1902Nopcsa ( , 1904) ) noting for the presence of three indeterminate sauropod species, including Titanosaurus, in Sibişel valley, near Sânpetru.Subsequently, Nopcsa (1915) provided an updated faunal list for the dinosaurs of Transylvania, where he confirmed the presence of Titanosaurus.In that work, he provided a brief description based on generally scattered and disarticulated sauropod material, and on a few partially associated specimens, from several sites of Transylvania.These remains included axial and appendicular elements, some of them being figured.He proposed a new species of the genus Titanosaurus, Titanosaurus dacus.Nopcsa (1915Nopcsa ( , 1923) ) suggested that the small size of that new taxon, and the presence of some characters tentatively referred as 'primitive', might be related with its evolution in an insular environment.Most of the specimens composing the paleontological collection of Nopcsa, including some studied by him, were sold to the British Museum of Natural History (Grigorescu 2010).Huene (1932) provided a re-analysis of these titanosaurian fossils, mainly corresponding to small individuals.He proposed a new genus, Magyarosaurus, to incorporate the sauropod remains found by Nopsca, proposing a new combination for the species of Nopcsa (1915), Magyarosaurus dacus, and establishing two new species, M. hungaricus and M. transsylvanicus.Some specimens were referred as Magyarosaurus sp.(Huene 1932).
After the death of Nopcsa in 1933, the search for new specimens and the systematic studies around the Late Cretaceous fauna of Romania suffered a significant decrease.A. Mamulea performed some geological work and collected some sauropod bones that were referred to Titanosaurus dacus by Mamulea (1953aMamulea ( , 1953b)).Since 1977, the identification of new fossil-sites in Upper Cretaceous outcrops of Transylvania provided a significant number of new specimens, which gave place to important collections such as the ones of the University of Bucharest, Geological Survey of Hungary, Babeş-Bolyai University and Department of Natural History of Deva Museum.The systematic analyses of old and new specimens, conducted by several researchers, resulted in a significant increase on the diversity and consequent understanding about the peculiar and unique Late Cretaceous vertebrate fauna of the Hațeg Island, including sauropods.The occurrence of sauropods has been noted in several Maastrichtian sites from the Hațeg (e.g.Grigorescu et al. 1985;Weishampel et al. 1991Weishampel et al. , 1999;;Smith et al. 2002;Van Itterbeeck et al. 2004;Grigorescu and Csiki 2008;Stein et al. 2010Stein et al. , 2010a;;Csiki et al. 2010cCsiki et al. , 2011;;Codrea and Solomon 2012;Codrea et al. 2013;Botfalvai et al. 2017Botfalvai et al. , 2019)), Rusca Montană (Codrea et al. 2009(Codrea et al. , 2012) ) and Transylvanian basins (e.g.Codrea and Mărginean 2007;Codrea et al. 2008Codrea et al. , 2010aCodrea et al. , 2010bCodrea et al. , 2013;;Brusatte et al. 2013aBrusatte et al. , 2013b;;Vremir et al. 2015a).However, only in a few cases, brief descriptions and preliminary systematic approaches were performed, except for the Nălaț-Vad sauropod (i.e. the holotype of Paludititan nalatzensis; see Csiki et al. 2010a).Here, we will refer to the most relevant sauropod occurrences published in recent years.Groza (1983) provided one of the first references to sauropods after the Nopcsa and Huene works, with the description of caudal vertebrae and appendicular elements that were attributed to Titanosaurus dacus.Posteriorly, Grigorescu et al. (1985) figured peg-like teeth attributed to Titanosauridae, discovered in the Pui area (Hațeg Basin).These teeth were subsequently referred to Magyarosaurus dacus by Grigorescu et al. (1999), as well as new teeth found in Valioara-Fântânele.Weishampel et al. (1991) described a new specimen deposited in the University of Bucharest corresponding to a partial sauropod skull of a juvenile individual found in Pui, being referred to Titanosauridae indet.The studies focused on new and old material allowed some authors to propose the grouping of all species of Magyarosaurus in only a valid one, Magyarosaurus dacus (e.g.Weishampel et al. 1991;Le Loeuff 1993;Jianu and Weishampel 1999;Wilson and Upchurch 2003;Upchurch et al. 2004).Csiki (1999) described a titanosaurian osteordem from the Maastrichtian of the Hațeg Basin, found in La Cărare (Sânpetru Formation).This specimen was firstly referred to Magyarosaurus dacus, but it was more recently reattributed to Titanosauria (Magyarosaurus dacus?) by Csiki (2006).Codrea and Mărginean (2007) referred for the presence of a sauropod tibia (tentatively referred to Magyarosaurus) found in Alba Iulia (Transylvanian Basin, Şard Fm), and deposited in the collections of the Aiud Natural Sciences Museum.It corresponds to the oldest find of sauropods in Romania (originally referred as a Paleogene large mammal, see Codrea and Mărginean 2007).The next year, an incomplete middle caudal vertebra found in Râpa Roşie (Transylvanian Basin, Șard Fm.) was referred to Titanosauriformes, its possible attribution to Magyarosaurus being indicated (Codrea et al. 2008).Recent research on the Maastrichtian continental deposits of the Rusca Montană Basin also allowed identifying the first sauropod remain documented in that basin: a single distal caudal vertebra referred to Sauropoda indet.(Codrea et al. 2009(Codrea et al. , 2012)).Codrea et al. (2009Codrea et al. ( , 2010aCodrea et al. ( , 2010bCodrea et al. ( , 2013) ) provided important data about the dinosaurs of southwestern Transylvania, pointing out the presence of titanosaurian remains in the Sebeș, Oarda de Jos and Lancrăm localities (Transylvanian Basin, Șard Fm.), including isolated bones (caudal vertebrae and fore-and hindlimb bones), which were briefly described and figured.The presence of procoelous caudal vertebrae was used by the authors to point for the presence of derived titanosaurs.
In 2002, a sauropod specimen composed of associated elements was discovered in Nălaț-Vad (Hațeg Basin, Sânpetru Formation) by Cristina Fărcaș and one of us (VAC), which was described by Csiki et al. (2010a).This specimen was composed of axial and appendicular bones, which allowed the establishment of a new titanosaurian taxon, Paludititan nalatzensis, recovered as a member of Lithostrotia.Sauropod material referred to Titanosauria was also reported in the Tuștea locality (Hațeg Basin, Densuş-Ciula Fm.), including a briefly described individual with associated caudal and sacral vertebrae, chevrons, and an ilium (Grigorescu et al. 2010a;Csiki-Sava et al. 2012;Botfalvai et al. 2017).Codrea and Solomon (Codrea et al. 2012(Codrea et al. , 2019)), Codrea et al. (2013) and Solomon and Codrea (2015) provided more data about sauropod remains recovered in the deposits of Pui (Hațeg Basin, Sânpetru Fm.), preliminary referred to Titanosauria indet., including caudal vertebrae and some appendicular elements.Some of them will be described herein in detail.
Considering all these finds, most sauropod specimens identified in the Maastrichtian continental deposits of Romania correspond to isolated and disarticulated bones.Only scarce discovers were found associated but they are still incomplete specimens.Furthermore, the detailed analysis of most of the new specimens found since the second half of the 20th century, as well as the revision of the sauropod specimens of Nopcsa referred to Magyarosaurus (see Mannion et al. 2019a) is still necessary.Nowadays, only two taxa are being considered as valid: M. dacus and Paludititan nalatzensis (e.g.McIntosh 1990;Le Loeuff 1993;Wilson and Upchurch 2003;Upchurch et al. 2004;Csiki et al. 2010a).The titanosaurian remains found by Nopcsa and other more recently discovered specimens have been referred to M. dacus (including the material referred to the two new species of Magyarosaurus proposed by Huene 1932: M. transsylvanicus and M. hungaricus).M. dacus is represented by a large amount of isolated cranial and postcranial remains (including osteoderms) from different sites and corresponding to at least 10-12 individuals (e.g.Weishampel et al. 1991;Upchurch et al. 2004;Mannion et al. 2019a), which resulted in a highly complex systematics, which need to be solved.In addition, Stein et al. (2010), based on histology, suggested that 'M'.hungaricus is a larger and different taxon from M. dacus.The systematic re-evaluation of the material referred to the genus Magyarosaurus (including the remains collected by Nopcsa) is being currently performed by P. D. Mannion and colleagues (see Mannion et al. 2019a).The presence of some still unnamed and distinct forms has been hypothesized for the Maastrichtian of Romania, with at least three or four different taxa living in the Hațeg Island during the Maastrichtian, including the presence of forms that are larger than M. dacus and P. nalatzensis (Csiki and Grigorescu 2004;Grigorescu 2005;Csiki et al. 2010a;Stein et al. 2010;Solomon and Codrea 2015;Mannion et al. 2019a).The presence of small-sized sauropod taxa such as M. dacus has been used as clue to the intense debate about the presence of island dwarfism in the dinosaurs of the Hațeg Island during the Late Cretaceous (e.g.Nopsca 1914Nopsca , 1923;;Weishampel et al. 1991;Jianu and Weishampel 1999;Benton et al. 2010;Stein et al. 2010), but the presence of medium-size sauropods uncovers a possible more complex evolutionary history.In addition, the sauropod taxa from the Maastrichtian of Romania have been incorporated in a few cladistic analyses, and their phylogenetic relationships are still poorly understood, but their inclusion within Lithostrotia seems to be consensual (Curry Rogers 2005;Csiki et al. 2010a;Mannion et al. 2019a;Mocho et al. 2019b).

Geological Setting
Following the Late Cretaceous tectonic pulse ('Laramide') in Transylvania (Romania) a series of sedimentary basins occurred: Hațeg, Rusca Montană, Transylvanian and possibly, Zlatna (Săndulescu 1984;Willingshofer et al. 2001;Csontos and Vörös 2004;Codrea and Dica 2005;Krézsek and Bally 2006;Codrea and Godefroit 2008;Țabără et al. 2022).All refer to collapsing areas of different already erected thrusting nappes of the Sothern Carpathians and Apuseni Mountains and are filled by terrestrial sediments (mainly of fluvial origin, sometimes with volcanic participation).Lithologic differences between these formations cropping out on various areas were controlled by the participation of the volcanic rocks (tuffs, lava flows etc.) or by the source areas of different clasts of the fluvial channel fills (Therrien et al. 2002;Van Itterbeeck et al. 2004;Therrien 2005Therrien , 2009)).In Rusca Montană Basin, the participation of the volcanic material is stronger (Codrea et al. 2009(Codrea et al. , 2012;;Popa et al. 2016), as well as in the northern area of the Hațeg Basin, in the Densuș-Ciula Formation (Popa et al. 2014 and references therein).On the opposite, on the southern margin of the Hațeg Basin, this participation is meager in the Sânpetru Fm. (including the 'Pui beds' sensu Therrien 2005).Same situation is in the basin of Transylvania in the Metaliferi sedimentary area or on the north-western side, in Jibou area (Codrea and Godefroit 2008).
The geological ages of the deposits are dominantly Maastrichtian (Codrea and Dica 2005), but in some sections of the Șard Formation, 'uppermost Campanian' deposits could be present too.A continuous sedimentation in Maastrichtian-Paleocene time span was not shown any where else but in the Jibou area, in the Jibou Fm. (Gheerbrant et al. 1999;Codrea and Godefroit 2008).In Rusca Montană Basin the presence of the Paleogene rocks was not documented by paleontological data (Codrea et al. 2017 and references therein) and in the Hațeg Basin, although Paleogene formations are figured on different geological maps (e.g.Grigorescu et al. 2010b), there is hardly any evidence about this age.On the southwestern side of the Transylvanian basin, in the Metaliferi sedimentary area, only the transgressing marine late Eocene (Priabonian) and early Oligocene (Rupelian) were proved (details in Codrea and Dica 2005 and references therein), lying in disconformity on the latest Cretaceous rocks, at Ighiu.In Zlatna Basin, the age of the red beds belonging to the Fața Băii Fm. are devoid of any fossil documenting a clear geological age (Codrea and Dica 2005).
The latest Cretaceous deposits are predominantly red beds.The mudstones of the floodplain were crossed by fluvial channels filled with various clasts, from microconglomerates to rough conglomerates, depending on their locations related to the sedimentary basin margins and the distance from the source areas of the clasts (Codrea et al. 2001(Codrea et al. , 2010b;;Van Itterbeeck et al. 2004;Therrien 2005).There is no evidence about extended uppermost Cretaceous lake environments, but ponds, abandoned channels and ox-bows are documented both in the Hațeg Basin (Codrea andSolomon 2012, Solomon andCodrea 2015) and the Metaliferi sedimentary area (Codrea et al. 2010a(Codrea et al. , 2010b(Codrea et al. , 2013)).
Concerning the paleogeography of this region during the Maastrichtian, many researchers agreed with an island realm in the eastern side of the Tethys Ocean (e.g.Benton et al. 2010;Csiki-Sava et al. 2015).This so-called 'Hațeg Island' is however not enough well outlined based on enough evidence, mainly due to successive erosion events that occurred after the latest Cretaceous and probably eroded large part of the sedimentary deposits.The island paleogeographic pattern during the Maastrichtian is an argument for a terrestrial realm settled by some 'dwarf' dinosaurs, but also by other land dwellers: fishes, amphibians, small reptiles, turtles, pterosaurs, birds and mammals (e.g.Codrea et al. 2010a;Weishampel et al. 2010;Csiki-Sava et al. 2015 and references therein;Venczel et al. 2016).Among them, sauropod dinosaur remains are often recorded, for instance related to two taxa only: Magyarosaurus dacus and Paludititan nalatzensis.However, their diversity should be larger.Most field contexts refer to isolated postcranial bones devoid of anatomical connexions; the skull remains, and teeth are extremely rare.The sauropod remains described and illustrated in this paper originate from various localities and sites as follows (Figure 1).In the Hațeg Basin: Nălaț-Vad (abbreviated UBB NVM), Pui (UBB P, UBB PL -Pui 'lens' and UBB PG -Pui Gater; details about these names in Codrea and Solomon 2012), Sânpetru (UBB SPM and UBB x).In the Metaliferi sedimentary area: Oarda de Jos, Alba Iulia town (Oarda A -UBB ODA, Oarda lens -UBB ODAN, Oarda "stock" -UBB ODS: this name and abbreviation are firstly introduced herein, referring to a stock of sedimentary rocks as gray sand, microconglomerate and red mudstone temporary and artificially erected during the works at Oarda de Jos for a platform targeted to host in future various constructions; it disappeared, being horizontally levelled) and Sebeș -Râpa Roșie (UBB RR).

Material and methods
A sample of caudal vertebrae recovered from different Maastrichtian continental fossil sites cropping out in the Hațeg and Transylvanian basins (Table 1) and deposited in the collections of the Babes-Bolyai University (Cluj-Napoca, Romania) is described in detail.We use 'Romerian' terms (Wilson 2006) for the anatomical structures (e.g.'centrum') and their orientation (e.g.'anterior').This study applies the landmark-based terminology of Wilson (1999Wilson ( , 2012) ) and Wilson et al. (2011) for vertebral laminae and fossae.The position used for the caudal vertebrae was based on Díez Díaz et al. (2013) and Tschopp et al. (2015): (i) anterior-most caudal vertebrae are vertebrae with triangular and deep transverse processes extending onto the neural arch (generally from the first to fourth/fifth caudal vertebrae), and a complexe set of vertebral laminae and fossae in the neural arch and the transverse processes, they also have rudimentary chevron factets when compared with the anterior caudal vertebrae; (ii) anterior caudal vertebrae correspond to the caudal vertebrae that preserves a simple transverse process (not extending to the lateral face of the neural arch as occur in the anterior-most caudal vertebrae), the chevron facets are welldevelopped, and the pattern of laminae and fossae is less complex; (iii) middle caudal vertebrae are vertebrae with no transverse processes, and a well-developed zygapophyseal processes and neural spine; and (iv) posterior caudal vertebrae are vertebrae with a reduced neural spine and zygapophyseal processes (the distal ones preserve a rod-like morphology).The measurements for the caudal vertebrae are provided in Table 2.The analyzed specimens were mainly compared with other titanosaurian taxa, in particularly, the ones from the European realm with preserved caudal vertebrae: Ampelosaurus atacis (Le Loeuff 1995Loeuff , 2005)), Atsinganosaurus velauciensis (García et al. 2010;Díez Díaz et al. 2018), Lirainosaurus astibiae (Sanz et al. 1999;Díez Díaz et al. 2013;pers. observ. PM 2016), Lohuecotitan pandafilandi (pers. observ. PM 2014-2020;Díez Díaz et al. 2016), Magyarosaurus dacus (Nopcsa 1915;Huene 1932;pers. observ. PM 2014) and Paludititan nalatzensis (Csiki et al. 2010a;pers. observ. PM 2018).In the following section of this study, Systematic Paleontology, the described fossils will be organized from specimens with a less inclusive systematic attribution to those with more inclusive ones.If more than one specimen is present for the same systematic attribution, they will be organized considering the position in the caudal series, from more anterior to more posterior elements.

Material
One posterior caudal vertebra and part of the right prezygapophyseal process of the subsequent vertebra found in articulation (UBB P-64, Figure 2 (a-f)).
Description: UBB P-64 is a posterior caudal vertebra (Figure 2 (af)).Its posterior edge is slightly broken in the left side and the prezygaphophyseal processes are incomplete.This vertebra is articulated with the distal end of the right prezygapophyseal process of the subsequent vertebra.This caudal vertebra is slightly amphicoelous, i.e. both the anterior and the posterior articular surfaces are sightly concave (the posterior one is almost flat).The anterior articular surface preserves a small convexity on its dorsal half, and a small circular pit below.The anterior face is quadrangular (slightly dorsoventrally longer than mediolaterally), with concave dorsal and ventral edges and convex-to-straight lateral edges (Figure 2d).The posterior face is sub-circular (slightly mediolaterally longer than dorsoventrally), with straight dorsal and ventral edges, and convex lateral ones (Figure 2f).The lateral face is anteroposteriorly concave and dorsoventrally convex.There is an anteroposteriorly elongated ridge at the level of the junction between the neural arch and the centrum, and an associated shallow, elliptical and small fossa below (Figure 2c and 2e).The ventral face is transversely flat at the midpoint of the centrum and transversely concave near the posterior edge.The ventral face is limited by the ventrolateral ridges, which are shallower at the midpoint (Figure 2b).The anterior and posterior chevron facets are present but rudimentary (the posterior ones are eroded).The neural arch is located on the anterior half of the centrum (Figure 2c and 2e) as in titanosauriforms and some non-neosauropod eusauropods (e.g.Upchurch and Martin 2003;D'Emic 2012), not reaching the anterior edge of the anterior articular surface.The prezygapophyseal processes are anterodorsally projected, and they are not dorsoventrally expanded in the distal end.
The lateral face of these processes is transversely convex.Between the zygapophyseal processes and lateral to the base of the neural spine, there is a shoulder-like region (i.e. a transversely broad ridge with a slightly excavated dorsal surface, which extends from the dorsal surface of the prezygapophyseal process), also present in UBB SPM-4, UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15.The prezygapophyseal facet is rudimentary and suboval.The anterior face of the neural arch has a short spinoprezygapophyseal lamina (sprl) which delimits a small spinoprezygapophseal fossa (sprf).The prezygapophyseal processes are medially connected by robust intraprezygapophyseal lamina (tprl), which is located posteriorly to the level of the anterior face of the centrum (Figure 2a).The postzygapophyses (located on the posterior half of the centrum) are posteriorly projected.
No spinopostzygapophyseal lamina (spol), postspinal lamina (posl) and spinopostzygapophyseal fossa (spof) are observed.The neural spine is reduced (i.e. it is dorsoventrally low) and anteroposteriorly long, resulting in a dorsal longitudinal crest.Pneumatic internal structures (i.e.camellae and camera) are absent in the centrum and neural arch.

Locality
Pui (Hațeg Basin); Sânpetru Formation, early Maastrichtian in age (Panaiotu and Panaiotu 2010).Description: UBB P-180 is a posterior caudal vertebra lacking the neural arch (Figure 2(g-l)).This caudal vertebra is slightly amphicoelous, i.e. both the anterior and the posterior articular sufaces are sightly concave, with robust and wide edges.The outlines of the anterior and posterior articular surfaces are subcircular, slightly mediolaterally wider than dorsoventrally, with convex edges (Figure 2h and 2j).The lateral face is anteroposteriorly concave and dorsoventrally convex.There is an anteroposteriorly elongated elliptical fossa at the level of the junction between the neural arch and centrum (Figure 2i).The ventral face is transversely flat, becoming slighlty concave near the posterior edge.That face is limited by rudimentary ventrolateral ridges (Figure 2k).The anterior chevron facets are absent.The posterior ones are eroded, but a subcircular outline is interpreted.The neural arch is absent, but it was located on the anterior half of the centrum as in titanosauriforms and some non-neosauropod eusauropods (e.g.Upchurch and Martin 2003;D'Emic 2012), possibly not reaching the anterior edge of the anterior articular surface of the centrum.Pneumatic internal structures are absent in the centrum.

Description
UBB ODAN-32 is a middle-posterior caudal vertebra with the anterior and posterior articular surfaces partially eroded and preserving part of the neural arch (the zygapophyseal processes are absent, Figure 3).This vertebra is interpreted as a middle-posterior caudal vertebra.This caudal vertebra is moderately-to-slightly opisthocoelous: the anterior face is moderately-to-slightly convex and the posterior one is concave (Figure 3b and 3d).The presence of opisthocoelous caudal vertebrae is uncommon within sauropods and diagnostic for some somphospondylan taxa, such as Opisthocoelicaudia skarzynskii (Borsuk-Bialynicka 1977), Abdarainurus barsboldi (Averianov and Lopatin 2020), Huabeisaurus allocotus (D'Emic et al. 2013) and Borealosaurus wimani (You et al. 2004), and present in one middle-posterior caudal vertebra of Magyarosaurus dacus (one element of NHMUK R.3865).The anterior articular surface is not constricted and lacks the presence of a rim and pits.The outlines of the anterior and posterior surfaces of the centrum are not totally preserved, but they seem to be sub-circular, with convex edges as in Magyarosaurus dacus (some elements of NHMUK R.3865; Huene 1932) and Paludititan nalatzensis (Csiki et al. 2010a).The posterior articular surface is wider in the ventral half than in the dorsal half.The lateral face is concave both anteriorposteriorly and dorsoventrally.It bears an anteroposteriorly elongated ridge at the level of the junction between the centrum and the neural arch.An elliptical and anteroposterioly elongated fossa is present in both sides ventrally to this ridge.The ventral face is taphonomically compressed, but it seems to preserve a vental keel, and the absence of a ventral hollow is interpreted.The anteroventral and posteroventral edges of the ventral face are eroded; however, two rough tuberosities are present in the posteroventral edge, which are interpreted as rudimentary posterior chevron facets.Some small foramina are present near the anterior articular surface of the centrum.The neural arch is on the anterior half of the centrum as in titanosauriforms and some nonneosauropod eusauropods (e.g.Wilson 2002;Upchurch and Martin 2003).The neural spine is transversely compressed and long anteroposteriorly.The anterior and posterior faces of the neural spine are badly preserved, but the base of the sprl can be observed, delimiting a small sprf.

Description
UBB NVM-22 is a partial centrum of an anterior-most caudal vertebra (Figure 4(a-c)).The centrum is slightly procoelous, i.e. the anterior articular surface is concave, and the posterior one is slightly convex (Figure 4a) as in UBB ODA-76 and UBB PL1-1.In lateral view, the anterior articular surface of the centrum is slightly anterodorsally inclined (Figure 4b), as in UBB ODA-52.The anterior articular surface is subcircular, with convex ventral and lateral edges (the dorsal edge is not complete).The lateral surface is badly preserved, but both sides are ventromedially directed and anteroposteriorly concave.The ventral face is transversely short and concave, resulting in a ventral hollow (Figure 4b) as occur in titanosaurs (Wilson 2002;D'Emic 2012), but it is shallower than in UBB ODA-76 and UBB PL1-1 (probably due the fact that UBB NVM-22 belongs to a more proximal position in the tail).The ventral hollow is delimited by well-developed ventrolateral ridges.The caudal rib is dorsoventrally deep and extends to the lateral surface of the neural arch.This morphology is indicative that UBB NVM-22 probably belongs to the anterior half of the anterior caudal series, probably between the second and fourth position.The neural arch is anteriorly displaced, reaching the anterior edge of the centrum.Pneumatic internal structures are absent in the centrum.

Description
ODA-76 is one of the last anterior-most caudal vertebra not preserving the neural arch, which is fractured (Figure 4(d-i)).The centrum is slightly procoelous, i.e. the anterior articular surface is concave, and the posterior one is slightly convex (Figure 4d and 4f).This posterior convexity is not so pronounced as in Paludititan nalatzensis (Csiki et al. 2010a), Magyarosaurus dacus (e.g.NHMUK R.3854, one element of NHMUK R.4898, the anterior-most vertebrae of NHMUK R.3849, but an anterior caudal vertebra of NHMUK R.3849 has a slightly convex posterior articular surface) and some of the herein described specimens (e.g.UBB NVM-25, UBB ODA-18 and UBB ODA-52).The posterior articular surface of the centrum is not constricted, and it is delimited by a rim (only slightly visible in the lateral side, Figure 4g), differing from the condition in P. nalatzensis (Csiki et al. 2010a), M. dacus (e.g.NHMUK R.3851, one element of NHMUK R.4898), and Lohuecotitan pandafilandi .No pits are present in the convex posterior articular surface.The anterior articular surface of the centrum is subcircular, wider in the dorsal half than in the ventral half, and mediolaterally longer than dorsoventrally (Figure 4e) as occur in some vertebrae referred to M. dacus (e.g. one element of NHMUK R.3852).In lateral view, the anterior articular surface is anterodorsally inclined as occur in UBB PL1-1, some caudal vertebrae of M. dacus (one element of NHMUK R.3852, one element of NHMUK R.4898, NHMUK R.10988; the anterior-most ones are subvertical, and this anterodorsally inclination is lost in the transition between the anterior and middle section of the tail, see MAFI ob.3091, Nopcsa 1915;Csiki et al. 2010a), Atsinganosaurus velauciensis (Díez Díaz et al. 2018), and in the anterior-most caudal vertebrae (MCNA 7458) of Lirainosaurus astibiae (in the last anterior caudal vertebrae of this taxon, the anterior edge is subvertical, see MCNA 14443 and MCNA 14446) differing from the more subvertical anterior articular surface shown in L. pandafilandi (HUE 3134/3-1, Díez Díaz et al. 2016).The anterior-most caudal centra of P. nalatzensis also seem to have an anterodorsally inclined anterior articular surface, which is subvertical in the last anterior caudal vertebrae (Csiki et al. 2010a).The anterior articular surface has a straight dorsal edge (posteriorly deflected, being concave in dorsal view), and convex lateral and ventral edges (the ventral edge is transversely short, Figure 4e).The posterior articular surface of the centrum is also sub-circular, with all edges being convex as in M. dacus (e.g. two elements of NHMUK R.3849, one element of NHMUK R.3852, one element of NHMUK R.4898; Nopcsa 1915), and the maximum diameter is located at its dorsoventrally midpoint (Figure 4g).The lateral face is ventromedially directed, anteroposteriorly concave, and dorsoventrally straight.The lateral and ventral faces are perforated by small foramina (1-2 mm), which are anteroposteriorly elongated (Figure 4d, f and i).The ventral face is transversely concave, resulting in a ventral hollow (Figure 4i), a common titanosaurian feature (Wilson 2002;Gomani 2005;Mannion and Calvo 2011;D'Emic 2012;Lacovara et al. 2014).That concavity is delimited by well-developed ventrolateral ridges, which are connected to the chevron facets.The posterior chevron facets are anteroventrally projected, being well-visible in posterior view.They show flat-to-concave surfaces and a semi-circular outline in ventral view.The anterior chevron facets are present but not as well-developed as the posterior ones.They preserve a rough surface.The caudal ribs are present but eroded.They are posterolaterally directed.An anteroposterior and striated ridge is located on the top of the caudal ribs, resulting in a bulge structure visible in anterior view (Figure 4e).This bulge presents a broad distribution within Sauropoda (e.g.Poropat et al. 2016;Mannion et al. 2019b), but it persists up to the posterior half of the anterior series of tail in a restrict number of taxa, being shared by several titanosaurian sauropods (e.g.Martínez et al. 2004;Kellner et al. 2005;Gorscak et al. 2014;González Riga et al. 2018), and present in L. pandafilandi (Díez Díaz et al. 2016), P. nalatzensis (Csiki et al. 2010a) andA. velauciensis (Díez Díaz et al. 2018).The caudal ribs are located at the midpoint of the anteroposterior length of the centrum (slightly displaced anteriorly), in the dorsal half of the lateral face.They are anteroventrally supported by a rudimentary anterior centrodiapophyseal lamina (acdl).The neural arch is anteriorly displaced and lacks pneumatic internal structures.

Description
UBB PL1-1 is an almost complete anterior caudal vertebra (interpreted as a sixth-to-eighth caudal vertebra) with eroded caudal ribs and the neural spine separated from the centrum (Figure 5a-f).The centrum is procoelous, i.e. the anterior articular surface is concave, and the posterior one is moderately convex (Figure 5a and 5c) as in UBB ODA-76, unlike other European titanosaurs with welldeveloped posterior condyles (e.g.Magyarosaurus dacus, Huene 1932; Lirainosaurus astibiae, Díez Díaz et al. 2013;Lohuecotitan pandafilandi, 2016;Atsinganosaurus velauciensis, 2018;Paludititan nalatzensis, Csiki et al. 2010a).The posterior face is not markedly constricted.It is delimited by a well-developed rim, more laterally marked than ventrally and dorsally (Figure 5d).No pits are present in the condyle.The anterior articular surface of the centrum is subcircular and mediolaterally longer than dorsoventrally.In lateral view, the anterior articular surface is anterodorsally inclined (Figure 5a and 5c) as occur in UBB ODA-76 and in some anterior-most and anterior caudal vertebrae of M. dacus (one element of NHMUK R.3852, NHMUK R.4898, NHMUK R.10988) and A. velauciensis (Díez Díaz et al. 2018).The dorsal edge is transversely convex-tostraight as occur in UBB ODA-76.It is posteriorly deflected, being concave in dorsal view.The lateral and ventral edges of the anterior articular surface are convex (Figure 5b).The posterior articular surface of the centrum is not complete, but is interpreted as subcircular, with convex lateral, dorsal and ventral edges.The lateral face is ventromedially directed, and markedly anteroposteriorly and dorsoventrally concave, resulting in a wide depression, as in UBB ODA-52.The ventral face is transversely short and concave, generating a well-developed ventral hollow (a synapomorphy of Titanosauria, see Wilson 2002;D'Emic 2012), and delimited by well-developed ventrolateral ridges, which are connected to the chevron facets (Figure 5f).The posterior chevron facets are ventrally projected, being well-visible in posterior view.The left one is convex and the right one is concave.The anterior chevron facets are also ventrally projected, and anteroposteriorly longer than the posterior ones.The caudal ribs are in the anterodorsal sector of the lateral face, and posteroventrally supported by a rudimentar posterior centrodiapophyseal lamina (pcdl).
The neural arch is anteriorly displaced, as occur in titanosauriforms and in some non-neosauropod eusauropods (Salgado et al. 1997;Upchurch and Martin 2003;Britt et al. 2017).The prezygapophyseal processes are significantly longer (same length as the centrum) and anterodorsally oriented.They are transversely compressed and the centroprezygapophyseal lamina (cprl) is dorsoventrally deep (Figure 5c).The lateral face of the prezygapophyseal processes is covered by rugosities, remarkably dense at the level of the prezygapophyseal facet (interpreted as a rudimentary preepipophyseal process), which is circular, flat and faces dorsomedially.The prezygapophyseal processes are medially connected by the tprl, which is located anteriorly to the anterior articulation of the centrum.The medial face of the prezygapophyseal process is transversely concave unlike in UBB ODA-52.The anterior face of the neural arch is badly preserved, but the sprl is not developed up to the prezygapophyseal face and seems to be restricted to the ventral half of the spine (being rudimentary to absent in the dorsal half).The postzygapophyses are slightly posteriorly displaced relative to the posterior edge of the spine, preserving a circular outline and a flat-to-dorsoventrally concave surface.The posterior face of the neural spine is marked by the presence of the spol in the ventral one third of the spine and connected to the dorsal edge of the postzygapophyses.The spol is absent in the dorsal two thirds of the spine.A small dorsoventrally elongated fossa is developed anteriorly to the postzygapophysis.It is interpreted as the postzygapophyseal centrodiapophyseal fossa (pocdf).No other important lamination is present in the lateral surface of the neural spine.The anterior surface of the dorsal area of the spine has a rough prespinal lamina (prsl), which occupies all the sprf in the preserved section.The neural spine is only slightly transversely expanded on the distal end, and a subvertical orientation is interpreted, similarly to those of Lohuecotitan pandafilandi (Díez Díaz et al. 2016) and Paludititan nalatzensis (Csiki et al. 2010a).The neural arch lacks pneumatic internal structures.

Material
Two anterior-most caudal vertebrae in articulation and some associated fragments (UBB NVM-25, Figure 6).

Description
UBB NVM-25 corresponds to a series of two anterior-most caudal vertebrae in articulation (Figure 6).They are interpreted as either caudal vertebrae 1 and 2 or 2 and 3 (the anterior-most caudal vertebrae identified for Paludititan nalatzensis are more posterior than UBB NVM-25, by one or two positions in the series).The anterior-most caudal vertebra lacks part of the transverse processes and the dorsal half of the neural spine.The edges of the anterior articular face of the centrum are also eroded.The second vertebra of this sequence lacks the left side of the centrum and the dorsal half of the neural spine.Sediment remains between both neural spines.
The centra are strongly procoelous, i.e. they show a welldeveloped concave anterior articular surface and a strongly convex posterior one (Figure 6c and 6e).The edges of the anterior articular face are damaged in both centra, and no accurate measurements can be obtained.However, the outline of the anterior articular face is interpreted as dorsoventrally compressed, subcircular to quadrangular, with a straight ventral edge and only slightly convex dorsal and lateral edges (Figure 6d), differing from the morphology observed in the caudal vertebrae of Magyarosaurus dacus (NHMUK R.3851), Lirainosaurus astibiae (MCNA 7458, MCNA 14444), and Ampelosaurus atacis (MDE C3-58, Díez Díaz et al. 2013).A more quadrangular outline is observed in the anterior-most caudal vertebrae of Lohuecotitan pandafilandi (Díez Díaz et al. 2016).The anterior articular surface is wider than tall, and the maximum mediolateral diameter is placed in the ventral half.The description of the posterior articular surface of the centrum is mainly based on the posteriormost element.As in the anterior articular surface, the posterior one is subcircular to quadrangular, with a ventral straight edge, and lightly convex-to-straight lateral edges.L. pandafilandi seems to preserve a similar morphology in some centra, but there is as clearly sub-circular morphology in others (pers.observ.PM 2014-2020).The anterior articular surface is larger than the posterior one.There is a pit, not totally preserved, in the center of the condyle.The condyle is constricted but no defined rim is present, being only slightly marked in the lateral side (Figure 6f).The posterior condyle is displaced to the dorsal half of the posterior articular surface of the centrum, and the apex of the convexity is only slightly dorsally displaced from the midpoint of the dorsoventral width of the centrum (Figure 6e), as in the anterior-most preserved caudal vertebrae of Paludititan nalatzensis (Csiki et al. 2010a).A posterior condyle with a dorsally displaced apex is present in Magyarosaurus dacus (e.g. one element of NHMUK R.3849, NHMUK R.3851), L. astibiae (Díez Díaz et al. 2013) and L. pandafilandi (also slightly dorsally displaced; see Díez Díaz et al. 2016), but it is absent in A. atacis (MDE C3-58; Le Loeuff et al. 2005) and Normanniasaurus genceyi (MHNH-2013.2.1.1;the condyle is slightly dorsally displaced but its apex is located at the midpoint of its dorsoventral length).
The lateral face is anteroposteriorly concave and dorsoventrally straight.Pneumatic foramina or fossae are absent, as well as nutritional foramina, as occur with the anterior-most caudal vertebrae of Magyarosaurus dacus (e.g.NHMUK R.3851, one element of NHMUK R.3852, NHMUK R.3849) but some anterior caudal vertebrae of that species develop a wide and shallow lateral fossa (e.g. one element of NHMUK R.3852), a condition shared with Lirainosaurus astibiae (Díez Díaz et al. 2013), but not with Lohuecotitan pandafilandi (pers. observ. PM 2014(pers. observ. PM -2020)).The ventral face is transversely flat in the more anterior centrum, and transversely concave in the subsequent one (Figure 6h).It is bordered by slightly developed ventrolateral ridges.The presence of a transversely convex or flat ventral surface is common in the anterior-most caudal vertebrae of titanosaurs (e.g.L. pandafilandi,   referred to M. dacus (e.g.NHMUK R.3854) also present a ventral hollow.If the assignation of these vertebrae to M. dacus is correct, this taxon seems to acquire a ventral hollow around the first caudal vertebrae, as in the taxon to which UBB NVM-25 and UBB ODA-18 belong.The ventral face seems be only slightly concave in the anterior-most caudal preserved vertebra in the holotype of Paludititan nalatzensis (Csiki et al. 2010a), being impossible to know where this concavity firstly appears.In lateral view, the ventral face is anteroposteriorly concave in both centra studied here.Some elliptical (anteroposteriorly elongated) and small foramina are present, probably corresponding to nutritional foramina.The anterior edges of both centra do not preserve evidence of chevron facets.The ventral edge of the posterior articular face of the centrum in the posteriormost caudal vertebra is eroded, but the presence of facets can be interpreted (this edge is not preserved in the more anterior caudal vertebra).Pneumatic internal structures are absent in the centrum (i.e.camellae are absent), which seems to differ from the condition in P. nalatzensis (in which the cavities are slightly larger than 2 mm).In the broken parts of the neural arch, it is possible to observe cavities bigger than 2 mm, interpreted as camellae.A similar condition is observed in the anterior-most caudal neural arches of the lithostrotians M. dixeyi (Wedel 2009), Neuquensaurus australis (Cerda et al. 2012;Zurriaguz and Cerda 2017) and the undefined Cenomanian lithostrotian titanosaur from the Spanish locality of Algora (Mocho et al. 2019a).
In these vertebrae, the pedicels of the neural arch are anteriorly displaced and occupy almost the entire dorsal surface of the centra, not reaching the posterior edge.The pedicels are slightly anterodorsally inclined.The anterior face of the neural arch is concave in the anterior-most caudal vertebra and almost flat in the posterior one.This concavity corresponds to a well-developed prezygapophyseal centrodiapophyseal fossa (prcdf), which is divided by a subvertical lamina, in two subfossae, the lateral one being a welldeveloped pneumatic foramen (Figure 6d).This foramen seems to be connected to the internal camellae of the neural arch.Small subcircular fossae are observed in the medialmost subfossa.The prezygapophyseal process is ventrally supported by a cprl, which is medially sharp, yielding a small rugosity near the prezygapophosis (possible homologous to the rugosities observed in Paludititan nalatzensis, pers.observ.PM 2018).The neural canal is taller than wider, subrectangular in anterior view (with a straight dorsal edge and being wider in its dorsal half), and elliptical in posterior view.The ventral face of the neural canal is transversely flat (only slightly excavated).The transverse processes are dorsally deep and extended to the lateral face of the neural arch.The dorsolateral edge of the transverse process is anteroposteriorly thick [homologous of the prezygodiapophyseal lamina (prdl)], being covered by rugosities.These rugosities are particularly pronounced laterally to the prezygapophysis, resulting in a rudimentary bulge.Posterior to this bulge, these rugosities are also pronounced at the base of the neural spine, resulting in a dorsally projected process.In the more anterior element, this process is located above the prdl, near the connection of the spinodiapophyseal lamina (spdl) to the prdl.In the more posterior one, this process is displaced to the sprl, giving place to  Lohuecotitan pandafilandi (HUE-03134/6), in which they are ventrolaterally projected.No round bulge is present in the dorsolaterally edge of the transverse process.
In lateral view, the distal end of the prezygapophyseal process is not ventrally deflected.The prezygapophysis is not dorsally and ventrally expanded, similarly to Magyarosaurus dacus (NHMUK R.3851).The medial face of the prezygapophyseal process is not visible due the articulation.The lateral face of the prezygapophyseal process is transversely convex.The prezygapophyseal articulation is flat and faces dorsomedially.The prezygapophyseal processes are medially connected by a tprl, which is located posterior to the anterior edge of the anterior articulation.The lateral face of the neural arch is marked by two excavations, which corresponds to possible pneumatic foramina, probably communicating with the internal structure of the neural arch.The ventral excavation is located anteroventrally to the postzgapophyses facet.It is round in outline and can be interpreted as the pocdf, which is common in several lithostrotians and Huabeisaurus allocotus (Mannion et al. 2013(Mannion et al. , 2019b)).The dorsal excavation is located just above the pocdf, anterodorsally to the postzygapophyseal facet.It is subtriangular in shape and is interpreted as the postzygapophyseal spinodiapophyseal fossa (posdf, Figure 6c) in the more anterior caudal vertebra, and as the spinodiapophyseal fossa (sdf, Figure 6b and 6e) in the more posterior one.Both facets are separated by a subhorizontal lamina, the postzygodiapophyseal lamina (podl), which is also present in Lirainosaurus astibiae, where it was considered as the only autapomorphy recognized in the holotype (Díez Díaz et al. 2013), and in UBB ODA-18 (see discussion of this character is Discussion); and is considered as synapomorphy of Lithostrotia (Mannion et al. 2013).The postzygapophyses are flat, laterally projected, and the postzygapophyseal surface faces ventrolaterally.They have a circular outline.Both postzygapophyses are medially connected by a short and subhorizontal intrapostzygapophyseal lamina (tpol), which differs from the V-shaped tpol of UBB ODA-18.
Only the ventral half of the neural spines of these vertebrae are preserved.The neural spine is dorsally oriented (being slightly anterodorsally inclined), which is markedly different from the condition in Magyarosaurus dacus (NHMUK R.3851), UBB ODA-18, Lirainosaurus astibiae (MCNA 7458) and Lohuecotitan pandafilandi (HUE-03134/6), with posterodorsally oriented neural spines.Ampelosaurus atacis (MDE C3-58) preserves a neural spine with a slight posterodorsal deflection.The anterior-most caudal vertebrae preserve a thick and short sprl, which departs from the distal end of the prezygapophyseal process and disappears in the base of the neural spine.In the more anterior caudal vertebra, from the dorsal edge of the transverse process departs a sharp lamina to the lateral face of the neural spine, interpreted as the spdl.Also in this vertebra, a subtriangular and shallow prezygapophyseal spinodiapophyseal fossa (prsdf) is recognized between the sprl, the spdl, and the prdl (Figure 6g).In the next vertebra, the spdl is possibly captured by the prezygapophyseal process, combining with the preexistent sprl (i.e. the spdl and prsdf disappear).The sprf is dorsoventrally elongated, and only well-developed in the base of the neural spine, becoming dorsally shallower.The sprf yields a thick, medially restricted, and well-developed prsl, with an anteriorly expanded surface marked by striation.The prsl is anteriorly projected and visible in lateral view.The spol can be observed in both caudal vertebrae.The spol is single and departs from the dorsal edge of the postzygapophysis.The spof is dorsoventrally elongated and ventrally deep, yielding a well-defined and medially restricted posl.The presence of medially restricted prsl and posl was recovered as a synapomorphy of Titanosauria (Mannion et al. 2013), but it is also present in several diplodocoids (Mannion et al. 2019b).
The surface of the posl is covered by several rugosities.In the last caudal vertebra, the posl becomes visible in anterior view.The ventral end of the posl is connected to the medial face of the postzygapophyses through a short and shallow lamina, referred as a ventral 'bifurcation' of the posl, which generates a small and round fossa below the posl.A similar 'bifurcation' is observed in the anterior-most caudal vertebrae of L. pandafilandi (HUE-03134/ 6) and A. atacis (MDE C3-58), but it is absent in L. astibiae (Díez Díaz et al. 2013), M. dacus (NHMUK R.3851) and UBB ODA-18.

Description
UBB ODA-18 is an anterior-most caudal vertebra, interpreted as a second caudal, lacking the caudal ribs, the distal half of the neural spine and the left prezygapophyseal process (Figure 7).The posterior articular surface of the centrum is partially eroded.The centrum is strongly procoelous, i.e. the anterior articular surface is concave, and the posterior one is markedly convex (Figure 7e).The posterior articular surface is not constricted, and no rim or pits are visible (Figure 7f).The apex of the posterior condyle is not dorsally displaced similarly to Ampelosaurus atacis (Le Loeuff 2005), but not in Magyarosaurus dacus (NHMUK R.3851), Lirainosaurus astibiae (Díez Díaz et al. 2013) and Lohuecotitan pandafilandi (in which the posterior convexity is slightly dorsally displaced; see Díez Díaz et al. 2016).The anterior articular surface of the centrum of UBB ODA-18 is sub-circular, with convex edges (Figure 7d).In lateral view, it is subvertical, differing from that in L. astibiae (MACN 7458; see Díez Díaz et al. 2013).The dorsal edge is slightly posteriorly deflected, being concave in dorsal view.The outline of the posterior articular surface of the centrum cannot be interpreted due to the poorly preservation of this region.The lateral surface of the centrum is anteroposteriorly concave and bears a shallower fossa below the caudal rib with some small foramina, as occur in L. pandafilandi (pers. observ. PM 2014(pers. observ. PM -2020)), but this fossa is absent in the preserved elements of L. astibiae (pers.observ.PM 2016).The ventral surface of the centrum is transversely flat and concave in the anterior and posterior half, respectively (Figure 7c).The absence of a developed ventral hollow is possibly related to the fact that UBB ODA-18 belongs to one of the first two caudal vertebrae of the tail.In several taxa, the ventral face of the first anterior caudal vertebrae are generally flat.Therefore, UBB ODA-18 might belong to a sauropod where the presence of a ventral hollow reaches the anterior-most section of the tail.This condition differs from the flat ventral surface common in the anterior-most caudal vertebrae of European titanosaurs such as L. pandafilandi (HUE-03134/6) and M. dacus (NHMUK R.3851).However, due the incompleteness of the later taxon, it is difficult to know exactly when the ventral concavity appears in the tail of this species.Rudimentary ventrolateral ridges can be identified in the posterior half of centrum.The anterior chevron facets are absent, and the posterior ones, if present, are not visible due the preservation.The neural canal is dorsoventrally elongated in anterior and posterior view as in UBB NVM-25, L. astibiae (see Díez Díaz et al. 2013) and L. pandafilandi (see Díez Díaz et al. 2016).The neural canal in M. dacus (based on the specimen NHMUK R.3851) is dorsoventrally compressed in anterior view.
The base of the caudal ribs is located at the anterodorsal sector of the lateral face of the centrum.They are dorsoventrally deep, extending to the lateral face of the neural arch (Figure 7d and 7f).The posteroventral edge of the caudal rib is supported by a thick and wide pcdl.The top of the caudal rib is covered by some rugosities.The neural arch is anteriorly displaced, not reaching the anterior articular face of the centrum.The prezygapophyseal processes are more dorsally projected than anteriorly, as occur in the holotype of Lirainosaurus astibiae (MCNA 7458), Lohuecotitan pandafilandi (see Díez Díaz et al. 2016) and Magyarosaurus dacus (NHMUK R.3851), and differing form the condition shown by the vertebrae MDE C3-58 from Ampelosaurus atacis and UBB NVM-25, which are considered as representing similar positions in the tail.The end of this process seems slightly deflected dorsally.This condition might be related with the dorsal projection of the prezygapophyseal facet as occur in the holotype of L. astibiae (MCNA 7458).In dorsal view, the prezygapophyseal processes are anterolaterally projected (Figure 7b).The distal end of the prezygapophyseal process is not located far beyond from the anterior edge of the centrum.The prezygapophyseal facet faces medially-to-slightly dorsomedially.The lateral face is transversely convex, the medial one is flat and slightly concave in the distal end, and the ventral edge is fractured.A ridge, interpreted as a rudimentary prdl, is located between the transition of the lateral face of the prezygapophyseal process and the dorsal face of the caudal rib.The prezygapophyseal processes are medially connected by a robust tprl, located at the level of anterior articular face of the centrum.There is a shallow fossa in the anterior face of the neural arch (lacking a developed cprl), between the neural canal and the rudimentary prdl, interpreted here as an incipient prcdf.The anterior face of the neural spine is marked by the presence of a well-developed sprl at least in the preserved section, which reaches the dorsal edge of the prezygapophyseal articulation.The 'sprl-process' is present just behind the prezygapophyseal facet (Figure 7a).This condition is also recognized in UBB NVM-25 and A. atacis (MDE C3-58) but it is absent in L. astibiae (MCNA 7458), L. pandafilandi (HUE-03134/ 6-5) and M. dacus (NHMUK R.3851).The anterior face of the spine yields a medially constricted prsl, which reach the base of the sprf.The sprf is wide and deep in the ventral half of the spine.There are some relatively large dorsoventrally elongated foramina in the surface of the sprf, laterally to the prsl.The postzygapophyses are located anteriorly to the edge of the posterior condyle.The postzygapohyseal facets are flat and dorsoventrally elongated.They are slightly posteriorly projected relative to the posterior edge of the spine, and they face ventrolaterally.The postzygapophyses are ventromedially connected by the V-shaped tpol.
The lateral face of the neural arch is relatively complex.A wide fossa is located anteriorly to the postzygapophyseal facet.This fossa is posteroventrally limited by an anteroventrallyposterodorsally oriented podl (Figure 7a), which was considered a synapomorphy of Lithostrotia by Mannion et al. (2013).This lamina is connecting the anterior edge of the postzygapophyseal facet to the posterior face of the transverse processes.A wide fossa is developed anterodorsally to the podl and the postzygapophyses, being anteriorly limited by the sprl and the prdl.It is interpreted as the sdf.This fossa is perforated by some foramina and bears some accessory and anteroventrally-posterodorsally rudimentary laminae (Figure 7a).A very shallow fossa is recognized ventrally to the postzygapophyseal facet and posterior to the podl.It is interpreted as the pocdf, which is shared with several lithostrotians and Huabeisaurus allocotus (Mannion et al. 2019b).The posterior face of the neural spine is delimited by the presence of the spol, which borders the spof.The spof is ventrally deep, but it is rapidly covered, above the level of the postzygapophyses, by the posl, which expands transversely.The posl reaches the ventral end of the spof but becomes thin and rudimentary.Two transverse laminae are observed in the lateral side of the posl, which are connecting the ventral end of the posl to the medial surface of the postzygapophyses.The neural spine is posterodorsally oriented as occur in Lirainosaurus astibiae (MCNA 7458).UBB ODA-18 differs from the slightly posterodorsally projected neural spines of Magyarosaurus dacus (NHMUK R.3851) and Lohuecotitan pandafilandi (HUE-03134/6-5), and from the subvertical neural spines of UBB NVM-25 and Ampelosaurus atacis (MDE C3-58).The neural arch preserves several cavities bigger than 2 mm of diameter, suggesting for the presence of internal camellate bone tissue at least in that anatomical region, as in some lithostrotian titanosaurs (e.g.Gomani 2005).

Description
UBB RR3 is a neural arch fragment corresponding to an anteriormost caudal vertebra (Figure 8(a-c)).The presence of a welldeveloped caudal rib associated to a prdl and a prcdf suggests that this neural arch belong to one of the first caudal vertebrae of the tail.The prezygapophyseal processes are anterodorsally projected (Figure 8a), similarly to UBB NVM-25 (not so dorsally projected than in UBB ODA-18), Magyarosaurus dacus (NHMUK R.3851) and Lirainosaurus astibiae (Díez Díaz et al. 2013).The distal end of the prezygapophyseal process is badly preserved.The anterior face of the of neural arch bears a robust cprl, which medially delimits the prcdf as occur in UBB NVM-25 and UBB ODA-18.The prcdf is relatively deep and it is laterally bordered by the prdl (Figure 8(ab)).The postzygapophyses are flat to slightly dorsoventrally concave (Figure 8c), as in UBB NVM-25.They bear a subcircular outline.There is a depression anterior to the postzygapophyses, interpreted as pocdf.This pocdf is dorsally limited by a ridge coming from the anterodorsal edge of the postzygapophyses, which corresponds to the podl (Figure 8a).The presence of pocdf and podl is shared by the lithostrotians (however, the non-titanosaurian titanosauriform Huabeisaurus allocotus also presents a pocdf in their anterior-most caudal vertebrae; Mannion et al. 2013Mannion et al. , 2019b)).The preservation of UBB RR3 does not allow to identify the presence of a sdf.The neural spine is badly preserved.The anterior face of neural spine is delimited by the sprl, which are poorly preserved, and by the sprf, well-developed at least in the ventral half of the spine.The sprf yields a transversely constricted prsl, a common feature in titanosaurs (Mannion et al. 2013).The posterior face of the neural arch is badly preserved.The orientation of the neural spine is unknown due to the preservation.

Description
UBB ODA-39 is a fragment of the neural arch of a caudal vertebra (Figure 8(d-f)).It corresponds to the base of the neural spine.Considering the similarities with the same element in UBB ODA-18, this neural spine is interpreted as belonging to one of the anterior-most caudal vertebrae.The anterior face is marked by the presence of the sprl (broken at the base), bordering the sprf (Figure 8(d-e)).A thin and medially constricted prsl is present as occur in titanosaurs (Mannion et al. 2013) and diplodocoids (Mannion et al. 2019b).The postzygapophyseal facets are not complete.Their surfaces are flat and face ventromedially.The spol are present, departing from the dorsal edge of the postzygapophyses and delimiting the spof (Figure 8f).The posterior face of the neural spines bears a thin posl, which slightly expands dorsally.Some dorsoventral scars are present in the surface of the spof lateral to the posl.The neural spine is posterodorsally projected as in UBB ODA-18.The observed internal cavities with more than 2 mm of diameter are interpreted as camellae, shared with other lithostrotian titanosaurs (Gomani 2005;Mocho et al. 2019a).

Description
These two elements were found in the same fossiliferous accumulation (together with an incomplete appendicular element), and their compatible size does not rule out the hypothesis of these two elements belong to the same individual.Both elements have a considerable size, bigger than the preserved caudal vertebrae of Magyarosaurus dacus (Nopsca 1915;Huene 1932) and Paludititan nalatzensis (Csiki et al. 2010a), and with similar size to the last preserved middle caudal vertebrae of Lohuecotitan pandafilandi (pers.observ.PM 2014-2020).For this reason, we will describe these two elements together.The first one is a neural arch fragment recognized as belonging to an anterior-most caudal vertebra (UBB ODS-3, Figure 9(a-c)) due the presence of a well-developed transverse process extending to the lateral face of the neural spine.The neural canal is oval and dorsoventrally taller than wide in posterior view, but subtriangular and transversely wide in anterior view.The crpl are thick and subvertical, bordering medially a deed prcdf.The prezygapophyses (not preserved) were medially connected by a tprl, which is posteriorly displaced, and delimits, ventrally, the sprf.The sprf preserves a subvertical and medially constricted prsl (Figure 9a).The postzygapophyses are ventrally supported by a welldefined centropostzygapophyseal lamina (cpol).There is moderately deep pocdf anterior to the cpol and anteroventral to the postzygapophyses, which represents a shared feature with several lithostrotians and Huabeisaurus allocotus (Mannion et al. 2019b).The pocdf is dorsally bordered by a well-developed podl, which delimits the sdf ventrally (Figure 9b).The postzygapophyses are flat-to-slightly concave.They face ventromedially and are connected by a transversely short tpol.The ventral sector of the spof is preserved, bearing a medially constricted and rough posl (Figure 9c).This posl is ventrally connected to the postzygapophysis by a shallow and thick lamina.The presence of medially constricted prsl and posl is common in titanosaurs (Mannion et al. 2013).The neural arch preserves an internal camellate bone tissue as some lithostrotian titanosaurs (Gomani 2005;Mocho et al. 2019a).
An almost complete posterior caudal vertebra (Figure 9(d-i)) was found associated to the previous described element.The articulations of the centrum are eroded, and the distal tip of the neural spine is broken.This caudal vertebra is platycoelous to amphicoelous, i.e. the anterior and posterior articular surfaces are flat to slightly concave as occur in the posteriormost middle caudal vertebrae of Paludititan nalatzensis (Csiki et al. 2010a) and in several middle-posterior and posterior caudal vertebrae referred to Magyarosaurus dacus (pers.observ.PM 2014).The anterior and posterior articular surface are wider than tall, preserving convex lateral and ventral edges, and a straight dorsal one.The lateral face of the centrum is concave anteroposteriorly and straight dorsoventrally.There is an anteroposteriorly elongated ridge located near the junction of the centrum with the neural arch (Figure 9d).The ventral face of the centrum preserves a shallow ventral hollow as occur in several titanosaurs but differing from the condition in the last preserved middle caudal vertebrae of P. nalatzensis (Csiki et al. 2010a).It is bordered by shallow ventrolateral ridges.No important fossae or foramina are observed in the lateral and ventral face of the centrum.The neural arch is located on the anterior half of the centrum (Figure 9d and 9f)), as occur in most titanosauriforms and some non-neosauropod eusauropods (Upchurch and Martin 2003;D'Emic 2012), not reaching the anterior edge of the anterior articular face.The sprl are present in the base of the neural spine, bordering a ventrally shallow sprf, which is restricted to the base of the spine (Figure 9h).The prezygapophyseal processes are medially connected by a tprl located on front of the anterior face of the centrum (Figure 9h) as in P. nalatzensis (Csiki et al. 2010a).The internal bone tissue of the centrum lacks pneumatic structures.

Description
UBB ODA-52 is an almost complete anterior caudal vertebra (Figure 5(g-l)).Based on the comparisons with partially complete caudal series (Campos et al. 2005;Lacovara et al. 2014;Otero et al. 2021), we interpreted this vertebra as corresponding to the sixth-to-eighth position.The centrum is procoelous, i.e. the anterior articular surface is broadly concave, and the posterior one is markedly convex.The posterior articular surface is not constricted but a non-pronounced rim is present (Figure 5j).A small depression is located on the dorsal half of the condyle.The anterior articular surface of the centrum is subcircular, with convex lateral and ventral edges, and a straight-tosmoothly concave dorsal edge (Figure 5h).This anterior articular surface is mediolaterally longer than dorsoventrally, similarly to the condition in UBB PL1-1, UBB ODA-76, Magyarosaurus dacus (anterior caudal vertebrae of NHMUK R.3849, one element of NHMUK R.4898) and Paludititan nalatzensis (Csiki et al. 2010a, UBB NVM1-2).In lateral view, the anterior articulation of the centrum is slightly anterodorsally inclined but not so pronounced as in UBB PL1-1 and UBB ODA-76.The dorsal edge of the anterior articular surface is posteriorly deflected, being concave in dorsal view.The posterior articular surface of the centrum is also subcircular, the dorsal edge is straight and the lateral and ventral edges are convex (Figure 5j).The lateral face of the centrum is slightly ventromedially directed, and markedly anteroposteriorly and dorsoventrally straight.A small, elliptical and anteroposteriorly elongated fossa, and some little foramina are developed below the caudal rib, as occur in Andesaurus delgadoi (Mannion and Calvo 2011).This feature was recovered as a potential synapomorphy of Titanosauriformes (Mannion et al. 2013).The ventral face is transversely concave as in other titanosaurs (Wilson 2002;D'Emic 2012), being wider than in UBB ODA-76 and UBB PL 1-1.The ventral hollow is delimited by well-developed ventrolateral ridges, which reach both chevron facets (Figure 5l).The posterior chevron facets are ventrally projected.The anterior chevron facets are also present but not so developed than the posterior ones.The surface of both facets is eroded.The base of the caudal rib is located at the anterodorsal sector of the lateral face of the centrum, extending to the the lateral face of the pedicles of the neural arch.A posterolateral orientation for the caudal ribs is interpreted (Figure 5k).The presence of posterolaterally oriented caudal ribs is shared by titanosauriforms, Haplocanthosaurus and some eusauropods (Mannion et al. 2019b).The neural arch is anteriorly displaced as occur in titanosauriforms (Salgado et al. 1997) and in the nonneosauropod eusauropods (not reaching the anterior articular surface; Upchurch and Martin 2003;Britt et al. 2017;Royo-Torres et al. 2017).The prezygapophyseal processes are significantly long (extending more than 60% of the anteroposterior length of the centrum excluding the condyle) and anterodorsally oriented (Figure 5i).They are not so transversely compressed than in UBB PL1-1.The lateral face of the prezygapophyseal processes is covered by some rugosities, at the level of the prezygapophyseal facet (slightly dorsoventrally expanded), resulting in a lateral bulge interpreted as the pre-epipophyseal process.This process can be observed in some titanosaurian taxa such as Lohuecotitan pandafilandi (pers.observ.PM 2014-2020), Mendozasaurus neguyelap (González Riga et al. 2018) and Notocolossus gonzalezparejasi (UNCUYO-LD 301, González Riga et al. 2016), and in the diplodocine Diplodocus (Tschopp et al. 2015).This boss structure is absent in Magyarosaurus dacus and rudimentary in Paludititan nalatzensis and UBB PL1-1.The prezygapophyseal processes are medially connected by robust tprl, which are located at the level of anterior articular surface of the centrum.The medial face of the prezygapophyseal process is transversely convex.The anterior face of the neural spine is delimited by the sprl, which is restricted to the ventral half of the spine (Figure 5(g-i)).The sprl is rudimentary in the dorsal half of the spine and not reaches the distal end of the prezygapophyseal process.Ventrally, the anterior face of the spine yields a rough and medially restricted prsl as in the members of Titanosauria (Mannion et al. 2013), which reach the base of the sprf.The postzygapophyses are located anterior to the edge of the posterior condyle.The postzygapohyseal facets are slightly posteriorly displaced relative to the posterior edge of the spine, preserving an elliptical and dorsoventrally elongated outline and a flat-todorsoventrally concave surface.There is a small dorsoventrally elongated fossa anterior to the postzygapophyseal facet, interpreted as the pocdf (Figure 5g and 5i), as occur in UBB PL1-1.The ventral edge of the postzygapophyses is medially connect by a short tpol, which present a circular and small pit.The posterior face of the neural spine is delimited by the spol, which is restricted to base of the neural spine, and bordering a dorsoventrally short spof.Dorsally to the spof, the neural spine is covered by a rough surface interpreted as the posl.The orientation of the neural spine is unknown based on its available section.This vertebra lacks internal pneumatic structures, but there are some internal cavities with more than 2 mm of diameter in the fractured areas of the neural spine and caudal ribs, suggesting for a rudimentary internal camellate bone tissue.

Description
UBB 15.548 is a caudal vertebra, which does not preserve the neural arch and part of the anterior articulation (Figure 10(a-f)).This element is interpreted as a middle caudal vertebra, due the absence of caudal ribs and relative moderate average aEI.This caudal vertebra is strongly procoelous, i.e. the anterior articular surface is deeply concave, and the posterior one is strongly convex (Figure 10c and 10e) as occur in most lithostrotian titanosaurs (Mocho et al. 2019a).The outline of the cotyle is not complete, but seems to be sub-circular, with convex ventral and lateral edges as in Magyarosaurus dacus (middle caudal vertebrae of NHMUK R.3865, some of them has a straight ventral edge).The condyle is subcircular but transversely wider in the dorsal half than in the ventral one.Its ventral edge is transversely short (Figure 10f).The ventral and lateral edges are convex and the dorsal one is transversely concave, which result in two round buttresses (Figure 10a and 10f) as in Paludititan nalatzensis (Csiki et al. 2010a) and Lohuecotitan pandafilandi (these processes extend to the dorsal surface of the centrum in this taxon; see Díez Díaz et al. 2016).A concave dorsal edge in the posterior articulation is absent in several of the middle vertebrae referred to M. dacus (Huene 1932), but present in some of the anterior caudal vertebrae of the species (e.g.MAFI Ob.3091, NHMUK R.4898).It is important to refer that the morphology of the articular surfaces of the centrum has some important variability in M. dacus when we consider all the referred material (Nopcsa 1915;Huene 1932;pers. observ. PM 2014).The condyle bears a ventrally and laterally developed rim, with a dorsoventrally elongated central pit in the dorsal half.The lateral face is concave anteroposteriorly and straight dorsoventrally (Figure 10f).The ventral surface of the centrum is transversely concave, being wider in the posterior half, and it is welldelimited by two ventrolateral ridges, one in each side (Figure 10b), as occur in several titanosaurs (Wilson 2002;D'Emic 2012).
No foramina or fossae are present in the lateral and ventral face of the centrum.The ventrolateral ridges ridges connect the anterior and posterior chevron facets.The posterior chevron facets are semi-circular and ventraly projected, being well-visible in posterior view.The left and right posterior chevron facet have a concave and flat surface, respectively.The anterior chevron facets are not so well-developed, being represented by a surface covered by several rugosities.The neural arch is not preserved, but it was located on the anterior half of the dorsal surface of the centrum (Figure 10a, c and 10e) as in most titanosauriforms and in some non-neosauropod eusauropods (Upchurch and Martin 2003;D'Emic 2012).The internal bone tissue of the centrum and that of the base of the neural arch lacks pneumatic internal structures.

Description
UBB NV-87 is a middle caudal vertebra, with an incomplete neural arch and a poorly preserved surface (Figure 10(g-j) and 10l).This caudal vertebra is procoelous, i.e. the anterior articular surface is slightly concave, and the posterior one bears a well-developed convexity (Figure 10g and 10i), as occur in most lithostrotian titanosaurs (Mocho et al. 2019a).The outline of the anterior and posterior articular surfaces is difficult to describe due their preservation.The posterior face seems to lack a developed rim and a pit.The lateral face is concave anteroposteriorly and straight dorsoventrally.There is an anteroposteriorly ridge located near the junction of the centrum with the neural arch.Below this ridge, the surface of the lateral face of the centrum is slightly concave.The ventral face of the centrum is badly preserved, and no major foramina are observed in the lateral and ventral face.The neural arch is located on the anterior half of the centrum (Figure 10g and 10i), as occur in most titanosauriforms (D'Emic 2012), not reaching the anterior edge of the anterior articular surface.The sprl is present in the base of the neural spine, bordering a ventrally and triangular sprf, which is also restricted to the base of the spine (Figure 10(h, l)).The prezygapophyseal processes are medially connected by a tprl located at the level of the anterior face of the centrum, in contrast with the condition in Paludititan nalatzensis (Csiki et al. 2010a).The internal bone tissue of the centrum and that of the base of the neural arch lacks pneumatic internal structures.

Description
UBB NVM-6 is a middle caudal vertebra with an incomplete posterior articular surface and lacking the neural arch (Figure 10k and 10(m-q)).This caudal vertebra is procoelous, i.e. the anterior articular surface is slightly concave, and the posterior one is moderately convex (Figure 10m and 10o); these features being shared with some deeply nested groups of lithostrotians and some titanosauriforms (D'Emic 2012; Mannion et al. 2013).The apex of the posterior convexity is slightly dorsally displaced as in UBB 15.548, unlike Magyarosaurus dacus (the middle caudal vertebrae of NHMUK.R.3865) and Lohuecotitan pandafilandi (Díez Díaz et al. 2016).The outline of the anterior and posterior articular surface is subcircular, with convex edges as in UBB 15.548.The posterior articular surface of the centrum seems to lack a developed rim but it has a small pit in the center.The lateral face is also poorly preserved, being concave anteroposteriorly and straight-to-convex dorsoventrally.No lateral ridge, fossae or foramina are present.The ventral face of the centrum is transversely flat and flat-to-slightly concave in the anterior and posterior half, respectively (Figure 10k).A rudimentary ventral hollow is present.This concavity on the posterior half of the ventral face is limited laterally by the ventrolateral ridges, which are connected to the posterior chevron facets.These facets are not so ventrally projected as in the anterior and anterior-middle caudal vertebrae but are wellvisible in posterior view.They preserve a semicircular-tosubtriangular outline.They are more developed than the anterior ones, which are rudimentary.The neural arch is located on the anterior half of the centrum (Figure 10m, o and 10q) as in other titanosauriforms and some non-neosauropod eusauropods (Wilson 2002;Upchurch and Martin 2003).The internal bone tissue of the centrum lacks pneumatic internal structures.

Description
UBB SPM-4 is an almost complete middle caudal vertebra, lacking a small section of both the neural spine and the centrum, which does not prevent to provide an almost complete description of the element (Figure 11).This caudal vertebra is transversely compressed due to the taphonomy.The centrum is slightly procoelous, i.e. the anterior articular surface is moderately concave, and the posterior one is sightly convex (Figure 11b and 11d), sharing this condition with Lithostrotia (D'Emic 2012; Mocho et al. 2019a).The apex of the posterior convexity is not dorsally displaced as in Magyarosaurus dacus (e.g.Huene 1932) and Lohuecotitan pandafilandi (Díez Díaz et al. 2016).The outline of the anterior and posterior articular surface is interpreted as subcircular, with convex edges as in M. dacus (e.g.Huene 1932) and Paludititan nalatzensis (Csiki et al. 2010a), and it is transversely compressed, being dorsoventrally longer than mediolaterally, due to taphonomy.The condyle preserves a laterally and ventrally well-developed rim and a circular pit in the center (Figure 11e).This rim in the posterior articulation surface is common in titanosaurs (e.g.Kellner et al. 2005;Díez Díaz et al. 2016).The lateral face is concave anteroposteriorly and flat dorsoventrally.There is an anteroposteriorly elongated ridge at the level of the junction between the neural arch and centrum (Figure 11b).There are some rugosities above this ridge, and an elliptical small fossa below.The ventral face of the centrum is transversely flat at the midpoint and flat-to-slightly concave near the anterior and posterior articular surfaces of the centrum (Figure 11e), as occur in P. nalatzensis (Csiki et al. 2010a); and differing from M. dacus (some caudal vertebrae of NHMUK R.3865), which seems to bear a ventral hollow on the middle-posterior section of the tail.This concavity is laterally bordered by incipient ventrolateral ridges connected to the anterior and posterior chevron facets.These facets are eroded.The neural arch is located on the anterior half of the centrum as in titanosauriforms and other non-neosauropod eusauropods (Upchurch and Martin 2003;D'Emic 2012), but the pedicels do not reach the anterior edge of the cotyle.The prezygapophyseal facet is circular, reduced, and flat.The prezygapophyseal processes are anteriorly projected and they slightly expand towards the distal end as in P. nalatzensis (Csiki et al. 2010a).The lateral face is transversely convex.These processes are long, reaching 50% of the total anteroposterior length of the centrum, excluding the condyle.Middle-posterior caudal vertebrae with anteroposteriorly eleongate prezygapophyseal process, which extend 20% or greater of the centrum length (excluding ball) beyond the anterior margin of the centrum was considered a synapomorphy of Somphospondyli/ Titanosauria by Mannion et al. (2013).A ridge departs from the dorsolateral edge of the prezygapophyses to the lateral face of the neural arch, similarly to the shoulder-like region described by Mannion et al. (2013) for Lusotitan atalaiensis.The anterior face of the neural spine preserves a short sprl which delimits a very reduced sprf.The anterior face of the neural spines is rough and covered by the prsl (Figure 11a).The prezygapophyseal processes are medially connected by a robust and anteriorly developed tprl, which is markedly anteriorly located relative to the cotyle (Figure 11a).This feature seems to be characteristic of some titanosaurs, including L. pandafilandi (pers.observ.PM 2014-2020) and P. nalatzensis (pers.observ.PM 2018).The postzygapophyses, located on the posterior half of the centrum, are posteriorly projected as in P. nalatzensis (Csiki et al. 2010a), and located posteriorly to the posterior edge of the spine (being at the level of the posterior edge of the spine in P. nalatzensis).The postzygapophyseal facets are subcircular and flat.Dorsally to these facets, the dorsoventrally short spol are delimiting a reduced spof, dorsally covered by some rugosities interpreted as the posl.The neural spine is vertically oriented, transversely compressed and not expanded anteroposteriorly nor transversely towards its end.Its anterior edge is subvertical and located anteriorly to the cotyle, and the posterior one is anterodorsally-posteroventrally oriented in lateral view, being subvertical in P. nalatzensis (see Csiki et al. 2010a).

Locality
Pui Gater (Hațeg Basin); Sânpetru Formation, early Maastrichtian in age (Panaiotu and Panaiotu 2010).Description UBB PGR1-6 (Figure 12(a-f)), UBB PGR1-14 (Figure 12(g-l)) and UBB PGR1-15 (Figure 12(m-r)) are a set of three posterior caudal vertebrae found in Pui Gater.The UBB PGR1-15 is posterior to the remaining ones, with a gap of a few vertebrae between them.These vertebrae are partially complete.The prezygaphophyseal processes, the posterior end of the neural spine, and the edges of the anterior and posterior articular surfaces of the centrum are broken in UBB PGR1-6 and UBB PGR1-14.The neural arch of UBB PGR1-15 is absent and interpreted as not fused to the centrum.These caudal vertebrae are slightly amphicoelous (Figure 12c, e, i, k, o and 12q), i.e. both the anterior and the posterior articular surfaces are sightly concave, with robust and wide edges.The outline of the anterior and posterior articular face is subcircular to subquadrangular with convex lateral edges and concave (UBB PGR1-6, Figure 12d and 12f) or straight (UBB PGR1-14 and UBB PGR1-15, Figure 12j, l, p and 12r) dorsal and ventral ones.Both articular faces are mediolaterally wider than dorsoventrally.The lateral face of the centrum is concave anteroposteriorly and convex dorsoventrally.There is an anteroposteriorly elongated ridge in UBB PGR1-6 and UBB PGR1-14 at the level of the junction between the neural arch and the centrum (Figure 12e and 12i)), which is absent in UBB PGR1-15.There is a shallow and elliptical small fossa bellow this ridge in all these vertebrae except in UBB PGR1-15.The ventral face of the centrum is transversely flat (Figure 12b, h and 12n).Only the anterior chevron facets are developed, with a semicircular outline.These facets are absent in UBB PGR1-15.The neural arch is located on the anterior half of the centrum, a condition shared with titanosauriforms and some non-neosauropod eusauropods (e.g.Upchurch and Martin 2003;D'Emic 2012), reaching the anterior edge of the anterior articular surface in UBB PGR1-14.The neural arch of UBB PGR1-15 is absent, but the sutures indicates that this structure was only slightly anteriorly displaced.The prezygapophyseal processes are anteriorly projected, but the distal end is not preserved.The lateral face of these processes is transversely convex.The lateral shoulderlike region described for UBB SPM-4 is also visible here.The anterior face of the neural spine preserves a very short sprl which delimits a very reduced sprf.As in UBB SPM-4 and Paludititan nalatzensis (Csiki et al. 2010a), the prezygapophyseal processes are medially connected by robust and anteriorly developed tprl, which is markedly anteriorly located relative to the cotyle.The postzygapophyses, located on the posterior half of the centrum, are posteriorly projected similarly to the condition in UBB SPM-4 and P. nalatzensis (Csiki et al. 2010a).The poszygapophyseal facets are subcircular and flat.A short spol departs from the dorsal edge of the postzygapophyseal facets and converge rapidly resulting in a dorsoventrally short spof.The neural spine is reduced, being dorsoventrally low.It is anteroposteriorly long, resulting in a dorsal longitudinal crest.The centra and the neural arch preserve no pneumatic internal structures.

Systematic approach
This work allows us to describe an important sample of caudal vertebrae from the Maastrichtian deposits of Romania.In this section we justify the proposed systematic attributions and provides a comprehensive discussion about how many morphotypes can be identified in this set from the Upper Cretaceous of Romania and about its taxonomic value.The systematic attribution proposed for some of the elements described and discussed here is treated with caution, however, we believe that all the considered remains probably belongs to Titanosauria, and specifically to the Lithostrotia clade.Titanosaurs are the only group of sauropod dinosaurs identified during the uppermost Upper Cretaceous fossil record worldwide (e.g.Upchurch et al. 2004).Anterior-most and anterior caudal vertebrae.UBB ODA-76 is an anterior-most caudal vertebra that shows a combination of characters exclusive for the members of Titanosauria including: (i) the presence of a ventral hollow bordered by ventrolateral ridges, recovered as synapomorphy of Titanosauria by Wilson (2002), D'Emic (2012) and Mannion et al. (2013; where the presence of a ventrolateral ridges in the centrum was recovered as synapomorphy of Titanosauria based on a LSDM matrix, and of 'Titanosauroidea' based on a LCDM matrix); (ii) a convex posterior articular surface, considered as a synapomorphy of Titanosauria (Wilson 2002), Lithostrotia (D'Emic 2012) and 'Titanosauroidea' (Mannion et al. 2013, based on the LCDM matrix); and (iii) the presence of a dorsal bulge in the transverse processes (Poropat et al. 2016).The co-occurrence of a convex posterior articular surface and a ventral hollow in UBB PL1-1 and UBB NVM-22 also supports their tentative assignation to the Titanosauria clade.
UBB NVM-25 and UBB ODA-18 preserve a combination which only occurs in the members of Lithostrotia.This combination includes synapomorphies of the Titanosauria clade or of more exclusive clades (e.g.Wilson 2002;D'Emic 2012;Mannion et al. 2013), such as the presence of a strongly convex posterior articular surface and that of a ventral hollow; and also some features that seem exclusive of some lithostrotian taxa, including the presence of: (i) internal camellate bone tissue in the neural arch of anterior-most caudal vertebrae (Gomani 2005;Mocho et al. 2019a); (ii) 'sprlprocess' in the anterior caudal vertebrae (D'Emic 2012); and (iii) sharp lipped posdf and pocdf, and the presence of a podl in anterior caudal vertebrae (see Mannion et al. 2013Mannion et al. , 2019b)).Besides the incompleteness of UBB ODA-39, the presence of a medially constricted prsl (synapomorphy of Titanosauria following Mannion et al. 2013), and internal camellate bone tissue in the neural arch (shared with lithostrotian members; Gomani 2005;Mocho et al. 2019a) suggest its assignation to Lithostrotia.The anterior neural arch UBB RR3 and UBB ODS-3 share with the titanosaurs a transversely constricted prsl (Mannion et al. 2013, synapomorphy of Titanosauria based on LSDM matrix, 2019b).UBB RR3 presents a well-developed pocdf, a common feature for the lithostrotians (Mannion et al. 2013); and UBB ODS-3 has an internal camellate bone tissue, shared by some lithostrotian titanosaurs (Powell 1992;Gomani 2005;Mocho et al. 2019a).Finally, UBB ODA-52 preserves a strongly convex posterior articular surface, with an associated slightly developed rim; a ventral hollow; a medially restricted prsl; and a pocdf.The presence of these characters supports its tentative assignation to Lithostrotia.Middle caudal vertebrae.The middle caudal vertebrae are less variable than the anterior caudal vertebrae, but some features have been identified as diagnostic in this sector of the tail for some European titanosaurs (Csiki et al. 2010a;Díez Díaz et al. 2013, 2016).The analyzed middle caudal vertebrae (UBB 15.548, UBB NV-87, UBB NVM-6, UBB SPM-4 and UBB ODAN-32) show an anteriorly displaced neural arch, which was considered as synapomorphy of Titanosauriformes (e.g.Wilson 2002) such as Giraffatitan brancai (Janensch 1950) and Saltasaurus loricatus (Powell 1992), but is also present in some non-neosauropod eusauropods, such as Cetiosaurus oxoniensis (Upchurch and Martin 2003).UBB 15.548, UBB NV-87, UBB NVM-6 and UBB SPM-4 can be referred to Lithostrotia due the presence of a strong procoelous centrum.The presence of this condition in middle and posterior caudal vertebrae was considered as a synapomorphy of more exclusive groups within Lithostrotia such as Nemegtosauridae + ['Titanosaurus' colberti + Saltasauridae] (Wilson 2002) and Alamosaurus + 'Saltasaurini' (D'Emic 2012); but also recovered as synapomorphy of 'Titanosauroidea' (Mannion et al. 2013).Some titanosauriforms might acquire a posterior convexity in the posterior caudal vertebrae such as Giraffatitan brancai (D'Emic 2012), Astrophocaudia slaughteri (D'Emic 2013), Gobititan shenzhouensis (You et al. 2003) or Brontomerus mcintoshi (Taylor et al. 2011).In addition, UBB 15.548 and UBB SPM-4 preserves a posterior convexity with well-developed rim and pit, which are common in several titanosaurs (e.g.Díez Díaz et al. 2016).UBB 15.548 and UBB NVM-6 also has a well-developed ventral hollow in the middle caudal vertebrae, bordered by ventrolateral ridges, which was recovered as synapomorphy of Lithostrotia by some authors (Wilson 2002;D'Emic 2012), and is present in middle caudal vertebrae of many lithostrotians such as Malawisaurus dixeyi (Gomani 2005), Opisthocoelicaudia skarzynskii (Borsuk-Bialynicka 1977), Saltasaurus loricatus (Powell 1992;pers. observ. PM 2019), Rapetosaurus krausei (Curry Rogers 2009;pers. observ. PM 2017) and Isisaurus colberti (Jain and Bandyopadhyay 1997).The cooccurrence of a strong procoelous condition, well-developed rim and pit in the posterior articular surface and a ventral hollow is a combination of features unique for lithostrotian titanosaurs (e.g.Mocho et al. 2019a).UBB SPM-4 has a similar morphology that of the same vertebrae of Paludititan nalatzensis (Csiki et al. 2010a), but those of the later present a flat posterior articulation.UBB SPM-4 and P. nalatzensis share an anteriorly displaced tprl, which is also shared with Lohuecotitan pandafilandi.The middle caudal vertebrae of L. pandafilandi are also characterized by the presence of round buttresses in the dorsal edge of the posterior articular surface of the centrum, which is absent in P. nalatzensis and UBB SPM-4.With the available information, we tentatively refer UBB SPM-4 to cf.Paludititan nalatzensis.
UBB ODAN-32 is characterized by a moderately-to-slightly convex anterior articular surface of the centrum.The presence of opisthocoelous caudal vertebrae is uncommon within the sauropods, being present in some titanosauriforms: the lithostrotian Opisthocoelicaudia skarzynskii (Borsuk-Bialynicka 1977), Rinconsaurus caudamirus (Calvo and González Riga 2003) and Sonidosaurus saihongaobiensis (Xu et al. 2006); the early branching titanosaur Abdarainurus barsboldi (Averianov and Lopatin 2020); an indeterminate titanosaur of Uzbekistan (Sues et al. 2015); the non-titanosaurian somphospondylan Borealosaurus wimani (You et al. 2004), Huabeisaurus allocotus (D'Emic et al. 2013) and Fukuititan nipponensis (Azuma and Shibata 2010); and in the nonneosauropod eusauropod Turiasaurus riodevensis (Royo-Torres et al. 2006).B. wimani was firstly referred to Opisthocoelicaudiinae due the presence of the opisthocoelous condition (see You et al. 2004).The occurrence of the opisthocoelous condition in these taxa has distinct distributions throughout the caudal series: (i) in the first 15 caudal vertebrae in O. skarzynskii (strong convex anterior articular surface, which become shallower in the last opisthocoelous vertebrae) and at least in the first anterior-most caudal vertebra of S. saihongaobiensis (slight opisthocoelous condition; Xu et al. 2006);(ii) Calvo and González Riga 2003).In addition, some of the referred taxa does not seem to preserve fully developed opisthocoelous condition, with a convex anterior articular surface and concave posterior one, such as T. riodevensis (pers.observ.PM 2014) and F. nipponensis (Azuma and Shibata 2010).The middle caudal vertebra UBB ODAN-32 lacks some synapomorphies of Lithostrotia such as the presence of a ventral hollow (Wilson 2002;D'Emic 2012).UBB ODAN-32 should be referred for the moment to an indeterminate somphospondylan.The presence of a ventral keel in the middle caudal vertebra is uncommon within Sauropoda, as should be diagnostic for this taxon that is still undescribed.Azuma and Shibata (2010) described the presence of a weak ridge in the posterior caudal vertebrae of F. nipponensis, which have been recovered as a titanosauriform of uncertain affinities by Azuma and Shibata (2010) and by Mannion et al. (2013) in the analysis based on its LSDM matrix and as a early branching 'titanosauroid' by Mannion et al. (2013) in the analyses based on the LSDM matrix plus implied weight and the LCDM matrix.Posterior caudal vertebrae.The posterior caudal vertebra UBB P-64 and UBB P-180 have less diagnostic information than the anterior and middle caudal vertebrae.They have an anteriorly displaced neural arch, a characteristic that was considered a synapomorphy of Titanosauriformes (e.g.D'Emic 2012).As was discussed before, some non-neosauropod eusauropods also preserved this feature (e.g.Upchurch and Martin 2003) but this anterior displacement is generally not so developed.In addition, no non-neosauropod eusauropods have been identified in the Upper Cretaceous deposits (e.g.Upchurch et al. 2004), supporting our tentative assignation of UBB P-64 and UBB P-180 to Titanosauriformes.The ventral face of both vertebra is flat and can be slightly concave near the anterior edge in UBB P-64 and near the posterior one in UBB P-180.This ventral face is also bordered by shallow ventrolateral ridges.The presence of a ventral hollow is diagnostic in the middle caudal vertebrae of the lithostrotians, which can reach the posterior section of the tail.However, this ventral hollow is not fully developed in UBB P-64 and UBB P-180, and the absence of a posterior condyle conditionate a more precise taxonomic approach.
The posterior caudal vertebrae UBB ODS-4 was found in strongly association with UBB ODS-3, which was referred to Lithostrotia indet.UBB ODS-4 is tentatively referred to Lithostrotia indet.because its size is compatible to the size of UBB ODS-3, suggesting that they might belong to same individual.In addition, UBB ODS-3 has an anteriorly displaced tprl, which suggested that it can be tentatively referred to cf.Paludititan nalatzensis.This individual seems to be a medium size individual, considerably bigger than the holotype of P. nalatzensis (Csiki et al. 2010a).This vertebra also presents a shallow ventral hollow bordered by shallow ventrolateral ridges, not evident in UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15, which corresponds to a much smaller individual.
The three posterior caudal vertebrae UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15 are tentatively referred to cf.Paludititan nalatzensis due the presence of an anteriorly displaced tprl.As we referred above, this feature is present in the middle caudal vertebrae of P. nalatzensis (Csiki et al. 2010a) and Lohuecotitan pandafilandi.Due to the fact that L. pandafilandi has not yet been discovered in the Upper Cretaceous of Romania.The middle caudal vertebrae of this taxon are also characterized by the presence of two round buttresses in the dorsal edge of the posterior articular surface, not present in the material studied here.

Accessing the diversity of Maastrichtian titanosaurs in Romania
The description of the new specimens studied here and their comparison with the material previously referred to Magyarosaurus dacus and Paludititan nalatzensis allow us to identify at least four different morphologies for the anterior-most caudal vertebrae (morphologies Am1, Am2, Am3 and Am4) for Romanian Maastrichtian titanosaurs.The anterior caudal vertebrae described here, and the ones referred to M. dacus and P. nalatzensis also present an important morphological variability with at least three distinct morphologies (morphologies A1, A2 and A3).They are analysed separatedly from the anterior-most caudal vertebrae, due the presence of important morphological changes that occur between the first and the last half of the anterior caudal series (loss of a well-developed transverse process and complex diapophyseal laminae and fossae, and the acquisition of transversely and markedly compressed neural spines).The available data also allow us to identify at least three different morphologies for the middle caudal vertebrae in our sample and M. dacus and P. nalatzensis (morphologies M1, M2 and M3).Finally, considering all the posterior caudal vertebrae studied here, we recognize at least two morphotypes (morphologies P1 and P2).For a resume see Table 3. Morphology Am1.One of the identified morphologies represented in our sample can be exemplified by UBB NVM-25 and UBB ODA-39 (Figure 6, 8 (a-c) and 13a and 13f), which are characterized by a vertical to slightly anteriorly oriented neural spine (Morphology Am1).This morphotype can also be characterized by a strong procoelous condition, dorsally displaced posterior condyle, internal camellate bone tissue, sharp lipped pocdf and posdf or sdf, and horizontal podl.In addition, the possible first caudal vertebra has a deep prcdf, which is divided by an accessory lamina.This morphology is markedly distinct from NHMUK R.3851 (Figure 13e and 13i), an anterior-most caudal vertebra referred to Magyarosaurus dacus, which are characterized by a posterodorsally oriented neural spine and a shallow and undivided prcdf.UBB NVM 25 and UBB ODA-39 cannot be directly compared with those of Paludititan nalatzensis, due to it lacks the first anterior-most caudal vertebrae (Csiki et al. 2010a); however, their last anterior-most caudal vertebrae (Figure 13d) also present a developed posterior condyle (dorsally displaced) and subvertical neural spines, suggesting for the possibility of UBB NVM-25 and UBB ODA-39 belong to P. nalatzensis.Morphology Am2.UBB ODA-18 (Figure 7) and UBB RR3 (Figure 8  (d-f)) are markedly distinct from UBB NVM 25 and UBB ODA-39.The neural spine is posterodorsally oriented as in Magyarosaurus dacus (Nopsca 1915;Huene 1932; is only slightly posterodorsally oriented).The 'sprl-process' of UBB-ODA-18 is absent in the anterior-most caudal vertebrae of M. dacus (but present in one of the anterior caudal vertebrae referred to NHMUK R.4898, Figure 13o).In addition, UBB ODA-18 differs from UBB NVM 25 due the presence of a subvertical podl, and more dorsally and laterally projected prezygaphophyses (also absent in M. dacus, Figure 13e).The posterior articular surface is not markedly dorsally projected contrasting with some anterior-most caudal vertebrae of M. dacus, and the ventral face is transversely concave, which differs from the flat ventral face of the anterior-most caudal vertebrae of M. dacus NHMUK R.3851 (Figure 13i).Besides the similarities between the spine orientation, some characters suggest that UBB ODA-18 cannot be referred for the moment to M. dacus, the later has dorsally oriented neural arch and laterally oriented caudal ribs.UBB ODA-18 and UBB RR3 represent a more anterior position in the tail than UBB ODA-76 and UBB PL1-1; however, the moderate convexity of the posterior articulation and the subvertical neural spine in UBB PL1-1 (Figure 5 (a-f) and 13k) suggests that they might belong to different taxa.Morphology Am3.UBB ODA-76 is one of the last anterior-most caudal vertebrae and lacks the neural spine (Figure 4(d-i), 13c and h).It shows a moderately-to-slightly developed convex posterior articular surface of the centrum, which is markedly different from the anterior caudal vertebrae of Paludititan nalatzensis (Csiki et al. 2010a), and the referred ones of Magyarosaurus dacus (e.g. the anterior-most caudals of NHMUK R.3849, NHMUK R.3854 ;Nopcsa 1915;Huene 1932).This vertebra is also characterized by an anterodorsally inclined anterior articular surface in lateral view and a well-developed ventral hollow.This vertebra is slightly posteriorly located than UBB NVM-25 and UBB ODA-18, and the hypothesis of this vertebrae corresponds to the same taxon of UBB NVM-25 or UBB ODA-18 seems unlikely, requiring an abrupt reduction of the posterior convexity of the centrum in Strongly convex posterior articular surface, which is slightly dorsally displaced; ventral hollow is present in the last anterior-most caudals.
Neural arch is anterodorsally oriented; caudal ribs are posteriorly projected; neural spine seems to have a vertical orientation.
Only the last anterior-most caudals are preserved and the Morphology Am1 cannot be differentiated from Paludititan nalatzensis.

Morphology A1
Moderately-to-slightly convex posterior articular surface, which is dorsally displaced; anterior articular surface is anteriorly inclined; ventral hollow is present.
UBB PL1-1 is attributed to this morphology, and possibly corresponds to Morphology Am3.

Morphology A2
Strongly convex posterior articular surface, which is not dorsally displaced; posterior condyle is constricted and marked by a rim; ventral hollow is present.
Cprl is not develloped; the lateral tuberosity in the prezygapophyseal processes is present; the neural spine is posterodorsally oriented.
UBB ODA-52 is attributed to this morphology.

Magyarosaurus dacus
Strongly convex or slightly convex posterior articular surface (dorsal displacement is variable); anterior articular surface is subvertical or anteriorly inclined; ventral hollow is present.
'Sprl-process' is present in some vertebrae; the morphology and orientation of the neural spine is markedly variable.
Based on the referred specimens NHMUK R.4898 (two elements of three), NHMUK R. the first half of the anterior series of caudal vertebrae.A moderate convexity is also present in UBB PL1-1 (Figure 5(a-f)), which belongs to the posterior half of the anterior series.This feature might be characteristic from the taxon or taxa represented by UBB ODA-76 and UBB PL1-1, which is a distinct taxon to P. nalatzensis and M. dacus, suggesting for the presence of an undescribed taxa in the Maastrichtian levels of Romania.Morphology Am4.-18), dorsally oriented neural arch (differing from the other remaining morphologies described herein), a shallower and undivided prcdf; laterally oriented caudal ribs (distinct from UBB NVM-25), and the presence of a ridge which connect the posterodorsal edge of the centrum with the dorsal tuberosity of the transverse process (Figure 13e and 13i).The centrum is characterized by a markedly convex posterior articular surface of the centrum, which is dorsally displaced; a ventral longitudinal hollow (except in the first anterior-most caudal vertebra); and a short aEI in the first caudal vertebrae.Morphology A1.UBB PL1-1 represents one of the morphologies observed in anterior caudal vertebrae (Figure 5(a-f) and 13k).It shares with UBB ODA-76 the moderate-to-slight procoelous condition.In addition, the anterior articular surface of the centrum is anterodorsally inclined as in UBB ODA-76 and also, as some anterior caudal vertebrae of Magyarosaurus dacus (e.g.NHMUK R.4898, Figure 13(n-o)), which seems markedly different from the subvertical anterior articular surface of Paludititan nalatzensis (Figure 13l).UBB PL1-1 has dorsoventrally concave postzygapophysis, resulting in the lateral projection of its ventral edge, feature that is absent in the already established taxa (Nopcsa 1915;Huene 1932;Csiki et al. 2010a); and it also shows a relatively deep cprl that distinguish this vertebra from UBB ODA-52, P. nalatzensis, and M. dacus.The prezygapophyseal processes present an extreme anterior development not observed in other anterior caudal vertebrae (M.dacus, P. nalatzensis and UBB ODA-52).Only one of the elements of NHMUK R.3849 referred to M. dacus can be referred to this morphology, characterized by a slightly convex posterior articular surface, and an anteriorly inclined anterior articular surface in lateral view.Morphology A2.UBB ODA-52 (Figure 5(g-l) and 13j), which occupies a similar position in the tail than UBB PL1-1, can be easily distinguished from it.The centrum has a well-developed and not dorsally displaced condyle with an associated rim; a ventral longitudinal hollow; and subvertical anterior articular surface of the centrum.The neural arch is characterized by a posterodorsal oriented neural spine, a dorsoventrally shorter crpl in lateral view, and the presence of a lateral tuberosity in the lateral side of the prezygapophyseal process.The posterodorsal orientation of the spine and a well-developed and not dorsally displaced condyle, with an associated rim, can be identified in some vertebrae of Magyarosaurus dacus (e.g. one element of NHMUK R.4889, Figure 13m).Morphology A3.Paludititan nalatzensis corresponds to a different morphology for the anterior caudal vertebrae (Figure 13l) and characterized by a strong-to-moderate convex posterior condyle and dorsally displaced, subvertical anterior articular surface of the centrum, ventral longitudinal hollow, and subvertical neural spines with anteroposteriorly expanded dorsal end (Csiki et al. 2010a;pers. observ. PM 2018).The lateral tuberosity in the lateral side of the tip of the prezygapophyseal process is rudimentary, differing from the condition of UBB ODA-52.
Anterior caudal vertebrae of Magyarosaurus dacus.The morphology of the referred material of Magyarosaurus dacus is highly variable (no morphology was established here).The dorsal displacement of the posterior condyle is variable in M. dacus from an apex located at the center (e.g. one of the vertebrae of NHMUK R.4889, Figure 13m) to dorsally displaced (e.g. the anterior caudal vertebrae of NHMUK R.4898, Figure 13(n-o)).The neural spine orientation Neural arch is anteriorly displaced; anteriorly displaced tprl.
Possibly corresponding to Morphology M2.

Magyarosaurus dacus
Posterior articular surface is strongly convex to slightly concave; anterior articular surface can be convex, flat or concave; ventral hollow can be present or absent.
Neural arch is anteriorly displaced; tprl located at the level of the anterior articular surface of the centrum.

Posterior caudal vertebrae
Morphology P1 Flat to concave posterior articular surface; shallow to rudimentary ventral hollow.
Neural arch is anteriorly displaced; tprl located at the level of the anterior articular surface of the centrum.
UBB P-64 is attributed to this morphology.

Morphology P2
Flat to concave posterior articular surface; absent or rudimentary ventral hollow.

Magyarosaurus dacus
Posterior articular surface can be convex, flat or concave; ventral hollow is present or absent.
Neural arch is anteriorly displaced.
is also problematic in M. dacus, which preserves subvertical (e.g. one of the vertebrae of NHMUK R.4889, Figure 13m), an anterodorsally (e.g. one of the vertebrae of NHMUK R.4898, Figure 13n) or a posterodorsally (e.g. one of the vertebrae of NHMUK R.4898, Figure 13m) oriented neural spines near the same position.If we accept that all this material belongs to the same taxon, M. dacus should be characterized by an important polymorphism in several morphological characters that have been incorporated on phylogenetic analyses and are used to codify the morphology of the caudal vertebrae (e.g.neural spine orientation).A similar magnitude of morphological variability in the caudal vertebrae was never described in other titanosaurian taxa with multiple tail specimens representing different individuals, such as Saltasaurus loricatus (Powell 1992(Powell , 2003)), Neuquensaurus australis (pers.observ.PM 2019) and Patagotitan mayorum (Carballido et al. 2017); and in partially complete tail series, such as Lohuecotitan pandafilandi (pers.observ.PM 2014-2020), Dreadnoughtus schrani (Lacovara et al. 2014), Mendozasaurus neguyelap (González Riga et al. 2018;pers. observ. PM 2019) or the indeterminate titanosaur MOZ-Pv 1221 (Otero et al. 2021).We believe that more than one taxon seems to be represented in the collections of the Natural History Museum UK [for example, the anterior-most caudal vertebrae figured by Huene (1932) in the plates 45 (figure 4) and 47 (figure 2), both referred by this author to M. dacus, could belong to two disitinct taxa].Other example is the anteroposteriorly expansion of the caudal neural spine in one of the vertebrae of NHMUK R.4889 (Figure 13m), which is similar to the morphology shown by the last anterior caudal vertebrae of Paludititan nalatzensis (Csiki et al. 2010a).The detailed description of M. dacus material is necessary to access their taxonomy and validity (Mannion et al. 2019a).The presence of a lateral tuberosity in the lateral side of the tip of the prezygapophyseal process in M. dacus is difficult to evaluate due the absence of the distal end of these processes.However, they can be observed in one of the specimens of NHMUK R.4898, being less pronounced than in UBB ODA-52.In P. nalatzensis these processes are also rudimentary.The anterior caudal vertebrae of Magyarosaurus dacus preserve two or three distinct morphologies, which possible correspond to two or three different taxa.Two of them are not represent in our sample (Figure 13n and 13o), and the other seems to fit in the morphology represented by P. nalatzensis (Figure 13m).Morphology M1.UBB 15.548 (Figure 10(a-f) and 13p), UBB NV-87 and UBB NVM-6 present a strongly convex posterior articular surface, a dorsally displaced condyle and bordered by a welldeveloped rim, a ventral longitudinal hollow, and an anteriorly displaced neural arch (Figure 10, and 13p).This morphology is partially shared with some of the middle caudal vertebrae referrered to Magyarosaurus dacus (one of the vertebrae of NHMUK R.3849, Figure 13s).Morphology M2.UBB SPM-4 represents a second morphology for the middle caudal vertebrae due to it lacks a fully developed ventral hollow and has a moderately convex condyle, a subvertical neural spine, a slightly dorsoventrally expanded prezygapophyseal processes, and an anteriorly displaced tprl (Figure 11 and 13u).An anteriorly displaced tprl is also present in taxa as Paludititan nalatzensis (Figure 13q; Csiki et al. 2010a;pers. observ. PM 2018) and Lohuecotitan pandafilandi (pers. observ. PM 2014(pers. observ. PM -2020)).The middle caudal vertebrae of P. nalatzensis (Figure 13q and 13t) preserves a similinar morphology to UBB SPM-4.In P. nalatzensis, the convexity of the posterior articular surface of the centrum disappears in the beginning of the middle series, adquiring a flat to shallowly concave surface.This morphology is also observed in some middle caudal vertebrae of Magyarosaurus dacus (e.g.some middle caudal vertebrae of NHMUK R.3865, Figure 13r).

Morphology M3.
A third morphology is represented by the middleposterior caudal vertebra UBB ODAN-32, with a convex anterior articular surface and a transversely convex and constricted ventral surface (Figure 3 and 13v).This morphotype is different from one slightly opisthocoelous middle-posterior caudal vertebra referred to Magyarosaurus dacus (one of the vertebrae referred to NHMUK R.3865, Figure 13w), which preserves a transversely concave ventral face in accordance with Opisthocoelicaudia skarzynskii (Borsuk-Bialynicka 1977) and Abdarainurus barsboldi (Averianov and Lopatin 2020).The ventral concavity is absent in the opisthocoelous caudal vertebra of Borealosaurus wimani (You et al. 2004), Turiasaurus riodevensis (pers.observ.PM 2014), Huabeisaurus allocotus (D'Emic et al. 2013) and Fukuititan nipponensis (Azuma and Shibata 2010).The presence of a opisthocoelous condition in B. wimani and O. skarzynskii was used by You et al. (2004) to suggest the presence of an unrecognized titanosaurian endemic lineage in the Cretaceous of Eastern Asia.However, as we saw above, the presence of this character state (a fully developed opisthocoelous condition, i.e. convex and concave anterior and posterior articular surfaces, respectively) has a wider distribution within Somphospondyli, and its assignation to members of Lithostrotia should be linked considering this character combined with other anatomical features, such as the presence of a ventral hollow and ventrolateral ridges.In addition, the development of the posterior convexity varies within the caudal series and can be restricted to some sectors of the tail [e.g. this anterior convexity is restricted to the posterior caudal vertebrae in Rinconsaurus caudamirus (Calvo and González Riga 2003)].
Middle caudal vertebrae of Magyarosaurus dacus.Several middle and posterior caudal vertebrae referred to M. dacus show a high morphological variability, specially concerning to the morphology of the posterior articular surface of the centrum, which can be strongly convex, moderately convex, or even shallowly concave; and in the development of the ventral face (Figure 13(r-s) and w; Nopcsa 1915;Huene 1932;persr. observ. PM 2014).We believe that they might belong to different taxa.In addition, the neural arch of the preserved elements is generally weakly preserved compromising an accurate comparison with the already identified morphologies.Some of the preserved vertebrae from NHMUK R.3865 can fit on the Morphology M1 or M2.One middle-posterior caudal vertebra can be identified in the set numbered as NHMUK R.3865 and characterized by an opisthoceolous centrum (see discussion above), but with a shallow ventral hollow, unlike the transversely convex ventral face of UBB ODAN-32.Morphology P1.UBB P-64 represents one of the identified morphologies for posterior caudal vertebrae, where the tprl is located at the level of anterior articular surface.The centrum is amphicoelous to platycoelous and preserves a shallow to rudimentary ventral longitudinal groove and ventrolateral ridges (Figure 2(a-f) and 13x and aa).Morphology P2.UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15 are featured by amphicoelous to platycoelous centra, the presence of an anteriorly displace tprl and no ventral longitudinal groove (Figure 12 and 13z and cc), which is present in the middle caudal vertebrae of Paludititan nalatzensis (Csiki et al., 2010a).UBB ODS-4 preserves this anteriorly displacement of the tprl (Figure 9 and 13y and bb) as P. nalatzensis, UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15, but it has a shallow ventral hollow, as in some posterior caudal vertebrae referred to Magyarosaurus dacus (e.g.NHMUK R.3858).In addition, the sized of UBB ODS-4 is significantly bigger from P. nalatzensis, UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15, suggesting for the possibility of a larger form with an anteriorly displaced tprl in posterior caudal vertebrae.However, these vertebrae are allocated to the same morphology pending the discovery of new materal.
Posterior caudal vertebrae of Magyarosaurus dacus.After examining part of the material referred to M. dacus, an important variability is observed concerning the morphology of posterior articular surface (convex, flat, or concave) and ventral face (ventral hollow can be present) of the centrum, and variable aEI, which can be related with the position each element in the posterior series.The absence of complete neural arches made the attribution to one of the above established morphologies difficult.
The presence of Magyarosaurus dacus is not clear in this new sample of caudal vertebrae, and Paludititan nalatzensis might be represented by UBB SPM-4 UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15 referred here as cf.P. nalatzensis and possibly UBB NVM 25 and UBB ODA-39, referred as Lithostrotia indet.The detailed description of titanosaurian individuals with more complete caudal series is essential to better characterize the variability of several of the characters described here at an intra-and interspecific level.In conclusion, the present study allows to identify four different morphologies in the anterior-most caudal vertebrae, at least three distinct morphologies in anterior and in middle caudal vertebrae and two morphologies in posterior caudal vertebrae for the titanosaurs of the Upper Cretaceous levels of Romania.This suggests for the possible presence of at least four taxa for the Maastrichtian record of Romania.This agrees with some previous studies, which considered that the sauropod diversity in the Hațeg Island is underestimated, being higher than the two established taxa, with three to four representatives (Csiki and Grigorescu 2004;Grigorescu 2005;Stein et al. 2010;Csiki et al. 2010a;Solomon and Codrea 2015;Mannion et al. 2019a).Mannion et al. (2019a) note for the possible presence of two other forms, including larger forms than M. dacus and P. nalatzensis, which agree with the presence of middleposterior and posterior vertebrae in our sample with an appreciable size (UBB ODAN-32 and UBB ODS-4) relative to those attributed to those taxa.UBB ODS-4 shares with P. nalatzensis and the smallsized posterior caudal vertebrae UBB PGR1-6, UBB PGR1-14 and UBB PGR1-15 the presence of an anteriorly displaced tprl, but the presence of a ventral hollow seems to differenciate it from P. nalatzensis.Two hypotheses can be considered: (i) all these Romanian Upper Cretaceous specimens with an anteriorly displaced tprl belongs to P. nalatzensis, which can reach larger sizes than previous thought (the histology of P. nalatzensis can be important to evaluate the ontogeny of the holotype individual and test this hypothesis), and the morphology of the ventral face of the centrum is variable in posterior caudal vertebrae; or (ii) an anteriorly displaced trpl characterizes a more inclusive clade, which includes P. nalatzensis and undescribed taxa from the Upper Cretaceous of Romania, and possibly the medium-sized lithostrotian Lohuecotitan pandafilandi, from the Campanian-Maastrichtian levels of the Villalba de la Sierra Formation of Spain (Díez Díaz et al. 2016).UBB ODS-4 is not referred to L. pandafilandi by the presence of a shallow ventral hollow, and the absence of dorsal buttresses in the dorsal edge of the posterior articular surface of the centrum.More material will be necessary to understand the distribution of these characters and supports this possible higher diversity in the Hațeg Island, with a mosaic of dwarf and mediumsized taxa.

Contributions for the understanding of some morphological characters
The description of the specimens presented here, and their comparison with remains of other titanosaurs, especially the Late Cretaceous European forms such as Magyarosaurus dacus (Nopsca 1915;Huene 1932), Paludititan nalatzensis (Csiki et al. 2010a), Lohuecotitan pandafilandi (Díez Díaz et al. 2016), Lirainosaurus astibiae (Sanz et al., 1999;Díez Díaz et al. 2013), Ampelosaurus atacis (Le Loeuff 1995Loeuff , 2005)), Atsinganosaurus velauciensis (García et al. 2010;Díez Díaz et al. 2018), Normanniasaurus genceyi (Le Loeuff et al. 2013) and the undefined lithostrotian titanosaur from Cenomanian levels of the Spanish Algora fossil site (Mocho et al. 2019a), allow us to better understand the distribution of some morphological characters within Titanosauria, especially considering the European members, and their morphological variability.Herein, we will briefly discuss some of these characters that might have some taxonomic significance, some of them already being used on the morphological data sets applied on cladistic analyses: (i) presence of the podl, pocdf and sdf in the anterior-most caudal vertebra; (ii) presence of a ventral hollow; and (iii) position of the tprl in the middle and posterior caudal vertebrae.
In our sample, a well-developed sharp-lipped pocdf is observed in UBB RR-3, UBB NVM-25, being shallower in UBB-ODA 18, UBB-ODA 52 and UBB PL1-1.The available material does not allow certifying the presence of a podl, pocdf and sdf in Paludititan nalatzensis (Csiki et al. 2010a) and in the Ibero-Armorican titanosaur Atsinganosaurus velauciensis (Díez Díaz et al. 2018).The anterior caudal vertebrae referred to Ampelosaurus atacis (Le Loeuff 2005) should be re-described to score these characters, but a podl seems to be present.The anterior-most caudal vertebra of Magyarosaurus dacus NHMUK R.3851 was partially reconstructed in the base of the neural arch, but both fossae seem to be present (is not clear the presence of the podl).The holotype of Lirainosaurus astibiae also presents sharp-lipped pocdf and sdf, and a podl (MCNA 7458;Díez Díaz et al. 2013;pers. observ. PM 2016).The sdf and podl are possibly present in MCNA 14444 (pers.observ.PM 2016), which was referred by Díez Díaz et al. (2013) to L. astibiae.The presence of a podl was considered as a diagnostic feature for L. astibiae.The holotype of Lohuecotitan pandafilandi preserves an elongated fossa in front of the postzygapophyses, which is not sharp-lipped, but it is shallowed subdivided by a rudimentary podl in the first anterior caudal vertebra.The presence of the podl has a wide distribution within Lithostrotia and should not be considered as a diagnostic feature for the L. astibiae, whose holotype lacks, at this moment, any other diagnostic feature.Presence of a ventral hollow.The presence of a ventral and longitudinal hollow bordered by ventrolateral ridges in caudal centra have been recognized as an informative anatomical feature and was incorporated in several morphological data sets.The older versions of this character considered the presence of a ventral longitudinal hollow only on the anterior caudal vertebrae (Upchurch 1998: C136;Upchurch et al. 2004: C182) or in the anterior and middle elements (Wilson 2002: C132;Carballido et al. 2011: C131;Mannion et al. 2013: C181;Carballido and Sander 2014: C132;Tschopp et al. 2015: C330).The presence of a ventral hollow was recovered as a synapomorphy of Titanosauria by Wilson (2002) and D'Emic (2012), being also identified as characteristic for some diplodocines such as Barosaurus lentus and Diplodocus spp.(Upchurch 1995(Upchurch , 1998)), and recovered as a synapomorphy of Diplodocinae by Carballido et al. (2011).Tschopp et al. (2015) recognized a wider distribution of this character within Diplodocoidea after its identification in some non-diplodocine diplodocoids.Mannion et al. (2019b) divided this character in two (C181 and C506) to better describe this feature in the anterior and middle caudal vertebrae.This ventral hollow generally appears in the anterior sector of tail, after the first caudal vertebrae, and might be developed up to the middle or even posterior region of the caudal series.The more recent scores show a wide distribution within Eusauropoda for the character when considering its presence on anterior caudal vertebrae (Mannion et al. 2019b), being shared by several titanosaurs (e.g.Powell 1992;Gomani 2005;Curry Rogers 2009;Mannion and Calvo 2011;Lacovara et al. 2014;Mannion et al. 2019b).The presence of this feature in the middle caudal vertebrae has a more restricted distribution within Titanosauria, being present in Andesaurus delgadoi (Mannion and Calvo 2011), Alamosaurus sanjuanensis, Huanghetitan liujiaxiaensis (Mannion et al. 2019b), Isisaurus coberti (Jain and Bandyopadhyay 1997), Malawisaurus dixeyi (Gomani 1999(Gomani , 2005)), Neuquensaurus australis (Salgado et al. 2005), Opisthocoelicaudia skarzynskii (Borsuk-Bialynicka 1977), Punatitan coughlini (Hechenleitner et al. 2020), Rapetosaurus krausei (Curry Rogers 2009), Rocasaurus muniozi (Salgado and Azpilicueta 2000), Saltasaurus loricatus (Powell 1992) and possibly Uberabatitan ribeiroi (Silva Junior et al. 2019); but it is also present in the somphospondylan Huabeisaurus allocotus (D'Emic et al. 2013) and in some diplodocines (e.g.Hatcher 1901;McIntosh 2005;Lucas et al. 2006;Remes 2006;Gallina et al. 2014;Tschopp et al. 2015).The comparison of the new specimens studied here (excluding the anterior-most caudal vertebrae, which can be flat and convex) with the caudal vertebrae of Paludititan nalatzensis and the material referred to Magyarosaurus dacus allows us to consider that almost all the caudal morphologies described for the titanosaurs of the Upper Cretaceous of Romania were characterized by the presence of a ventral hollow, at least in the anterior section of the tail.Some of these taxa loses this concavity in the transition between the anterior and middle caudal vertebrae such as P. nalatzensis.However, other taxa preserve a fully developed longitudinal ventral hollow up to the middle or even posterior caudal vertebrae (e.g.some caudal vertebrae of M. dacus: NHMUK R.3858, some vertebrae of NHMUK R.3865).
The anterior caudal vertebrae of the Spanish Campanian-Maastrichtian titanosaurs Lirainosaurus astibiae and Lohuecotitan pandafilandi also present a ventral hollow.L. pandafilandi preserves a ventral concavity in the last anterior caudal vertebrae, which rapidly disappears in the first middle caudal vertebrae (pers.observ.PM 2014-2020).L. astibiae was established based on isolated material found in a bone-bed accumulation in Laño (Condado de Treviño, Spain) from more than one individual (Sanz et al. 1999;Pereda Suberbiola et al. 2000), some of them being slightly deformed or poorly preserved.The firsthand study of the holotype of that species and of referred material allows us to conclude that the anterior-most caudal vertebrae lack this ventral hollow.However, vertebrae from Laño corresponding to the posterior half of the anterior series preserve a shallow concavity bordered by longitudinal ridges (e.g.MCNA 14446, MCNA 1882, and MCNA 13388, which is an anterior-middle caudal vertebra).The available material from Laño does not allow us to understand clearly where this ventral hollow disappears.However, some middle and posterior caudal vertebrae preserve a flat to transversely convex ventral face (e.g.MCNA 7457, MCNA 8807, MCNA 13851, MCNA 14435, MCNA 14449), and other are characterized by rudimentary ventrolateral ridges bordering a very shallow longitudinal groove (e.g.MCNA 2211, MCNA 7454, MCNA 13853, MCNA 14437, MCNA 14447, MCNA 13857).In addition, some of the last anterior caudal vertebrae have the ventral hollow restricted to the posterior half of the centrum (MCNA 14439).If the attribution of all the material of Laño to a single taxon (i.e.L. astibiae) is correct, we need to assume that this feature is subject to an important intraspecific variability.Therefore, in that case, it should be analyzed with caution when it is used as a character with taxonomic value and in the scoring of morphological data sets.The detailed description of all elements from Laño is necessary to understand if more than one taxon was present in the locality.
Three titanosaurian taxa were established for the uppermost Upper Cretaceous record of France.Only two of them preserve caudal vertebrae, Ampelosaurus atacis and Atsinganosaurus velauciensis.A. atacis have been described from remains found in a bonebed accumulation from the Bellevue locality (Aude).The presence of a high morphological variability in the collected remains have been used by some authors to hypothesize the presence of at least two taxa (e.g.Vila et al. 2012;Díez Díaz et al. 2021;Páramo et al. 2022).The detailed description of the holotype and the referred specimens of A. atacis is still necessary.Based on the available information, the ventral longitudinal hollow is well-developed in the last anterior caudal vertebrae, but it is not fully developed in the middle caudals, being restricted to the posterior half (see Le Loeuff 2005: figure 4.9). A. velauciensis was found in Velaux-La Bastide Neuve locality (Bouches-du-Rhone), in its main bone-rich accumulation (sequence 2, B2 and B3), where the material was found isolated (only the holotype was found in articulation, Gallina and Apesteguía 2015).New remains of A. velauciensis were described by Díez Díaz et al. (2018), and the presence of a ventral longitudinal groove and ridges seems to be restricted to the anterior half of the anterior series.
The position in caudal series and the morphology of the ventral hollow seems to be variable between different taxa [e.g. it appears from the fifth caudal in Lohuecotitan pandafilandi; it appears around the sixth caudal in Dreadnoughtus schrani, but the ventral hollow is only developed in the posterior half of the ventral surface (Lacovara et al. 2014); it appears from the second caudal in Neuquensaurus australis (Salgado et al. 2005)].In addition, the disappearance of this feature can be variable, being absent in the middle caudal vertebrae on L. pandafilandi but remaining present up to the posterior caudal vertebrae in Saltasaurus loricatus (pers.observ.PM 2019).UBB NVM-25 and UBB ODA-18 belong to a sauropod where the ventral hollow appears in the anterior-most section of the tail, between the second and the third caudal.Besides the presence of a transverse convex-to-flat ventral surface in NHMUK R.3851, some of the anterior-most anterior caudal vertebrae referred to Magyarosaurus dacus (NHMUK R.3854) also present a ventral hollow.If their assignation to M. dacus is correct, this taxon seems to acquire a ventral hollow around the second caudal, similarly to UBB NVM-25 and UBB ODA-18.In the anterior-most caudal vertebra preserved in the holotype of Paludititan nalatzensis, the ventral face seems be only slightly concave (Csiki et al. 2010a), being impossible to know where this concavity firstly appears.
The detailed description of the ventral face along the caudal series is crucial to know how to score it and, eventually, to propose a re-codification of the previously proposed morphological characters.It will be also important to understand how this morphological feature varies with ontogeny.Following that goal, it will be important to provide data about the more anterior and more posterior appearance of the ventral hollow in the caudal series.In addition, other characters that can be present within the ventral hollow such as subfossae, foramina and laminae should be incorporated because they can be useful features to characterize and differenciate some titanosaurs (e.g.Saltasaurus loricatus and Rocasaurus muniozi; Zurriaguz and Cerda 2017).Furthermore, the definition of the ventral hollow should be clarified, and eventually restricted to a fully developed ventral longitudinal depression (present along the total anteroposteriorly width of the centrum) and bordered by ventrolateral ridges also fully developed.Tschopp et al. (2015) proposed a new character (C331) based on Curtice (1996), focused on the depth of the ventral hollow.The application of this character might help to better characterize the ventral hollow in the lithostrotians, with some taxa preserving a considerably deep ventral excavation (e.g. S. loricatus, pers.observ.PM 2019).Position of the tprl in the middle and posterior caudal vertebrae.The tprl is a lamina that medially connects the prezyagpophyseal processes.In the caudal vertebrae, this lamina is generally restricted to the base of these processes, being located at the level of the anterior articular surface.In some cases, the tprl can be slightly anteriorly displaced in the anterior and middle caudal vertebrae due to the extreme anterior displacement of the neural arch and/or the presence of an anteriorly inclined anterior articular surface of the centrum.First, middle and posterior caudal vertebrae from many titanosaurian taxa with extreme anteriorly displaced neural arch preserve a tprl at the level of the anterior articular surface of the centrum, such as Saltasaurus loricatus (PVL 4017, pers. observ. PM 2019), Mendozasaurus neguyelap (slightly anteriorly displaced due the presence of a slightly inclined anterior articular surface, González Riga et al. 2018), Dreadnoughtus schrani (Lacovara et al. 2014), Uberabatitan ribeiroi (Silva Junior et al. 2019) and Baurutitan britoi (Kellner et al. 2005).In our sample, the presence of an anteriorly displaced tprl in the middle and posterior caudal vertebrae has been recognized in some of the analyzed specimens with a rudimentary or no inclination of the anterior articular surface of the centrum (UBB SPM-4, UBB ODS-4, UBB PGR1-14), suggesting that this feature is not necessarily related with the anterior inclination of that articular surface.An anteriorly displaced tprl is also present in the middle caudal vertebrae of the holotype of Paludititan nalatzensis (Csiki et al. 2010a) and can be observed in the middle caudal vertebrae of Lohuecotitan pandafilandi (the anterior section of the neural arch is fractured, but the precense of an anteriorly displaced tprl is clear) and Ampelosaurus atacis (Le Loeuff 2005).All these middle caudal vertebrae preserve a subvertical anterior articular surface of centrum.The prezygapophyseal processes are poorly preserved in Lirainosaurus astibiae, but the available referred remains exclude for the moment the presence of this feature in the species.The figured material of Atsinganosaurus velauciensis suggests for the presence of an anteriorly displaced tprl in some specimens (MMS/VBN.09.D.003; Díez Díaz et al. 2018).However, the presence of this character still needs to be confirmed in the taxon.This feature can represent a possible synapomorphy of a European subclade of lithostrotian titanosaurs, containing P. nalatzensis, L. pandafilandi, A. atacis and possibly A. velauciensis.
The morphological datasets of Calvo et al. (2007a, b) incorporated a character which can be related with the anterior displacement of the tprl proposed herein.This character was described as: 'well developed interprezygapophyseal lamina in middle caudal vertebrae: absent (0); present (1)'.Those authors identified the apomorphic condition in Rinconsaurus, Gondwanatitan, Muyelensaurus and Aelosaurus rionegrinus.However, no more detailed data was provided about how to score this character, and detailed descriptions of the caudal vertebrae of those taxa are still necessary.Aelosaurus rionegrinus is composed by a series of anterior caudal vertebrae with an important anterior inclination of the anterior articular surface of the centrum and a slightly anterior displaced tprl.No middle and posterior caudal vertebrae have been identified for this taxon.In the case of Gondwanatitan faustoi, a posterior caudal vertebra figured by Kellner and Azevedo (1999) presents a trpl placed at the level of the anterior articular surface of the centrum.
The incorporation of this feature in the morphological data sets will be important to understand its distribution.For this reason, we propose here the following character: 'Middle and posterior caudal neural arches (excluding the first middle caudal vertebrae) intraprezygapophyseal lamina (tprl): located at the level of the anterior articular surface (0); anteriorly displaced and located forward to the anterior articular surface (1)'.To avoid the impact of the anterior inclination of the anterior articular surface of the centrum in our scoring, we should trace a line through the ventralmost and dorsalmost point of the anterior articulation, if the tprl is located at the level of this line, this character should be scored as '0', if this tprl is markedly anteriorly placed, this character should be scored as '1'.

Conclusions
In the present work, we performed a detailed description of several caudal vertebrae found isolated or in association in the Upper Cretaceous levels of Romania.This is the first detailed work on Romanian titanosaurs after the description of the Nălaț-Vad sauropod, resulting in the establishment Paludititan nalatzensis in 2010.The systematics analyses and the obtention of well-supported phylogenetic approaches for the Romanian titanosaurs is hampered by the fragmentary nature of the collected specimens, mostly found as isolated remains.In addition, the detailed systematic review of the material referred to Magyarosaurus dacus requires to be performed (Mannion et al. 2019a), and a new diagnosis need to be proposed to support its validity.
The comparison of the set of caudal vertebrae studied here with those previously documented for Magyarosaurus dacus and Paludititan nalatzensis allow us identifying distinct morphologies in the Maastrichtian levels of Romania (four anterior-most caudal morphologies: Am1, Am2, Am3 and Am4; at least three anterior caudal morphologies: A1, A2 and A3; at least three middle caudal morphologies: M1, M2 and M3; and at least two posterior caudal morphologies: P1 and P2).The possible presence of at least four different taxa inhabiting the Hațeg Island during the Late Cretaceous is suggested here, including at least one form with opisthocoelous condition in the middle and posterior caudal vertebrae, which does not coincide with any of the previously described taxon for the Upper Cretaceous of Romania.
The morphological variability shown by the caudal vertebrae of Magyarosaurus dacus is high, and we believe that more than one taxon is represented by the material so far attributed to it, supporting the hypotheses previously proposed by other authors (Csiki and Grigorescu 2004;Grigorescu 2005;Csiki et al. 2010a;Stein et al. 2010;Solomon and Codrea 2015;Mannion et al. 2019a).The analyses of the vertebrae presented here suggest for the possibility of four different tail morphologies, which might belong to four different taxa (supporting the previous proposal of some authors), including Paludititan nalatzensis, M. dacus and at least two other species.Some of the analyzed vertebrae preserve a size compatible with the medium size sauropods found in the Villalba de la Sierra Fm., in Spain (e.g.Lohuecotitan pandafilandi, Díez Díez et al. 2016).All the studied vertebrae possibly belong to members of Titanosauria, the only sauropod clade represented in the Maastrichtian worldwide.We can identify Titanosauria indet., Lithostrotia indet., and cf.Paludititan nalatzensis in our sample.However, the available anatomical data for some elements is restricted, being attributed to a more exclusive clade than Titanosauria (Titanosauriformes or Somphospondyli).
In this work we pointed and discussed the significance of some morphological characters of the caudal vertebrae.The codification and scoring of the ventral longitudinal hollow should consider the variability that might exists throughout the anterior caudal series (e.g. the point of appearance might change between taxa), the transition between the anterior and middle series, and its development up to the posterior caudal vertebrae.The presence of podl, pocdf and sdf has a wide distribution within Lithostrotia, and the only autapomorphy shown by the holotype of Lirainosaurus astibiae (i.e.MCNA 7458) should be reconsidered.A new morphological character describing the position of the tprl in the middle and posterior caudal vertebrae is proposed.An anteriorly displaced tprl seems to characterize some titanosaurian taxa of Europe (Paludititan nalatzensis, Lohuecotitan pandafilandi and, possibly, Ampelosaurus atacis and Atsinganosaurus velauciensis), and its incorporation on the morphological data sets might shed light about its distribution within Lithostrotia.

Table 1 .
List of sauropod caudal vertebrae analysed in the present work and colleted in the levels of the Upper Cretaceous of Romania.This material is deposited in the Babes-Bolyai University, Cluj-Napoca, Romania.

Table 2 .
Measurements (in millimeters)for the sauropod caudal vertebrae analysed in the present work and collected in the levels of the Upper Cretaceous of Romania.This material is deposited in the Babes-Bolyai University, Cluj-Napoca, Romania.Column labels are represented by the number of the specimens, which should be preceded by the institutional abbreviation UBB.*Approximate measurement.

Table 3 .
Overview of the distinct morphologies of sauropod caudal vertebrae (anterior-most, anterior, middle and posterior caudal vertebrae) identified for the studied sample, Magyarosaurus dacus and Paludititan nalazensis.