The extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) in the Upper Cretaceous of central-western Russia—The road to easternmost peri-Tethyan seas

Isolated teeth belonging to the genus Ptychodus Agassiz, 1834 (Chondrichthyes; Elasmobranchii) from the Upper Cretaceous of the Ryazan and Moscow Oblast regions (European Russia) are described and discussed in detail herein. The taxonomic composition of the Ptychodus assemblage from the Ryazan region is very diverse including the first records of the cuspidate species P. altior and P. anonymus, which thus is largely consistent with those from other contemporaneous European localities. Ptychodus ubiquitously inhabited epicontinental seas of Europe during most of the Cretaceous with the most diverse assemblages coming from southern England, northern Italy, Belgium, and European Russia. Additionally, the material documented here from the Cenomanian of Varavinsky ravine area (Moscow Oblast) represents the northernmost occurrence of Ptychodus hitherto reported from Europe. It is evident that the Late Cretaceous shallow seas of the Russian platform represented a crucial pathway for the dispersal of Ptychodus from the European peri-Tethys to the eastern margins of the Neo-Tethyan Ocean. The Albian–Campanian records of Ptychodus from Europe indicate that its dominance in the peri-Tethys persisted for most of its evolutionary history. A local temperature drop across most of the European shallow seas probably contributed to the narrowing of its geographic range in the peri-Tethyan seas towards the end of the Mesozoic Era. The fossil remains of Ptychodus documented herein are accordingly of utmost importance for better understanding the taxonomic composition of Russian fossil ichthyofaunas and also inform about the dispersal of Ptychodus towards western and eastern peri-Tethyan seas during the Late Cretaceous.


INTRODUCTION
The Late Cretaceous represents a time of tremendous global changes in the history of Earth, such as paleogeographic modifications and climate fluctuations, with relevant implications in the course towards current conditions (e.g., Núñez-Useche et al., 2014;Stanley and Luczaj, 2015). In particular, the Cretaceous global and regional shifts in available aquatic habitats driven by these environmental and climatic changes probably affected the geographic distribution and the evolution of marine communities (e.g., Kriwet and Klug, 2008;Peterson and Lieberman, 2012;Sorenson et al., 2014;Poyato-Ariza and Martin-Abad, 2016). Additionally, Late Cretaceous elasmobranchs particularly experienced one of the greatest bursts of diversity in their evolutionary history, also developing various trophic adaptations and strategies (e.g., durophagous feeding; Walker and Brett, 2002;Summers et al., 2004;Underwood, 2006;Sorenson et al., 2014).
The northeastern peri-Tethys represented a crucial connection between the various peripheral areas of the Neo-Tethys Ocean with the Russian Platform during the Late Cretaceous that had a major role in shaping aquatic environments, and possibly the distribution of marine faunas, across these epicontinental sea areas (e.g., Baraboshkin et al., 2003;Solonin et al., 2021b;Zorina, 2022). Sharks from the Upper Cretaceous of central Russia have not yet been extensively studied and those studies available mainly are limited to Cenomanian deposits (Kiprijanoff, 1852(Kiprijanoff, , 1881Glikman, 1980;Eremin, 1998;Olferev and Alekseev, 2005;Starodubtseva et al., 2008). Kiprijanoff (1852) reported for the first time elasmobranch faunas from the Cenomanian of Kursk Oblast (western European Russia). Later, elasmobranch teeth were described from the Upper Cretaceous of several other localities of European Russia (e.g., Moscow, Saratov, and Volga areas;Glikman, 1980;Eremin, 1998;Olferev and Alekseev, 2005;Starodubtseva et al., 2008). Recently, Solonin et al. (2020) provide a first preliminary assessment of an elasmobranch fauna, including Ptychodus, from the Ryazan Oblast area (European Russia; Vodorezov and Solonin, 2017; see also Fig. 1).
In the present paper, we present the first detailed assessment of a very diverse and relatively abundant ptychodontid assemblage from the Upper Cretaceous of European Russia. In particular, isolated teeth of Ptychodus from the ?Cenomanian-Santonian of the Malyy Prolom area (Ryazan Oblast) are documented and discussed in detail herein, contributing to our knowledge of Upper Cretaceous elasmobranch faunas from European Russia. Moreover, unknown fragmentary teeth of Ptychodus from the Cenomanian of the Varavinsky ravine (Moscow Oblast) are described here for comparative purposes. The species richness of various Russian ptychodontid assemblages is also compared here with those of the main European elasmobranch faunas including Ptychodus that previously were documented. We therefore also present short reviews of European occurrences of Ptychodus. Additionally, we propose a hypothesis on possible dispersal patterns of Ptychodus towards the northeastern peri-Tethyan basin during the Late Cretaceous and tentatively correlate possible migration patterns with possible abiotic factors.

PREVIOUS RECORDS OF PTYCHODUS
EUROPEAN RUSSIA Cretaceous ptychodontids from European Russia have not been studied in adequate detail up to now. The first description of teeth of Ptychodus from central Russia dates back to the second half of the 19th century (Kiprijanoff, 1852(Kiprijanoff, , 1881Nikitin, 1888;Sinzow, 1899). However, there are only very few reports on findings of ptychodontid remains from central Russia in the following decades. Kiprijanoff (1852) established the presence of P. latissimus Agassiz, 1835, andP. polygyrus Agassiz, 1835, from the Cretaceous sandy marls or sands in the Kursk Governorate (now Kursk Oblast), and P. decurrens Agassiz, 1838, P. latissimus, P. mammillaris Agassiz, 1835, from the same deposits in Oryol Governorate (now Oryol Oblast). Later, Kiprijanoff (1881) added information about new findings of P. oweni Dixon, 1850, and P. decurrens teeth from the same lithostratigraphic level in Oryol Oblast. All the above-mentioned specimens of Ptychodus were found directly below or directly above the so-called 'Kursk samorod' (Kiprijanoff, 1852) or 'Siwerischen Osteolith' (Kiprijanoff, 1881). Both terms are synonyms used to describe a layer of quartz-glauconite sands with numerous phosphorite pebbles. Based on their foraminiferous content, the layer is dated as early Cenomanian (e.g., Olferev and Alekseev, 2005).
Ptychodus teeth also occur in the northern part of Moscow Oblast where teeth of P. polygyrus, P. mammillaris, and Ptychodus sp. were found in quartz sands with phosphorite concretions in the Varavinsky ravine (near Sergiev Posad). Based on ammonites, the layer is dated as early-middle Cenomanian (Nikitin, 1888;Olferev and Alekseev, 2005). Ptychodus teeth are rare in all the above-mentioned fossil sites and coexist with much more abundant assemblages of teeth belonging to various other elasmobranch groups (compare Glickman, 1953;Olferev and Alekseev, 2005;Starodubtseva et al., 2008;Popov, 2016).

GEOLOGICAL SETTING
In general, the Upper Cretaceous stratigraphy of European Russia is quite problematic due to the regular erosion of sediments, which had gradually accumulated from the Cenomanian to the Santonian. Periodic regressions of the sea might have occurred, during which sediments were eroded and fossils (e.g., shark teeth) re-deposited at the base of the strata. The widespread marine transgression in the early-middle Cenomanian was replaced by a significant regression in the late Cenomanian-early Turonian. This exposed and destroyed most Cenomanian deposits, after which numerous transgressive-regressive cycles began during the Turonian-Santonian (Baraboshkin et al., 2003;Solonin et al., 2021b).

Ryazan Oblast (Malyy Prolom)
The teeth of Ptychodus documented herein have been collected from an active sand quarry near the village of Malyy Prolom, located about 5.0 km northwest of Shatsk in the southeastern part of Ryazan Oblast area (see Figs. 1,2). The area of Ryazan Oblast geologically is located in the southeastern part (Chuchkovo depression) of the Moscow Syneclise in the central part of the Russian Platform (East European Platform; Kuzmin et al., 2015). Large stratigraphic gaps including Turonian-Coniacian and Campanian-Maastrichtian stages characterize the Upper Cretaceous stratigraphy of the Ryazan region (Sahagian et al., 1996;Fadeeva and Iosifova, 1998 Drutskoi and Fadeeva, 2001;Olferev and Alekseev, 2005;Kuzmin et al., 2015). Both Cenomanian formations and the Dmitrov Fm. were deposited in marine shoreface conditions, while the Zagorsk Fm. were formed in offshore-deep marine conditions (Sahagian et al., 1996).
In the Malyy Prolom area, directly below Quaternary sediments, upper Santonian quartz sands and sandstones (Dmitrov Fm.) occur, which are underlain by lower Cenomanian quartz sands of the Yakhroma Fm. and upper Albian clays and silts of the Paramanovsk Fm., respectively (Drutskoi and Fadeeva, 2001;Kuzmin et al., 2015;see also Fig. 2). Deposits of different geological stages are separated by distinct erosional unconformities (Sidorenko, 1971;Nikitin et al., 1984;Kuzmin et al., 2015). The Malyy Prolom quarry exposes 4.5 m fine-grained Cretaceous quartz sands or quartz-glauconite sands covered by 1.6 m of Quaternary sediments (Fig. 2). Cretaceous rocks do not contain body fossils with the exception of a layer of coarsegrained sand in the upper part of this unit (0.1-0.6 m). Its paleontological content mainly includes vertebrate remains consisting of numerous elasmobranch and actinopterygian teeth, rare vertebrae as well as extremely rare teeth of marine reptiles (plesiosaurians, ichthyosaurians, mosasaurians) and pterosaurs (see also Solonin et al., 2021aSolonin et al., , 2021b. In the same layer, rare fragmentary remains of the bivalves (e.g., Neithea and pectenids), sponges, bryozoans, calcareous algae, and fragments of ichnofossil burrows were documented (see also Solonin et al., 2021aSolonin et al., , 2021b. This fossiliferous layer also differs from underlying and overlying sand layers in larger grain diameters and the presence of phosphatic pebbles of sandstones and large siliceous sandstone concretions. This fossiliferous layer is the only Cretaceous bed, which lies on a strongly eroded surface (Krivtsov et al., 2018).
Unfortunately, the Cretaceous sands of the Malyy Prolom quarry have not yielded any stratigraphically important taxa up to now. However, analyses of the available geological maps of the region (Fadeeva and Iosifova, 1998;Kuzmin et al., 2015) and geological profiles of nearby boreholes from Karnauhovo (3 km east of the studied quarry) prove that the uppermost part of the Cretaceous sands (including the fossiliferous layer) of the Malyy Prolom quarry, which is separated from underlying beds by an erosional surface, is certainly late Santonian (Dmitrov Fm.) in age, while the underlying sand layers are of early Cenomanian (Yahroma Fm.) age (see Fig. 2).
The low degree of preservation exhibited by the fossils from the Upper Cretaceous of Ryazan Oblast (European Russia) indicates post-mortem re-deposition for most of these specimens (see the 'Taphonomic remarks' section, below). However, re-deposition events could also have involved better-preserved teeth, such as those described here for Ptychodus. Although previous preliminary analyses indicate the presence of Santonian nannofossils embedded within the sandstone from which the examined teeth come, the age of the specimens cannot be conclusively established (Olferev and Alekseev, 2005). The teeth of Ptychodus from the Ryazan Oblast are therefore considered here to be ?Cenomanian-Santonian in age. and Ptychodus sp. have been recovered in the deposits of the Yakhroma Fm. (Olferev and Alekseev, 2005).  Tables S1 and S2 in Supplemental Data, for more details on all examined material).

Methods
The material described herein was obtained by bulk sampling and subsequent screen washing of fossiliferous sediments from the Malyy Prolom sand quarry with the use of sieves of different mesh sizes (0.5-5 mm). Some of the specimens were collected using a BM-51-2 binocular stereo microscope (magnification 8.75×) from concentrates of the sieved samples, while others were surface-collected. All specimens were cleaned with water.
In order to ensure a rigorous comparison, five Russian localities have been selected (Moscow Oblast, Ryazan Oblast, Oryol Oblast, Kursk Oblast, and Saratov Oblast) and the remaining European occurrences of Ptychodus were subdivided into 19 geographic areas (southern Spain, southern England, northern France, western France, southeastern France, Belgium, western Germany, eastern Germany, northern Czech Republic, northern Switzerland, eastern Austria, northern Italy, central Italy, southern Italy, southern Sweden, Denmark, southern Poland, central Lithuania, and central Romania). Dubious occurrences with non-described and/or non-figured specimens have been discarded for the species richness comparison. The anatomical and odontological terminologies mostly follow Cappetta (2012), Shimada (2012), and Amadori et al. (2019bAmadori et al. ( , 2020a. Synonymy lists follow the standards proposed by Matthews (1973), Bengtson (1988), and Sigovini et al. (2016), but are not intended to be complete; only important references are listed.

I ).
A deep posterior sulcus characterizes the posterior side and a well-developed anterior protuberance elongates the anterior margin of the tooth crown. The tooth apex is almost completely abraded, whereas the marginal ornamentation exhibits an almost concentric pattern around the cusp (see Fig. 3A). In lateral view ( Fig. 3A II ), the dental cusp is well-developed with an anterior outline markedly inclined and a posterior side almost perpendicular to the crown base. A small portion of tooth root still is preserved. In occlusal view (Fig. 3B), RSU DGE 2020 RO MP-39 shows an irregular and asymmetrical crown outline with a left marginal area almost straight and a curved, convex tooth edge on the right one; the latter is slightly damaged posteriorly.  Remarks-The tooth crown of the isolated specimens identified as belonging to this species is generally well preserved with few exceptions (e.g., RSU DGE 2020 RO MP-27). Likely, the teeth all belong to lateral rows of the dentition with the exception of RSU DGE 2021 RO MP-1. The latter is probably a symphyseal tooth due to its symmetrical crown outline. RSU DGE 2021 RO MP-1 ( Fig. 3F-F II ) exhibits a rounded, broad dental cusp similar to those characterizing Ptychodus rugosus (e.g., Hamm 2020:figs. 72-75; Amadori et al. 2019b: fig. 15A-A II ). Nevertheless, RSU DGE 2021 RO MP-1 exhibits completely smooth cusp sides, which is a species-specific feature of P. altior. The symmetric outline of RSU DGE 2021 RO MP-16 indicates its original placement along the medial tooth rows; in general, its occlusal shape resembles that depicted in fig. 11, pl. 25b by Agassiz (1838). The features characterizing the upper and the lower dental plates of P. altior have not been hitherto defined due to the lack of articulated dentitions. The attribution to the maxillary or mandibular tooth plate thus remains doubtful for the teeth documented herein. The occlusal wear on the cusp apex of some teeth confirms their involvement in prey processing (see 'functional teeth' sensu Shimada, 2012; see also Amadori et al., 2019bAmadori et al., , 2020a. †PTYCHODUS ANONYMUS Williston, 1900a (Fig. 4) (Selected synonyms) p.Ptychodus anonymus: Williston, 1900a:32, pl. 11, figs. 5, 6, 16-18, 24 (non figs. 7, 8, 21-22 4A-A III ) exhibits an irregular, asymmetrical outline with the anterior and the right edges merged to each other, forming a single curved margin. In occlusal view (Fig. 4A), the anterior protuberance is rounded and the posterior sulcus is deep. The left margin of the crown is slightly concave. A rounded cusp crossed by six transversal, thin ridges characterizes the occlusal surface of the crown; concentric, thin wrinkles cover the entire marginal area. In posterior view ( Fig. 4A II ), the cusp is narrow and the crown base is flat; the root is bilobate with a wide, shallow antero-posterior sulcus (see also Fig Most of the right side of the RSU DGE 2020 RO MP-13 is missing and its outline seems quite symmetrical, also exhibiting a wide posterior sulcus. The cusp is rounded and crossed by around 10 thin ridges with lateral end curved anteriorly and merging to each other on the right side ('loops'). The marginal area is covered by thin, concentric wrinkles. The crowns of RSU DGE 2020 RO MP-18 and RSU DGE 2021 RO MP-3 show approximately 11 occlusal ridges and all lateral edges rounded. The cusp of RSU DGE 2020 RO MP-18 is more developed and higher than that of RSU DGE 2020 RO MP-3 (   Fig. 4H-H III ) a crown with a central, rounded cusp and two lateral lobes; the right tooth edge is broken. The cusp is entirely crossed by 9-10 thin ridges, whereas the marginal area shows a concentric ornamentation. The cusp apex is slightly abraded.
Remarks-Articulated dentitions of Ptychodus anonymus Williston, 1900a, are not presently known. Nevertheless, dental mesiodistal patterns are clearly recognizable in the associated tooth set FHSM-VP 19170 from the Upper Cretaceous Jetmore Chalk of Kansas (U.S.A.; see Hamm, 2019). The American tooth set exhibits symmetrical, symphyseal teeth with high cusps and lateral teeth with rounded and distally moved cusps (see also Hamm, 2020a:figs. 23, 24).  fig. 4 (non fig. 3 Hamm (2020a:26). Species Stratigraphic Range-Ptychodus decurrens is a wellknown species from the late Cenomanian-middle Turonian of numerous localities around the world, but it has been also rarely reported and figured from Albian deposits of Europe and North America (e.g., Priem, 1912;Welton and Farish, 1993 Description-RSU DGE 2020 RO MP-10 ( Fig. 5A-A IV ) is an isolated, polygonal (almost rectangular) tooth with an asymmetrical occlusal outline. In occlusal view (Fig. 5A), the right dental edge is almost straight and slightly depressed, whereas the left one is curved and enlarged posteriorly. The posterior sulcus is shallow and wide. The anterior protuberance is well developed. The crown is largely covered by seven thin, parallel ridges, which branch at their ends and reach the lateral crown margins (see Fig. 5A). Fine granulations without a diagnostic pattern cover the anterior marginal area. In posterior view ( Fig. 5A II ), the crown is gently raised at its center. The thick, bilobate root exhibits a deep antero-posterior sulcus; although the right lobe is more developed and wider than the other. In lateral view (Fig. 5A III ), the raised portion of the occlusal surface has a posterior outline perpendicular to the crown base; the anterior outline of the crown is inclined instead. The crown juts out from the root on all sides. The lateral outline of the root is straight posteriorly and tilted back on the anterior side (see Fig. 5A III ). Numerous foramina are placed along the crown/root boundary. In inferior view ( Fig. 5A IV ), the root is rectangular. In occlusal view (Fig. 5B), RSU DGE 2020 RO MP-11 exhibits a polygonal, asymmetrical outline. The crown has a deep posterior sulcus and a slight anterior protuberance. The right marginal area is more developed than the left one. Eight to nine thin, parallel ridges cross transversally on the occlusal surface, branching and reaching the lateral tooth edges (see Fig. 5B). The anterior marginal area is poorly developed and covered by fine granulation. In posterior view (Fig. 5B I ), the left side of the occlusal crown surface is gently raised. The root is bilobate with a shallow antero-posterior sulcus. In the inferior view ( Fig. 5B II ), the root outline follows the shape of the crown. RSU DGE 2020 RO MP-21 (Fig. 5C, C I ) exhibits an almost triangular outline with the anterior tooth edge narrow and merged to the right one in a single margin curved posteriorly; the left side is instead slightly convex. The posterior sulcus is shallow. In occlusal view, 8-9 thin, parallel ridges cross the dental crown and reach the lateral tooth edge with branched ends. A fine granulation lacking in diagnostic patterns covers the anterior marginal area of RSU DGE 2020 RO MP-21 (see  Fig. 5F). Nine to 10 thin, transversal ridges cross the occlusal surface branching on the marginal areas and reaching the lateral tooth edges (see Fig. 5F). Fine granulation covers the rest of the crown without a diagnostic pattern. In anterior (Fig. 5F I ) and posterior (Fig. 5F II ) views, the crown of RSU DGE 2021 RO MP-11 is markedly raised and bulged at the center with lateral side externally tilted. In lateral view (Fig. 5F III , F IV ), the anterior side of the crown is inclined downwards. The dental root is missing. RSU DGE 2021 RO MP-13 has a rectangular outline with shallow posterior sulcus and wide, rounded anterior protuberance (see Fig. 5G). The lateral surface of the left side shows two depressed areas (posterior and anterior), whereas the right one has a quite rounded outline. Nine to 10 thin ridges transversally cross the occlusal tooth surface, branching at their ends and reaching the lateral crown edges. Fine granulation and wrinkles characterize the anterior protuberance (see Fig. 5G). In posterior view (Fig. 5G I ), the dental crown is bulged in the center with inclined lateral marginal areas; the root is bilobate with a shallow antero-posterior sulcus. In lateral view (Fig. 5G II ), the anterior crown outline is tilted downwards; the tooth root exhibits a posterior side perpendicular to the crown base, whereas its anterior outline is inclined. In inferior view (Fig. 5G IV ), the root follows the rectangular outline of the crown. Irregular, unbranched, and whitish traces are on occlusal and posterior sides of RSU DGE 2021 RO MP-11 (see black arrows in Fig.  5F-F III ) and RSU DGE 2021 RO MP-13 (see black arrows in Fig. 5G-G II ). KP NVF 19956/16 and RSU DGE 2021 RO MP-6 ( Fig. 5E, EI) are similar to each other and exhibit only the dental crown with a raised central area. Nine thin, transversal ridges cross the entire occlusal surface, reaching the marginal area. The ridges branch at their ends at the lateral tooth edges and are slightly abraded at the center of the tooth crown. Fine wrinkles cross the anterior protuberance.
Referred Material-A single tooth (SS106-1), housed in the collection of the Sasovo secondary school n. 106.
Description-Specimen SS106-1 (Fig. 6A-A IV ) is an isolated tooth characterized by an almost symmetrical outline. Both anterior protuberance and posterior sulcus are poorly developed. In occlusal view (Fig. 6A), eight thick, sharp ridges cross the dental surface. The anterior-most and posterior-most ridges are interrupted and finer than the others. All ridges terminate abruptly without reaching the lateral tooth edges. The anteriormost ridges are slightly damaged, but overall dental wear is absent or very limited. Large granular bumps surround the crested area, whereas thin granules prolong the ends of the ridges. Nevertheless, the crested area is clearly separated from the marginal ornamentations. The marginal area is covered by coarse granulations without diagnostic patterns. In anterior view (Fig. 6A I ), the center of the thick crown is bulged. The preserved tooth root is bilobate with a shallow antero-posterior sulcus. The lateral root sides slightly converge inwards. In inferior view (Fig. 6A IV ), the tooth crown overhangs the root on all sides. The root is rectangular and partially damaged.
Remarks-The material presented herein is well preserved; both crown and root of SS106-1 (Fig. 6A-A IV ) are almost complete. Moreover, this large tooth was probably situated within the lateral area of the dentition and close to the symphyseal row. The occlusal ridges are only slightly damaged and the observed scratches do not conform to the typical dental wear commonly related to the prey processing in this species (also see Amadori et al., 2020aAmadori et al., , 2020b. In addition, Vodorezov and Solonin (2017: fig. 2) described and figured an isolated tooth RSU DGE 2017 RO MP-34 from the Malyy Prolom quarry; the specimen is currently lost. RSU DGE 2017 RO MP-34 has an asymmetrical outline and eight ridges transversally arranged on the center of the occlusal surface. The ridges are abruptly interrupted at their ends and well separated from the marginal ornamentation. Coarse granules cover the marginal area. Although only one image (latero-occlusal view; see Vodorezov and Solonin, 2017: fig. 2) is currently available for RSU DGE 2017 RO MP-34, its asymmetrical outline would indicate its arrangement along one of the lateral tooth rows; moreover, the small size and thinned ridges of this tooth indicate its distal position within the dentition. †PTYCHODUS MAMMILLARIS Agassiz, 1835 ( Fig. 7) (Selected synonyms) p."Teeth allied to Diodon": Mantell, 1822:231, pl. 32, fig. 20 (non figs. 17-19, 21, 23-25, 27, 29 fig. 3 (non figs. 1, 2, 4-8). Description-RSU DGE 2020 RO MP-6 ( Fig. 7A-A IV ) and RSU DGE 2020 RO MP-12 (Fig. 7B, B I ) have quadrangular crowns similar to each other with asymmetrical outline and a single knob-like cusp moved on the left side. In both teeth, the anterior protuberance is rounded and the left dental edge curved backwards. Their cusps are crossed by 9-10 thin ridges, which are well-separated by marginal ornamentations. Their marginal areas are well developed and entirely covered by thin, concentric wrinkles. In occlusal view (Fig. 7A), the right tooth edges of RSU DGE 2020 RO MP-6 are characterized by a lowered margin (see also 'f' in  Fig. 7F). The right tooth edge is slightly depressed at its external margin. The apical surface of the knob-like cusp exhibits nine thin, transversal ridges. The crown juts out from a bilobate root on all sides. In posterior view (Fig. 7F I ), the left lobe of the dental root is the smallest and its external side is inclined towards the center of the root. In inferior view (Fig. 7F III ), the tooth root is square and follows the crown occlusal outline. In occlusal view (Fig. 7G), the crown of SS106-7 is almost symmetrical with a straight left tooth edge and a rounded outline on the opposite side; the tooth cusp is narrow anteriorly. In anterior view (Fig. 7G I ), the cusp is almost flat and the bilobate root is broken on the left. RSU DGE 2021 RO MP-14, SS106-4, and SS106-7 exhibit slight wear traces on the occlusal ridges.
Referred Material-Two isolated teeth (SS106-2 and SS106-3) belonging to the fossil collection of the Sasovo secondary school n. 106.
Description-Specimen SS106-2 ( Fig. 8A-A IV ) is antero-posteriorly elongated in general shape. In occlusal view (Fig. 8A), its crown exhibits an almost rectangular outline with a rounded anterior margin; the posterior side is larger than the opposite one with a shallow posterior sulcus. The mesio-distally compressed occlusal surface shows three short and irregular ridges with anastomoses (no concentric 'loops'); thin wrinkles cover the anterior marginal area, whereas fine granulations characterize the posterior occlusal area. In anterior and posterior views ( Fig. 8A I , A II ), the crown of SS106-2 is thin and narrow and the lateral edges are characterized by depressed and tilted areas. The root is thick and monolobate with a rounded and large end; both lateral root sides are inclined externally (see also Fig. 8A I , A II ). The crown base juts out from the root on all the sides. In lateral view (see Fig. 8A III , A IV ), the crown base is flat and the crested area is raised; the root is markedly tilted posteriorly. The dental root is slightly damaged anteriorly. In occlusal view (Fig. 8B-B III ), SS106-3 has an asymmetrical and trapezoidal outline with the anterior side of the crown markedly wider than the posterior one. Both anterior protuberance and posterior sulcus are poorly developed. The right tooth edge is rounded and convex, whereas the left one is slightly concave. Eight ridges characterized the center of the crown occlusal surface; the ridge ends become thinner without reaching the tooth edges (see Fig. 8B). The ridges curve anteriorly and gradually blend with fine concentric wrinkles and granulations, covering the entire marginal area. The anterior marginal area is well developed, whereas the posterior one is almost absent. A depressed area is recognizable along the left marginal area (see f in Fig. 8B). Although the occlusal ridges are slightly damaged, traces of dental wear are clearly observable on the left side of the crested area (see Fig. 8B, B II ). In anterior view (Fig. 8B I ), the lateral crown edges are inclined downwards, whereas the crested area is gently raised and moved on the right side. The dental root is thick and bilobate with both lateral sides inclined towards the center of the tooth; the antero-posterior sulcus is shallow (see Fig. 8B I ). The inferior portion of SS106-3 is cracked anteriorly. In lateral view (Fig.  8B II ), the dental crown is slightly raised with a posterior outline parallel to the tooth base and the anterior side curved downwards. The posterior side of the tooth root is straight, whereas the anterior one is tilted back (see also Fig. 8B II ). In inferior view (Fig. 8B III ), the crown protrudes from the root on all sides; the root follows the polygonal outline of the dental crown.
Referred Material-A single isolated tooth RSU DGE 2021 RO MP-10 belonging to the fossil collections of the Ryazan State University.
Description-Specimen RSU DGE 2021 RO MP-10 exhibits a poorly preserved crown and it lacks a root. In occlusal view (Fig.  9A-A IV ), RSU DGE 2021 RO MP-10 has a rectangular and symmetrical outline with a wide and rounded anterior protuberance and a shallow posterior sulcus. Ten to 11 thick ridges cross the tooth crown, but fail to reach the lateral dental edges; large bumps surround the crested area. The ridges curve anteriorly and thin at their ends; a coarse granulation covers the marginal areas. In anterior and posterior views (Fig. 9A I , A II ), the occlusal surface is convex and rounded with later edges slightly inclined downwards. The crested area of RSU DGE 2021 RO MP-10 is markedly abraded (see Fig. 9A).
Ptychodus sp.: Aleksandrovich, 2019:265, fig. 1 Description-RSU DGE 2018 RO MP-41 ( Fig. 10A-A III ) and KP NVF 19956/215 exhibit rectangular, asymmetrical tooth crowns almost completely covered by 10-11 thick transversal ridges. Both anterior protuberance and posterior sulcus are not marked. The right tooth edges of KP NVF 19956/215 are broken. In occlusal view (Fig. 10A), the ridges curve anteriorly reaching the lateral dental edges; anastomoses and loops are also recognizable at the right ends of the ridges. Coarse granulations without a diagnostic pattern cover the anterior marginal area. In anterior view (Fig. 10A II ), both teeth have massive, slightly raised tooth crowns; the roots are bilobate and thick with a shallow antero-posterior sulcus. In lateral view (Fig. 10A III ), the outline of the occlusal surface is inclined anteriorly; the root has a posterior side perpendicular to the crown base, whereas the anterior outline is tilted posteriorly. The crown of RSU DGE 2018 RO MP-41 has a slight lateral depression on the side of the left edge (see Fig. 10A, A II ). RSU DGE 2021 RO MP-19 (Fig. 10B-B III ) has a rectangular crown with both posterior sulcus and anterior protuberance poorly developed. The right crown edge is almost straight, whereas the left one is curved. Ten transverse, thin ridges cross the occlusal surface, branching at their ends and reaching the lateral tooth margins (see Fig. 10B). Fine granulation covers the anterior marginal area without a diagnostic pattern. In posterior view (Fig. B II ), RSU DGE 2021 RO MP-19 exhibits a flat crown. The root has a shallow antero-posterior sulcus and the outline of the left side is markedly inclined. In lateral view (Fig. B III ), the anterior sides of both crown and root are tilted. The posterior outline of the tooth is instead straight.
Remarks-The bilateral asymmetry and the raised occlusal surface shown by both specimens (RSU DGE 2018 RO MP-41 and KP NVF 19956/215) indicate their belonging to the lateral rows of the lower dentition. However, they were probably arranged within opposite sides of the dentition. Based on its . This lateral imprint was probably useful for interlocking with adjacent teeth (juxtaposition). Similar lateral 'facets' are also observable in other un-cuspidate species (e.g., Ptychodus decurrens; Woodward, 1887; Amadori et al., 2019a;Hamm, 2020a; this paper, see above).
Additional fragmentary and/or abraded teeth (see Fig. S1) are described in the Supplemental Data.

Taphonomic Remarks
Post-Paleozoic elasmobranchs (sharks, skates, and rays) are well known for their fragmentary record, which is largely composed of isolated teeth with a wide range of sizes and preservational degrees (e.g., Cappetta, 2012;Underwood et al., 2015). Consequently, most studies over the last century focused on articulated material and large teeth in order to obtain higher quality and more reliable information (Underwood et al., 2015). In this regard, ptychodontid sharks are no exception with a huge amount of heavily broken or badly preserved isolated teeth representing their fossil record (Woodward, 1912;D'Erasmo, 1922;Herman, 1977;Cappetta, 2012;Hamm, 2020a). Indeed, much of the material that has been documented for all Ptychodontid genera so far (Heteroptychodus, Ptychodus, and Paraptychodus) consists of isolated teeth, often lacking the root and/or part of the dental crown (e.g., Cappetta, 2012;Hamm, 2020a). Only in recent decades, an increasing amount of isolated material, both unknown and previously collected, has been extensively studied (Underwood et al., 2015).
Most of the shark and ray teeth collected from the Upper Cretaceous of Ryazan Oblast (European Russia) up to now are fragmentary. Only about 8% of the non-ptychodontid teeth are completely preserved, whereas about 50% of tooth crowns are broken and in 92% of the specimens the root is poorly preserved or totally missing. Furthermore, about 80% of the teeth exhibit moderate abrasions on both crowns and roots. Actinopterygian teeth from the same fossil site are mostly damaged, broken and/or worn as well.
Among the isolated teeth of Ptychodus from the Upper Cretaceous of Ryazan Oblast (European Russia) that are here examined, only 27% of the specimens are fragmentary, whereas the remaining specimens represent more or less well-preserved tooth crowns (see also Table S1 in Supplemental Data). The teeth of Ptychodus examined here therefore show a relatively good state of preservation compared with those usually observed in this group and in other elasmobranch assemblages. Moreover, whitish irregular signs characterize the occlusal surface of RSU DGE 2020 RO MP-14 (see black arrows in Fig. 4E-E III ), RSU DGE 2020 RO MP-11 (see black arrows in Fig. 5F, F II , F III ), and RSU DGE 2021 RO MP-13 (see black arrows in Fig. 5G-G II ). Similar traces with rounded extremities and irregular patterns were documented for various isolated shark teeth from the mid-Cretaceous of eastern England and on tooth sets of Ptychodus from the Upper Cretaceous of northern Italy (see Amadori et al., 2020a: fig. 18 A, C-E, H, I; Underwood et al., 1999: fig. 1b-h). These whitish traces probably result from bioerosion by endolithic organisms and are morphologically close to the fungal borings characterizing the ichnospecies Mycelites ossifragus (Roux, 1887;Underwood et al., 1999;Amadori et al., 2020a).
The isolated teeth of Ptychodus from the Cenomanian of the Varavinsky Ravine locality (Moscow Oblast) are poorly preserved with only broken tooth crowns and no specimens exhibit the root.
Rare articulated skeletal remains of Mesozoic elasmobranchs are considered to provide detailed morphological information, which is essential for better understanding their evolution and ecology (e.g., Shimada, 2005;Shimada et al., 2009;Cappetta, 2012;Underwood et al., 2015;Collareta et al., 2017;Amalfitano et al., 2017;Amadori et al., 2020a). However, skeletal remains of Ptychodus are extremely rare. Nevertheless, the massive sampling and rigorous study of an increasing number of isolated teeth of elasmobranch fishes has led to a tremendous improvement in our knowledge about their diversity and distribution patterns as well as their systematic affinities (Reis, 2005;Cappetta, 2012;Underwood et al., 2015;Bogan et al., 2016;Marramà et al., 2018). Isolated teeth of Ptychodus species therefore also have the potential to address various paleobiological topics going far beyond pure taxonomic aspects.
About half of the isolated teeth of Ptychodus identified from the Upper Cretaceous of Malyy Prolom (Ryazan Oblast) exhibit dental wear patterns with various degrees of abrasion related to the procession of shelled prey items. Interestingly, most of the worn teeth (ca. 87%) are assigned to cuspidate species (see Table S1 in Supplemental Data). Therefore, teeth with occlusal cusps might not offer a significant advantage in preventing prey abrasion. However, the loss and subsequent replacement of teeth in Ptychodus might not be strictly dependent on the increasing occurrence of damaged and/or not fully functional teeth. The tooth plate might thus progress anteriorly out of the oral cavity leading to the removal and replacement of both worn and intact teeth. In addition, a possible sampling bias could have influenced the result of the preliminary analysis presented here due to the limited number of low-crowned teeth within the Ryazan assemblage. Further analyses of a larger number of cuspidate and un-cuspidate teeth of Ptychodus are thus necessary and recommended in order to better clarify the dental renewal and the durophagous tooth adaptations of this enigmatic predator.
Regardless of possible differences in dentition efficiency and functionalities between cuspidate and un-cuspidate species, the different morphologies identified in Ptychodus from Russia, as well as from other European localities, confirm a complex and intricate scenario for the trophic ecology of this marine predator (see also Amadori et al., 2019bAmadori et al., , 2020bAmadori et al., , 2022. Moreover, the various degrees of tooth wear documented for cuspidate and un-cuspidate Ptychodus from numerous other localities around the world indicate durophagy as one of the main feeding strategies for this extinct predatory elasmobranch, reaching even a high degree of specialization in some species (e.g., Woodward, 1887Woodward, , 1912Shimada, 2012;Diedrich, 2013;Amadori et al., 2019bAmadori et al., , 2020aAmadori et al., , 2022Hamm, 2020a;present paper). In Ptychodus, diversification in tooth morphology and, consequently, in diet preferences could therefore have reduced competition for food (e.g., shell-covered prey) within the genus (niche specialization), as well as with other possible durophagous groups.

Species Richness across Europe: A Preliminary Comparison
Ptychodus is well known from the Upper Cretaceous of Europe with numerous isolated and associated teeth coming from the Cenomanian-Campanian of various localities, such as England, France, Belgium, Germany, Poland, and Italy (e.g., Agassiz, 1835;Leriche, 1902Leriche, , 1906Woodward, 1912;Herman, 1977;Niedzẃiedzki and Kalina, 2003;Diedrich, 2013;Amadori et al., 2019aAmadori et al., , 2019bAmadori et al., , 2020a see Table S3 in Supplemental Data, for the updated record of Ptychodus from Europe). The most abundant assemblages of Ptychodus are reported from England and France (e.g., Leriche, 1902Leriche, , 1906Woodward, 1912;Herman, 1977). Ptychodus from England is rather species diverse with nine species reported from the Cenomanian-Campanian, but the occurrences are mainly limited to the southern area of England (see Table 1). Northern France shows a higher diversity than the southern regions of France, with nine species of Ptychodus being identified in the Cenomanian-Campanian (see Table 1). An isolated tooth (coll. Déchaux) of P. decurrens from the Albian of Clansayes (Auvergne-Rhône-Alpes region, southern France) and housed in the fossil collection at the "Faculté des Sciences de Grenoble" (Université Grenoble Alpes, France; see Priem, 1912: 255) represents the oldest unambiguous occurrence of the genus that has been documented from Europe up to now. In addition, other isolated tooth fragments from the Gault Formation (Albian) of Cucheron (Chartreuse area, Auvergne-Rhône-Alpes region, southeastern France) and belonging to the Collections géologiques de l'Observatoire des Sciences de l'Univers de Grenoble are claerly assignable to Ptychodus (see also Breistroffer, 1933). Ptychodus also was reported from the Albian of Rencurel in the Auvergne-Rhône-Alpes region of southern France (Priem, 1912) and the Albian-Turonian (Bristow, 1998) of the Mondrepuis area in northern France (Leriche, 1906;Herman, 1977). The cited material, however, was not figured and it therefore is not possible to correctly identify the specimens. These occurrences are therefore discarded from the present study until a future and desirable revision of the French records of Ptychodus has been conducted.
Although the fossil record of Ptychodus from northern Italy and Belgium is relatively rare compared with that from England and France, 8-9 taxa of Ptychodus previously were identified from the Cenomanian-Santonian (possibly Campanian) of these areas (see Table 1). Ptychodus remains are well documented from both western and eastern German localities with abundant, but less diverse, assemblages from the Cenomanian to Campanian; in particular, cuspidate taxa are quite rare (see Table 1). A relatively high number of species of Ptychodus (5-6) have been documented from the Cenomanian-Turonian of northern Czech Republic and southern Poland, respectively, up to now (see Table 1), based on very rare isolated teeth.
Based on previous records and the material documented in the present study, six species of Ptychodus certainty occurred in the Cenomanian-?Santonian of European Russia (see also Table 2). Additionally, ?P. marginalis and ?P. mediterraneus are documented only from the Ryazan and Saratov regions. Un-cuspidate taxa, such as P. decurrens, P. latissimus, and P. polygyrus, are the most widespread in western European Russia together with the cuspidate P. mammillaris. The rarest species of Ptychodus from the Upper Cretaceous of Russia are P. altior and ? P. marginalis and their distribution seems to be limited to the Ryazan Oblast area (see also Table 2). New selachian dental remains, including an isolated tooth of P. rugosus, from the Upper Cretaceous (probably Santonian-Campanian) of the Orenburg region (eastern European Russia) is currently under reassessment (Jambura et al., in prep.). This confirms the presence of P. rugosus in European Russia, which previously had been reported from the Saratov area (see Sinzow, 1899).
A maximum of 10 species of Ptychodus (P. altior, P. anonymus, P. decurrens, P. latissimus, P. mammillaris, P. marginalis, P. mediterraneus, P. mortoni, P. polygyrus, and P. rugosus) are currently known from Europe and reported in 15 countries (see Tables 1 and 2), with P. decurrens, P. latissimus, and P. mammillaris being the most widespread taxa (see also Table  S3 in Supplemental Data). Ptychodus mortoni is instead very rare with a few isolated teeth only occurring in England, Belgium, and Italy (see also Table 1). Moreover, P. mortoni has not been identified from any Russian locality so far (see also Table 2). Ptychodus mediterraneus only has been reported from northern France and northern Italy up to now. This species, however, possibly reached European Russia as well (see also Tables 1 and 2). Due to the dubious provenance of the Ptychodus specimens reported from southern Sweden (see also Siverson, 1993), the teeth described here from the Cenomanian of the Varavinsky Ravine locality (Moscow Oblast, European Russia) seem to represent the northernmost occurrences documented in Europe for this durophagous predator in the early Late Cretaceous so far (see Fig. 11A). The diversity hot spots for Ptychodus in Europe are southern England, northern France, and northern Italy (see Fig. 11B). In addition, the Ptychodus assemblage from the Upper Cretaceous of the Ryazan Oblast described in the present study seems to be the most diverse among those from the Upper Cretaceous of European Russia (see Fig. 11B). Both cuspidate and un-cuspidate species were well distributed across all the examined European areas, with a few exceptions (see Fig. 11B). Among the Russian localities, no cuspidate species has been reported from the Kursk Oblast (European Russia) so far (see Fig. 11B; see also Table 2).

Paleoenvironment and Paleobiogeography
Scenarios for modern marine ecosystems based on models of climate and environmental change and community responses are increasingly alarming (Bruno et al., 2018;Ullah et al., 2018;Babcock et al., 2019). Drastic fluctuations in environmental conditions, as well as loss and degradation of suitable habitats, significantly affect the distribution patterns and dispersal performances of extant elasmobranchs (Musick et al., 2000;De Angelis et al., 2008;Field, 2009;Knip et al., 2010). For instance, the water temperature and salinity represent crucial environmental factors that drive distributions and habitat preferences of extant elasmobranchs, also directly influencing their physiology (e.g., muscle performance and metabolism; Bernal et al., 2012;Schlaff et al., 2014;Munroe et al., 2014;Bernal and Lowe, 2015;Meese and Lowe, 2019). Furthermore, multiple biotic variables, such as prey density and availability, can trigger distribution changes and habitat choice for both specialized (e.g., durophagous batoids) and generalist elasmobranchs (Torres et al., 2006;Pimiento et al., 2016;Serrano-Flores et al., 2019). In particular, variations in biotic and abiotic factors severely threaten the survival of specialized taxa, and identifying specialized predators is thus crucial to properly evaluate the vulnerability of elasmobranch communities (Munroe et al., 2014;Pimiento et al., 2016). However, abiotic stressors may alter the abundance and distribution of their primary prey, indirectly   Tables 1 and 2, and Tables S3 and Table S4  influencing dispersal patterns (Schlaff et al., 2014). Such strong connections between elasmobranchs, including some durophagous taxa, and their environment can be assumed throughout most of their evolutionary history (e.g., Kriwet and Klug, 2008;Sorenson et al., 2014;Maduna et al., 2020). Valuable information on paleogeographic and climatic stressors driving the distribution and evolution of a fauna over long temporal scales can be inferred from its paleobiogeographic trends (Brooks and McLennan, 1991;Brown and Lomolino, 1998;Lieberman, 2000;Myers and Lieberman, 2010;Maduna et al., 2020). Studies on spatial and temporal distribution patterns based on fossil occurrences thus play a key role in understanding evolutionary processes, as well as in estimating conservation potential, for land and marine fauna communities (e.g., Lieberman, 2008Lieberman, , 2012Louys, 2012;Peterson and Lieberman, 2012;Cranbrook and Piper, 2013;Dombrosky, 2015;Maduna et al., 2020). As a result of the Cenomanian transgression, an epicontinental sea covered a vast area of the Eastern European Platform (Russian Platform), though for the most part it was shallow with rapidly changing currents. In the south, there were large islands that weathered and contributed sediment and debris to the seabed. During the Cenomanian, the Russian sea was widely connected in the south to the Caucasus, in the east to the Kopet Dagh seas, and in the west by seaways to Central Europe (Vinogradov, 1975). The upper Cenomanian sediments on the Russian Platform were partially eroded during the latest Cenomanian-early Turonian regression. During the early Turonian, erosion resulted in part from a sea-level fall in most regions of the Russian, Scythian, and Turonian platforms. Only in the southernmost areas did the sedimentation continue. The next transgressive episode began in the middle Turonian with a maximum in the late Turonian and Coniacian (Baraboshkin et al., 2003). Although there were episodes of regression during the Coniacian, the marine basin on the Russian Platform was somewhat reduced, especially on the outskirts of synclines and anticlines (Vinogradov, 1975). In the late Coniacian to Santonian, a vast transgression of the sea from the east and southeast covered almost completely the central Russian Platform. During the Santonian, the eastern edge of the platform was most intensively submerged and here, a narrow strait formed that connected the Russian with the boreal seas (Kuzmin et al., 2015). In the Eastern European Platform, all Late Cretaceous units with marine sediments were deposited in the northeastern peri-Tethyan basin, which was well connected to the West Siberian Sea and to the marine basins of Central Europe (Baraboshkin et al., 2003).
The early Campanian featured the largest transgression on the Russian Platform during the Upper Cretaceous (Baraboshkin et al., 2003). However, since the middle or late Campanian, the Russian Sea decreased in size and finally the central part of the Russian platform fell dry. This is clearly observable in the area of the Moscow syncline and in the east of the platform, where the direct connection to the boreal and south seas probably ceased. However, in other parts of the platform, the situation continued to be the same as before (Vinogradov, 1975).
The late Maastrichtian featured a regressive phase on the Russian Platform. The northern coastline moved south over several hundred kilometres, and sediments on coastal parts of the basin were later eroded (Baraboshkin et al., 2003). In general, frequent shallowing of the marine basin and depositional hiatuses occurred during the Upper Cretaceous on the Russian platform as a result of fluctuations in sea levels evidenced by the presence of the phosphorite content of Upper Cretaceous sediments (Sidorenko et al, 1971;Vinogradov, 1975;Baraboshkin et al., 2003;Kuzmin et al., 2015). In the center of the eastern European Platform, in a shallow-water environment with a zone of increased water turbulence, sandstones accumulated, which were saturated with inclusions of grus, sandstone gravel, and phosphorite pebbles rich in fossils including skeletal elements and teeth of marine reptiles. In addition to the remains of herpetofauna in the formations of that age, the vertebrae and teeth of cartilaginous fishes, mainly elasmobranchs are present (Sidorenko et al., 1971;Glikman, 1980;Arkhangelsky et al., 2008;Kuzmin et al., 2015).
In the Cretaceous, Ptychodus spanned most of the peri-Tethys with a stratigraphic distribution ranging from the Albian-Cenomanian to the Campanian and thus survived for around 30-40 million years (see also 'Spatial and temporal distribution of Ptychodus from Europe' section, above). The oldest European specimens of Ptychodus from the Albian of Auvergne-Rhône-Alpes region (southeastern France) indicate that the rise of the peri-Tethyan domain of this durophagous predator would have started in shallow open marine environments (see also Herrle et al., 2003;Granier et al., 2017). In particular, the Early Cretaceous spatial distribution of Ptychodus included the epicontinental sea area of Vercors platforms (see Fig. 12). The central position of the epicontinental seas in southern French and their multiple connections with the rest of the marginal areas of the western Neo-Tethys Ocean during the Late Cretaceous would have greatly favored a subsequent spread of Ptychodus species throughout the entire peri-Tethys (see Fig. 13). During the Cenomanian-Turonian, Ptychodus was well diversified with both cuspidate and un-cuspidate species being widely distributed throughout European epicontinental seas (see Fig. 13A). In particular, this durophagous elasmobranch reached the northeastern area of the Russian platform area (RP in Fig. 13A) in the early FIGURE 12. Lower Cretaceous paleogeographic maps with occurrences of Ptychodus from the Albian of Clansayes (Auvergne-Rhône-Alpes region, southern France; modified from Granier, 2017; see also Table  S3 and Table S4 in Supplemental Data).  also Table S3 and Table S4 in Supplemental Data). Asterisks indicate the areas from which the material examined in this study comes. Question marks indicate occurrences with uncertain age. Late Cretaceous, probably using this epicontinental seaway to migrate along the peripheral areas of the Neo-Tethys Ocean (e.g., Asian peri-Tethys) and to access the northwestern margin of the paleo-Pacific Ocean. Ptychodus was reported from the Turonian-Santonian of western Kazakhstan by Glickman et al. (1970) and Zhelezko and Glikman (1971; see also Popov, 2016), and the Cenomanian-Turonian to ?Campanian of Japan by Goto et al. (1996). In the Santonian, the Anglo-Paris basin, the Adria platform, and the southern Central Polish basin certainly were part of the epicontinental areas inhabited by Ptychodus (see Fig. 13B). The evolutionary history of Ptychodus in Europe extends into the Campanian of the Anglo-Paris (APB in Fig. 13C) and western North German basins (NGB in Fig.  13C). Other Campanian epicontinental seas occupied by this predatory elasmobranch include areas of present-day eastern Austria and, possibly, southern Lithuania. Additional studies of specimens of Ptychodus from the Upper Cretaceous of European Russia nevertheless are mandatory to establish the persistence of this durophagous elasmobranch in the Russian platform area after the Cenomanian. A serious reduction in the geographic distribution of Ptychodus in the peri-Tethyan seas undeniably occurred towards the end of the Upper Cretaceous. Jouve et al. (2017:fig. 4) reconstructed a global decrease in sea surface temperature alternating with temporary water heating during the Late Cretaceous. The highest heating peak in the shallow seas occurred in the Turonian, whereas the late Late Cretaceous saw a dramatic cooling that ended at the end of the Mesozoic era; although the temperature decline of surface waters already started in the late Turonian, a temporary, but noticeable, warming of the shallow seas occurred in the Coniacian-upper Santonian (Jouve et al., 2017). Furthermore, a local cooling and a salinity drop in surface waters (0-1000 m depth) recently was hypothesized throughout the Upper Cretaceous in most peri-Tethyan seas at a latitude ranging between 30-60°N (see Ladant et al., 2020:fig. 10d). However, the surface water temperature was relatively stable in the westernmost peri-Tethys, also rising in some areas (e.g., North German Basin; see Ladant et al., 2020:fig. 9a). The heating of these shallow water areas probably were due to the superficial 'proto-Gulf stream', which distributed warm surface waters from the Gulf of Mexico towards northwestern Europe during the Late Cretaceous (see Ladant et al., 2020;Wilmsen et al., 2021).
Variations in temperature and salinity represent typical abiotic stressors affecting the geographic distribution and migration patterns of extant elasmobranchs including durophagous taxa, such as Myliobatis (eagle rays) and Mustelus (smooth-hound sharks; Bernal et al., 2012;Schlaff et al., 2014;Bernal and Lowe, 2015;Meese and Lowe, 2019). The cooling of the epicontinental shallow seas inhabited by Ptychodus (see above) was probably involved in the narrowing of the European geographic range of this durophagous elasmobranch throughout the Late Cretaceous.
Conversely, the warmer waters of the 'proto-Gulf stream', which accessed the peri-Tethyan seas through the northern Anglo-Paris and the Aquitaine basins (see also Wilmsen et al., 2021: fig. 10), allowed Ptychodus species to persist in the northwestern peri-Tethys until the Campanian. However, the cooling of the sea surface alone can hardly explain the demise of Ptychodus in Europe after the Campanian. For example, the Late Cretaceous distribution and/or abundance of possible shelled prey (biotic factors), such as ammonites, bivalves, and crustaceans, may have been an additional factor playing a key role in the disappearance of this durophagous predator. Therefore, future surveys on possible correlations between paleobiogeography and diversity/abundance fluctuations of Ptychodus species and its prey in the peri-Tethys are required for confirming or discarding the hypothesis proposed in the present study.

CONCLUSIONS
The new Late Cretaceous assemblages of Ptychodus from European Russia presented herein indicate the occurrence of at least seven species, P. altior, P. anonymus, P. decurrens, P. latissimus, P. mammillaris, P. marginalis, and P. polygyrus within the ?Cenomanian-Santonian shallow water environments of the Russian platform (northeastern peri-Tethys). The occurrence of an eighth additional taxon (P. mediterraneus) is likely, but requires further investigation. Two of the species here documented, P. altior and P. anonymus are reported here for the first time from the Russian localities. Ptychodus appeared in Europe in the late Early Cretaceous, rapidly colonizing most of the epicontinental seas of the western peri-Tethys throughout the Late Cretaceous. In the Late Cretaceous, Ptychodus started to migrate towards the easternmost peripheral areas of the Neo-Tethyan Ocean across the Russian platform. The highly diverse fauna of Ptychodus from the Malyy Prolom area indicates that the Russian platform provided a conducive environment (mainly shallow epicontinental seas) for the spread and diversification of this durophagous predator with various cuspidate and un-cuspidate taxa probably playing different roles in the trophic web (niche partitioning) of the northeastern peri-Tethys.
Additionally, local variations in abiotic factors, such as temperature and habitat availability driven by coastal line changes, might have trigged the narrowing of the geographic range of Ptychodus within relatively limited areas during the Late Cretaceous epicontinental seas (e.g., Neo-Tethyan margins). Nevertheless, this durophagous predator was probably sensitive to multiple environmental stressors. For instance, prey abundance and distribution could have been additional factors in shaping the evolution and dispersal of the most specialized taxa within the genus Ptychodus. Further studies on unknown fossil fish material from Russian localities might reveal an even more complex scenario for the Cretaceous diversification and dispersal of Ptychodus, as well as for other elasmobranch predators, in eastern peri-Tethyan seas.