Robustoconus tisljari n . gen . , n . sp . , a new larger benthic foraminifer from the Middle Jurassic ( Early Bajocian ) of the Adriatic Carbonate Platform of Croatia

Larger benthic foraminifera are widely distributed in Jurassic shallow-water carbonates of the Adriatic Carbonate Platform, i.e. the Karst Dinarides (e.g. RADOIČIĆ, 1966; NIK LER & SOKAČ, 1968; GUŠIĆ, 1969a, b, 1977; VELIĆ, 1977, 2007; GUŠIĆ & VELIĆ, 1978; VELIĆ & SOKAČ, 1978; SCHLAGINTWEIT & VELIĆ, 2011). During recent investigations of Aalenian–Bajocian limestones of Biokovo Mt. and Dubrovnik area, new and poorly known taxa were described by SCHLAGINTWEIT & VELIĆ (2011). Previously, the Middle Jurassic shallow-water carbonates of the Karst Dinarides were considered to be poor in benthic foraminifera. The aforementioned investigations however, have shown that these are distinctly richer in taxa than the Lower Jurassic strata, with many of these representing index forms (VELIĆ, 2007). In the framework of on-going studies, another new Middle Jurassic larger benthic foraminiferan is recognized, described as Robustoconus tisljari n. gen., n. sp.


GEOLOGICAL SETTING
The Middle Jurassic platform carbonates containing Robustoconus tisljari n. gen., n. sp.crop out near Osojnik in the vicinity of Dubrovnik, southern Croatia (Fig. 1).Tectonically, they belong to the southwestern marginal area of the Adriatic Carbonate Platform (VLAHOVIĆ et al., 2005).These carbonates were deposited in shallow water environments strongly influenced by currents and waves, with the

Ab STRA CT
The new larger benthic foraminifer Robustoconus tisljari n. gen., n. sp. is described from Middle Jurassic (Early Bajocian) high-energy shallow-water limestones of the Dubrovnik area in the south-eastern part of the Adriatic Carbonate Platform.Robustoconus represents a large dimorphic agglutinated foraminiferan with a coarse exoskeleton typical of representatives of the Family Hauraniidae.Differences to the allied genera Spiraloconulus ALLEMANN & SCHROEDER and Bostia BASSOULLET are discussed.The occurrence of a further new genus of larger foraminifera provides additional evidence for a high origination rate in the Middle Jurassic.The restriction of Robustoconus tisljari to the Timodonella sarda taxon-range zone highlights its biostratigraphic interest.

SYSTEMATIC PALAEONTOLOGY
The suprageneric classification of agglutinated-conical benthic foraminifera (= pro parte the so-called "larger agglutinated foraminifera") provided by LOEBLICH & TAP-PAN (1987) was criticised recently by VECCHIO & HOT-TINGER (2007) as not taking into account the architecture of the exo-and endoskeletal elements and their pattern.Such a scheme was introduced some decades ago by SEPTFON-TAINE (1988) and was incorporated in the "year 2000 classification" of KAMINSKI (2004) that is followed here.Origin of the name: robustus (lat.)= robust and conus (lat.)= cone.Named after the high-conical test (A-and most B-forms), combined with the thick, coarse hypodermal network.

Class Foraminiferida
Diagnosis: Test conical to cylindrical, with an early plani spiral and involute coiled stage, later rectilinear.Cone base slightly arched in the juvenile stage, becoming distinctly convex in adult chambers.Macrospheric specimen with a small simple subspherical proloculus.Microspheric tests, large and may be slightly flabelliform.Wall microgranular calcareous, agglutinated, and may include large grains especially in the central part.Chambers subdivided into a marginal and central zone.Coarse exoskeleton of radial (beams and intercalary beams) and horizontal partitions (rafters) forming a network of chamberlets in the marginal zone.The complex cen tral zone consists of anastomizing septal excrescences (with constrictions and swellings) that reach into the cham- ber interior, but do not span between two succesive septa, leaving instead a continuous open space at the proximal parts of the chambers.This peculiar type of endoskeleton that strong ly reduces the volume of the chamber interior may be obscured in specimens where large amounts of agglutinated particles are incorporated in the test.Intracameral foramina single, interiomarginal in the coiled part, becoming multiple, irregularly distributed in the central zone of the rectilinear part.
The differences of Robustoconus to both Spiraloconulus and Bostia are more delicate needing comprehensive discussion.Spiraloconulus differs from Robustoconus by the construction of its exoskeleton consisting of a rather narrow marginal zone of thin-walled chamberlets and a large central zone with septa typically agglutinating large grains (ALLE- MANN & SCHROEDER, 1972; CHERCHI & SCHRO-EDER, 1981) (Fig. 4).In fact, the central zone is "relatively simple" in Spiraloconulus where the "partly coarse agglutination of the chamber floors (remark: roofs!) can easily be mistaken for a complex structure" (ALLEMANN & SC- HROEDER, 1972: p. 207).This statement was given for the type-species S. perconigi where the agglutination of foreign material is concentrated within the septa and the chamber interior is more or less free.In S. giganteus, the chamber interior of micropsheric specimens may be largely filled with ooids connected to the septa and to adjacent ooids by means of very thin walls as a biomineralized part of the test.Notwithstanding, BOUDAGHER-FADEL (2008: p. 179) defines the genus Spiraloconulus as possesing an "endoskeleton(s) of pillars from septum to septum" obviously adopting the classification of LOEBLICH & TAPPAN (1987), placing the genus within the pillaroid family Spirocyclinidae.In fact, Spiraloconulus (and also Bostia) is placed in the subfamily Amijiellinae (SEPTFONTAINE, 1988;KAMINSKI, 2004) where representatives usually do not develop pillars.The central zone of Robustoconus is made up of a complex structure of anastomizing micritic septal excrescences (with con-strictions and swellings) that reach deep into the chamber lumen (= trabécules micritiques of BASSOULLET, 1998).However, they do not span between two succesive septa, instead leaving a continuous open space at the proximal parts of the chambers.Therefore these cannot be termed interseptal pillars (see HOTTINGER, 2006).This type of endoskeleton is often obscured in specimens where large amounts of agglutinated particles are incorporated in the tests of Robustoconus.In Bostia, BASSOULLET (1996) speaks of an agglutinated endoskeleton.For the genus Limognella (see below), SEPTFONTAINE (1988: p. 244) assumes that such an endoskeletal structure is only pretend due to the highly  agglutinated character.In this respect, returning to the genus Spiraloconulus, the type-species S. perconigi can be considered as a species with no endoskeleton, whereas the short micritic walls connecting adjacent agglutinated grains in S. giganteus can be considered as endoskeletal structures.
Concerning the development of the macrospheric embryo, it is uncertain whether in Robustoconus it is unilocular or bilocular, e.g. as in Spiraloconulus perconigi (see ALLE-MANN & SCHROEDER, 1972).In any case it is not complex as in Timidonella, Orbitopsella, Cymbriaella or Bostia, composed of a rather large single spherical proloculus that is enclosed by a thin wall and a large spheroconch (e.g.BA-SSOULLET et al., 1974;BASSOULLET, 1998;FUGA-GNOLI, 1999).In most cases, a single subspherical proloculus is observable in Robustoconus with a diameter of 0.1 to 0.2 mm, distinctly smaller than the giant spheroconch of Bostia with a size of 0.6 to 1.0 mm (BASSOULLET, 1996: p. 192).In rare cases, a second crescent-shaped chamber, slightly separated from the proloculus is discernible in Robustoconus that is interpreted as the first post-embryonic chamber rather than a deuterconch.Let us remember that structural differences in the embryonic apparatus are widely accepted criteria in larger benthic foraminifera.Besides the giant complex embryonic apparatus in Bostia, this species also differs from Robustoconus by the distinctly reduced to absent initial spire.It should be mentioned that KAMINSKI (2000: p. 269) considers Bostia "to represent a more advanced stage in the evolution of the Spiraloconulus lineage".
Diagnosis: Being monospecific so far, the species diagnosis corresponds to the genus diagnosis.
The test height of adult specimens is most frequently within the range of 1.7 mm to 2.5 mm.The greatest observed specimen measures 5.9 mm in height and 1.54 mm in diameter (Pl.1a).
The short initial planispirally involute coiled part (height ~0.3 to 0.5 mm) consists of about one (? one and a half) whorl(s) of low crescent-shaped chambers (5 to 10 in number).This part may be apical or eccentric in position.Transverse sections through the enrolled part are approximately circular to slightly elliptical.In the coiled part, the intracameral foramina are single and interio-marginal in position (Fig. 7b).The macrospheric proloculus is simple with a diameter of ~0.1 mm to ~0.2 mm (most values between 0.13 and 0.16 mm).In sections parallel to the plane of coiling the shape of the proloculus is subspherical, whereas in sections perpendicular to the plane of coiling it appears ovally compressed (Fig. 6a).The proloculus is followed by a second crescent-shaped chamber closely attaching the proloculus (Pl.2, Fig. m).It is interpreted as the second post-embryonic chamber rather than as a deuteroconch.The uncoiled recti- linear stage corresponds to the main part of the test with up to about 15 uniserial chambers.During ontogeny, the almost flat chamber base successively becomes distinctly convex as appears also the ventral surface of the cone.In transverse sections, the rectilinear part is round, occasionally slightly elliptical (Pls.3-4) and in rare cases also with some marginal embayments visible in transverse section (Fig. 8f).Other test irregularities may result from very large agglutinated particles (Fig. 8c).
Each chamber is subdivided into a marginal zone and a central zone .The marginal zone consists of radial partitions (beams and 1 to 2 intercalary beams) and short horizontal partitions forming a network of chamberlets (Fig. 9).The diameter of these chamberlets exhibiting polygonal outlines varies between 0.06 and 0.08 mm in shallow tangential sections.Towards the chamber interior, the chamberlets increase in size along with a thickening of the exoskeletal elements.There may be 2 to 3 (rarely 4) rows of cham berlets per chamber displaying an irregular alternating position (Fig. 8c, upper part).
The central zone is made up of rather thick micritic septal excrescences (with constrictions and swellings) showing a general subparallel orientation toward the test axis (e.g.Pl. 2f).As visible in transverse sections, they form an anastomosing network (Pls.3-4).This structure forms a peculiar type of endoskeleton that may occupy large parts of the chamber lumen (e.g., Pl. 4b).The amount and also the size of agglutinated grains is rather variable.In many cases these grains are clearly recognizable as small lithoclasts with micritic rims or small benthic foraminifera (Fig. 8c, Pl. 3i, p).Homogeneous micritic grains (peloids) may also be agglutinated and are often indistinguishable from thickenings of the endoskeletal elements.In longitudinal sections, a continuous open space in the proximal part of the chamber base is discernible (Fig. 6, Pl. 1c-e).No interseptal structures (pillars) are present.Laterally, the endoskeletal elements fuse with those of the exoskeleton.
In the uniserial stage, there are numerous tiny intercameral foramina (width about 0.01 to 0.03 mm) piercing the septa and exhibiting an irregular distribution throughout the middle part of the central zone (Pl. 4c,l).
Remarks: Irregular aberrant tests as observed in Robustoconus are reported for instance from the Middle Jurassic Bostia irregularis (BASSOULLET, 1998), the Upper Jurassic Anchispirocyclina (RAMALHO, 1971) or the Lower Cretaceous Torremiroella (BRUN & CANÉROT, 1979).The reasons for these abnormalities are unknown.Tests of Spiraloconulus giganteus displaying both irregular internal struc tures and external morphologies were reported by CHERCHI & SCHROEDER (1981) resulting from large agglutinated ooids.In another case, partial test damage of Robustoconus tisljari (Fig. 8e) was the most likely cause of unusual growth.Test abnormalities are also reported from modern benthic foraminifera.They may be related to environmental stress conditions, e.g., warm water sources in deep sea areas (MERIÇ et al., 2003) or other factors (see for example, MUKHOPADHYAY, 2012).
A test dimorphism is assumed for Robustoconus.This can be suggested from specimens with a clear pronounced proloculus in a given size range (Pl.2), whereas in other specimens it is not detectable in longitudinal sections.Megalospheric forms are usually high-cylindroconical, whereas slightly flabelliform tests can be referred to B-forms (Fig. 8d-e).Another indication was obtained from the size dimensions.In thin-sections, 242 transverse sections were measured resulting in a range from 0.32 to 1.54 mm for the test width (Fig. 11).The data show two maxima, from which the smaller one (between 0.6 and 0.65 mm) is attributed to macrospheric specimens.However, it must be noted that this is only an interpretation, as from the inner structure the two generations cannot be distinguished.Therefore the data are assumed as representing a mixture of both generations and adult/juvenile specimens within a given assemblage.
Stratigraphy: According to the foraminiferal assemblage at the type locality (Fig. 3), the stratigraphic range of Robustoconus tisljari n. gen.n. sp.can be considered as Ear ly Bajocian.
Microfacies: Sample 0-26 represents an intraclastic grain stone clearly dominated by the overall presence of tests of Robustoconus tisljari (Fig. 5).Other foraminifera include common textulariids and some rare specimens of Bosniella bassoulleti SCHLAGINTWEIT & VELIĆ.Calcareous algae are very rare and are only represented by scattered porostromate algae and rare debris of an unknown tiny Clypeina species.Remains of gastropods (including nerineids) are common.

ACKNOWLEDGMENT
Field and laboratory investigations were supported by the Croatian Geological Survey and the Croatian Ministry of Science, Education and Sports of the Republic of Croatia, through projects 195-1953068-0242 "Microfossil assemblages in carbonate deposits of the Karst Dinarides" and 181-1811096-1093" Basic Geological Map of the Republic of Croatia 1:50.000".Antonietta CHERCHI (Cagliari) and Kemal TASLI (Mercin) are thanked for their remarks that encouraged us to better clarify some details of the manuscript.We would also like to thank Julie ROBSON (Open University) for the language review.

Figure 1 :
Figure 1: Geographic position of the investigated Osojnik profile ca 5 km north of Dubrovnik, in southern Croatia (asterisk).

Figure 3 :
Figure 3: Lithostratigraphy of the Osojnik section and the distribution of important larger benthic foraminifers, with the position of Robustoconus tis ljari n. gen.n. sp.(sample O-26).

Figure 4 :
Figure 4: Spiraloconulus giganteus CHERCHI & SCHROEDER from the Bajocian of Mount Biokovo, Croatia.a) Oolitic grainstone with two specimens.b) Longitudinal section.Note the simple central zone composed of large individual ooids that in the lower test portion stretch between two successive septula.c) Longitudinal section.Note ooids (white arrow) and small benthic foraminifer (black arrow) attached to the chamber rooves.Scale bars 0.5 mm.

Figure 5 :
Figure 5: a) Grainstone with abundant Robustoconus tisljari showing moderate parallel orientation of their cylindroconical tests.Thin-section 0-26/5.Scale bar 1 mm.b) Two specimens of Robustoconus tisljari in longitudinal section, the left one showing an initial spire with protoconch (p).Note the different dimensions and the difference in chamber convexity resulting in a more flattened (left) or convex apertual face (right) in large specimen.The specimen on the right (outlined by the dashed white-line) occurs in a packstone intraclast.Thin-section 0-26/6.Scale bars 0.5 mm.
regarding Limognella a synonym of Alzonella BERNIER & NEUMANN, 1970.However, PELISSIÉ & PEYBERNÈS (1982: Pl. 2, Fig. 7) illustrated a single transverse section of a microspheric forms of Limognella exhibiting a flattened test with subparallel opposite sides.Transverse sections of Spiraloconulus are almost exlusively round (cy lin drical enrolled part), and slightly elliptical shapes can only be observed in exceptional cases.There are also examples where test compression is used for genus differentiation, e.g.Planisepta (SEPTFON-TAINE in KAMINSKI, 2000) versus Lituosepta CATI, 1959.The occurrence of both orbitoliniform (S. perconigi) and ammobaculitoid forms (S. giganteus) within the same genus, however, makes the test morphology highly problematic as a generic criterion.Robustoconus tisljari nov.sp.(Figs.5-10; Pls.1-4) Origin of the name: In memory and dedication to Josip Tišljar for his numerous contributions to geology, especially to the carbonate sedimentology of the Adriatic carbonate platform (i.e.Karst Dinarides in Croatia, Slovenia, Bosnia and Herzegovina, and Montenegro).Holotype: Longitudinal section illustrated in Fig. 6a, c thin-section 0-26/3.Paratypes: All other specimens illustrated in the present paper.

Figure 6 :
Figure 6: Robustoconus tisljari nov.gen., n. sp.from the Early Bajocian of Croatia.a) Longitudinal specimen of a macrospheric specimen with short initial spire, holotype.The upper part passes obliquely through the reticular network.Thin-section 0-26/3.Scale bar 1 mm.b) Longitudinal section.Thinsection 0-26.Scale bar 1 mm.c) Detail from a showing four chambers without endoskeletal pillars connecting subsequent septa.Note the numerous horizontal marginal plates.Scale bar 0.3 mm.d) Detail from c showing the free spaces at the proximal parts of the chambers.Scale bars 0.3 mm.

Figure 8 :
Figure 8: Robustoconus tisljari nov.gen., n. sp.from the Early Bajocian of Croatia.a) Oblique section of a large microspheric flabelliform test.Thin-section 0-26/2.b) Oblique axial section of a megalospheric specimen passing through the initial spire and the proloculus.Note the aberrant test excrescence (branching).Thin-section 0-26/3.c) Oblique longitudinal section of a microspheric specimen with large agglutinated micritized lithoclasts.The arrow points to a lithoclast incorporated into the marginal zone that lacks a network in this part.Note the irregular test growth.Thin-section 0-26.d) Oblique section of a slightly flabelliform test.Thin-section O 26-6.e) Specimen with irregular morphology, most likely resulting from partial test damage (arrows) that was not fatal enabling ongoing chamber formation.Thin-section O 26-7.f) Oblique transverse section of a specimen exhibiting a marginal test irregularity (upper side).Thin-section O 26-8.Scale bars 0.5 mm.

Figure 9 :
Figure 9: Robustoconus tisljari nov.gen., n. sp.from the Early Bajocian of Croatia.a-c) Oblique sections, most probably all microspheric specimens.Note the thick dense and vermiform network of partitions in b.In c, four chambers are obliquely transacted and the network becomes continuously coarser from the marginal parts towards the chamber interior.Thin-sections 0-26/8, 0-26/4 and 0-26/7.Scale bars 0.5 mm.

Figure 11 :
Figure 11: Test diameter (mm) of Robustoconus tisljari in transverse sections showing two separated maxima and their interpretation in terms of different generations (A-or macrospheric and B-or microspheric).Number of measured specimens = 242.Further explanations in the text.