 Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) of the Adriatic Carbonate Platform of Croatia  Felix Schlagintweit1, Ivo Velić2 and Branko Sokač3 1 Lerchenauerstr. 167, D-80935 München, Germany; (Felix.Schlagintweit@gmx.de) 2 Croatian Geological Summer School and Geolog d.o.o., Pančićeva 5, HR-10000 Zagreb, Croatia (ivo. velic@zg.t-com.hr) 3 Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia doi: 10.4154/gc.2013.02 Geologia Croatica 66/1 15–28 11 Figs. 4 Pls. Zagreb 2013 Geologia CroaticaGeologia Croatica 1. INTRODUCTION Larger benthic foraminifera are widely distributed in Juras- sic 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). Previ- ously, the Middle Jurassic shallow-water carbonates of the Karst Dinarides were considered to be poor in benthic fora- minifera. 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, an- other new Middle Jurassic larger benthic foraminiferan is recognized, described as Robustoconus tisljari n. gen., n. sp. 2. GEOLOGICAL SETTING The Middle Jurassic platform carbonates containing Robus- toconus tisljari n. gen., n. sp. crop out near Osojnik in the vicinity of Dubrovnik, southern Croatia (Fig.1). Tectoni- cally, they belong to the southwestern marginal area of the Adriatic Carbonate Platform (VLAHOVIĆ et al., 2005). These carbonates were deposited in shallow water environ- ments 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 Ba- jocian) high-energy shallow-water limestones of the Dubrovnik area in the south-eastern part of the Adriatic Carbo- nate Platform. Robustoconus represents a large dimorphic agglutinated foraminiferan with a coarse exoskeleton typi- cal 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 foramini- fera 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. Keywords: Benthic foraminifera, systematics, Middle Jurassic, Adriatic carbonate platform, Croatia Geologia Croatica 66/1Geologia Croatica 16 occasional development and destruction of patch reefs (see SCHLAGINTWEIT & VELIĆ, 2011, for more details). In general, they are represented by various types of skeletal- intraclastic/bioclastic grainstones and oncoidal facies, with abundant tests of larger benthic foraminifera. This Early Middle Jurassic profile lies ca. 1 km south- southeast of Osojnik, on the road connecting the village with Dubrovnik (Figs. 1–2). Abundant specimens of R. tisljari n. gen., n. sp. occur in thick bedded skeletal-intraclastic/bio- clastic grainstones (sample O-26; Fig. 3), ca. 20 metres above the last occurrence of Timidonella sarda BASSOUL- LET, CHABRIER & FOURCADE (see Osojnik section – SCHLAGINTWEIT & VELIĆ, 2011). 3. SYSTEMATIC PALAEONTOLOGY The suprageneric classification of agglutinated-conical benthic foraminifera (= pro parte the so-called „larger ag- glutinated 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 clas- sification” of KAMINSKI (2004) that is followed here. Class Foraminiferida Order Loftusiida KAMINSKI & MIKHALEVICH, 2004 Suborder Orbitolinina KAMINSKI, 2004 Superfamily Pfenderinacea SMOUT & SUDGEN, 1962 Family Hauraniidae SEPTFONTAINE, 1988 Subfamily Amijiellinae SEPTFONTAINE, 1988 Genus Robustoconus nov. gen. 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 net- work. Diagnosis: Test conical to cylindrical, with an early pla- ni spiral and involute coiled stage, later rectilinear. Cone base slightly arched in the juvenile stage, becoming distinctly con- vex 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 cen- tral zone. Coarse exoskeleton of radial (beams and interca- lary beams) and horizontal partitions (rafters) forming a net- work 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- Figure 1: Geographic position of the investigated Osojnik profile ca 5 km north of Dubrovnik, in southern Croatia (asterisk). Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 17 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. Type species: Robustoconus tisljari nov. sp. Comparisons: Robustoconus belongs to a group of large ben thic foraminifera exhibiting a „structure exosqueletique complexe grossiere“ (HOTTINGER, 1967) or hauraniiform subepidermal network (réseaux hypodermique) (SEPTFON- TAINE, 1980). These forms are included in the family Hau- raniidae SEPTFONTAINE (see KAMINSKI, 2004). Robu- stoconus can be compared with Spiraloconulus ALLEMANN & SCHROEDER, 1972, and the allied genus Bostia BASSO- ULLET, 1998, as well as with Timidonella BASSOULLET, CHABRIER & FOURCADE, 1974. With respect to Timidonella, only cylindriform macro- spheric specimens (e.g. VELIĆ, 2007: pl. 6, fig. 3) can be taken for comparison given that the microspheric generation is characterized by discoidal tests (BASSOULLET et al., 1974). Timidonella possesses an endoskeleton of pillars connecting consecutive septa. The differences of Robustoconus to both Spiraloconulus and Bostia are more delicate needing comprehensive discu- ssion. Spiraloconulus differs from Robustoconus by the con- struction 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- Figure 2: Geographic map (Dubrovnik sheet, no. 625-3-1), scale 1:25.000 showing the location of sample O-26 con- taining the holotype of Robustoconus tisljari n. gen., n. sp. near Osojnik. Geologia Croatica 66/1Geologia Croatica 18 MANN & SCHROEDER, 1972; CHERCHI & SCHRO- EDER, 1981) (Fig. 4). In fact, the central zone is „relatively simple“ in Spiraloconulus where the „partly coarse aggluti- nation 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 in- terior of micropsheric specimens may be largely filled with 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). Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 19 ooids connected to the septa and to adjacent ooids by means of very thin walls as a biomineralized part of the test. Not- withstanding, 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 struc- ture 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, in- stead leaving a continuous open space at the proximal parts of the chambers. Therefore these cannot be termed intersep- tal pillars (see HOTTINGER, 2006). This type of endoske- leton is often obscured in specimens where large amounts of agglutinated particles are incorporated in the tests of Robu- stoconus. 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 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 sep- tula. 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: a) Grainstone with abundant Robustoconus tisljari showing moderate parallel orientation of their cylindroconical tests. Thin-section 0-26/5. Sca- le 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. Geologia Croatica 66/1Geologia Croatica 20 agglutinated character. In this respect, returning to the genus Spiraloconulus, the type-species S. perconigi can be consi- dered 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 em- bryo, 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 com- plex 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 prolocu- lus is observable in Robustoconus with a diameter of 0.1 to 0.2 mm, distinctly smaller than the giant spheroconch of Bo- stia with a size of 0.6 to 1.0 mm (BASSOULLET, 1996: p. 192). In rare cases, a second crescent-shaped chamber, sli- ghtly separated from the proloculus is discernible in Robu- stoconus that is interpreted as the first post-embryonic cham- ber 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 advan- ced stage in the evolution of the Spiraloconulus lineage“. Last but not least, we also have to mention the genus Limognella PELISSIÉ & PEYBERNÈS, 1982. Whereas CHERCHI & SCHROEDER (1983) consider Limognella a synonym of Spiraloconulus, BASSOULLET (1999: p. 193) stresses morphological differences. A third view was expre- ssed by SEPTFONTAINE (1988: p. 244) regarding Limo- gnella 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 al- most exlusively round (cy lin drical enrolled part), and sli- ghtly 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. Material: From sample 0–26, 11 thin-sections (2.5 x 7.5 cm) were made, numbered 0–26, 0–26/1 to 0–26/10 each containing about 40 to ~70 variously sectioned specimens. Depository: Croatian Natural History Museum, Deme- trova 1, 10000 Zagreb, Croatia. 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. Thin- section 0-26. Scale bar 1 mm. c) Detail from a showing four chambers with- out 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 7: Robustoconus tisljari nov. gen., n. sp. from the Early Bajocian of Croatia. a) Slightly oblique axial section through the planispiral involute initial spire of a megalospheric specimen. Thin-section 0-26/1. Scale bar 0.3 mm. b) Equatorial section through a juvenile specimen only consisting of the initial planispiral part. Note the interio-marginal intercameral foramina (arrows). Thin-section 0-26/5. Scale bars 0.3 mm. Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 21 PLATE 1 Robustoconus tisljari, different longitudinal, tangential (a-j) and oblique sections (k-m), mostly of supposedly microspheric specimens; Figure j shows a megalospheric specimen. Early Bajocian of Croatia. a Two specimens in oblique longitudinal section. Note the irregular growth of the uppermost part of the test (arrows) of the specimen on the right. Thin-section 0-26/5. b Oblique longitudinal section. Thin-section 0-26/10. c Oblique longitudinal section. Thin-section 0-26/2. d Longitudinal section. Thin-section 0-26/2. e Longitudinal section. Thin-section 0-26/4. f Longitudinal section; note the initial test bending. Thin-section 0-26/5. g Oblique longitudinal to tangential section. Thin-section 0-26/4. h Oblique longitudinal section. Thin-section 0-26/4. i Longitudinal-tangential section. Thin-section 0-26/10. j Longitudianl section of a juvenile specimen showing initial spire with proloculus. Thin-section 0-26/8. k Oblique section. Note the micritic endoskeletal structure of the central zone devoid of agglutinated material (see also Fig. l). Thin-section 0-26/1. l Oblique section of a test with an elliptical enrolled part. Thin-section 0-26/2. m Oblique section, thin-section 0-26/1. Scale bars 0.5 mm. Geologia Croatica 66/1Geologia Croatica 22 PLATE 2 Robustoconus tisljari, different sections of macrospheric specimens. Early Bajocian of Croatia. a Oblique longitudinal section of specimen showing initial spire with proloculus. Thin-section 0-26/5. b Longitudinal section of specimen showing initial spire and proloculus. Thin-section 0-26/10. c Oblique longitudinal section passing through the proloculus. Thin-section 0-26. d Oblique longitudinal section of specimen showing initial spire with proloculus. Thin- section 0-26/2. e Oblique section passing through the proloculus. Thin-section 0-26/5. f Longitudinal-tangential section cutting the proloculus marginally. Note the subparallel longitudinal orientation of the micritic endoskeletal elements devoid of agglutinated particles (see also Fig. d). Thin-section 0-26/8. g Oblique section passing through the initial spire and proloculus, thin-section 0-26. h-i Longitudinal section cutting the initial spire (with about 9 chambers in h) and proloculus. Thin-sections 0-26/5 and O-26/8. j-l Oblique section through the initial part with proloculus. Thin-sections 0-26/4, and O- 26/2 (k-l). m Note the sickle-shaped first post-embryonic chamber (pec). Thin-section 0-26/2. Scale bars 0.5 mm. Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 23 PLATE 3 Robustoconus tisljari, transverse to slightly oblique transverse sections of small specimens. Early Bajocian of Croatia. a Thin-section 0-26/7. b Thin-section 0-26/2. c Thin-section 0-26. d Thin-section 0-26/2. e Thin-section 0-26/3. f Thin-section 0-26. g Thin-section 0-26/9. h Thin-section 0-26/4. i Note the large agglutinated grains (amongst a small textulariid foraminifer) in the central zone. Thin-section 0-26/5. j Thin-section 0-26/5. k Thin-section 0-26/5. l Thin-section 0-26/6. m Thin-section 0-26/6. n Thin-section 0-26/7. o Thin-section 0-26/8. p Thin-section 0-26/9. q Thin-section 0-26/8. r Thin-section 0-26/10. s Thin-section 0-26/8. Scale bars 0.5 mm. Geologia Croatica 66/1Geologia Croatica 24 PLATE 4 Robustoconus tisljari, transverse to slightly oblique transverse sections of large specimens. Early Bajocian of Croatia. a Thin-section 0-26/4. b Note the strongly reduced chamber volume resulting from the dense and thick anastomizing endoskeletal network. Thin-section 0-26/2. c Note the transect of part of the septum with irregular distributed tiny foramina (white dashed circle). Thin-section 0-26/2. d Thin-section 0-26/3. e Thin-section 0-26/3. f Thin-section 0-26/6. G Thin-section 0-26/7. h Note the anastomosing septulae (as) of the central zone. Thin-section 0-26/5. i Thin-section 0-26/7. j Thin-section 0-26/7. k Thin-section 0-26/9. l Note the transect of part of the septum with irregular distributed tiny foramina (white dashed circle). Thin-section 0-26/8. Scale bars 0.5 mm. Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 25 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 ir- regularity (upper side). Thin-section O 26-8. Scale bars 0.5 mm. 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 continuous- ly coarser from the marginal parts towards the chamber interior. Thin-sec- tions 0-26/8, 0-26/4 and 0-26/7. Scale bars 0.5 mm. Type-locality: the road cut on the eastern slope of Sv. Ilija hill, ca. 1 km SSE from Osojnik, on the road from Oso- jnik to Dubrovnik (GPS, x=6506425, y=4728797; Fig. 2). Type-level: thick bedded, light-brownish coloured to white skeletal-intraclastic/bioclastic grainstone/rudstone li- me stone of Lower Bajocian age. Diagnosis: Being monospecific so far, the species diag- nosis corresponds to the genus diagnosis. Description: Test high conical to cylindrical, with an early planispiral and involute coiled stage, later rectilinear in specimens attributed to megalospheric forms (e.g., Fig. 6a, Pl. 2b, d). Occasionally, tests with irregular protuberances („pseudo-branching”) at the ventral side (Fig. 8b) or slightly flabelliform outline (Figs. 8d-e) are observed in addition. Geologia Croatica 66/1Geologia Croatica 26 The test height of adult specimens is most frequently within the range of 1.7 mm to 2.5 mm. The greatest observed spec- imen 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 num- ber). This part may be apical or eccentric in position. Trans- verse sections through the enrolled part are approximately circular to slightly elliptical. In the coiled part, the intraca- meral foramina are single and interio-marginal in position (Fig. 7b). The macrospheric proloculus is simple with a di- ameter 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 com- pressed (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- Figure 10: Robustoconus tisljari nov. gen., n. sp. from the Early Bajocian of Croatia. a-j) Oblique and tangential sections passing through the epidermal network of beams and rafters. From the marginal part towards the chamber interior, the network becomes successively coarser. Note also the thickening at the junction points. s = septum (f ). Thin-sections 0-26/5 (a, e), 0-26/4 (b), 0-26/7 (c), 0-26/9 (d, I), 0-26/1 (f ), 0-26/8 (g), 0-26/10 (h, j). Scale bars 0.5 mm. 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 parti- cles (Fig. 8c). Each chamber is subdivided into a marginal zone and a central zone (Pls. 3–4). 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 out- lines varies between 0.06 and 0.08 mm in shallow tangential sections. Towards the chamber interior, the chamberlets in- crease in size along with a thickening of the exoskeletal ele- ments. There may be 2 to 3 (rarely 4) rows of cham berlets per chamber displaying an irregular alternating position (Fig. 8c, upper part). Schlagintweit et al.: Robustoconus tisljari n. gen., n. sp., a new larger benthic foraminifer from the Middle Jurassic (Early Bajocian) ... Geologia Croatica 27 The central zone is made up of rather thick micritic sep- tal 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 anasto- mosing 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 thicke- nings 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 structu- res (pillars) are present. Laterally, the endoskeletal elements fuse with those of the exoskeleton. In the uniserial stage, there are numerous tiny intercam- eral 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 Robus- toconus are reported for instance from the Middle Jurassic Bostia irregularis (BASSOULLET, 1998), the Upper Juras- sic Anchispirocyclina (RAMALHO, 1971) or the Lower Cre- taceous Torremiroella (BRUN & CANÉROT, 1979). The reasons for these abnormalities are unknown. Tests of Spi- raloconulus giganteus displaying both irregular internal struc tures and external morphologies were reported by CHERCHI & SCHROEDER (1981) resulting from large ag- glutinated ooids. In another case, partial test damage of Ro- bustoconus 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 envi- ronmental stress conditions, e.g., warm water sources in deep sea areas (MERIÇ et al., 2003) or other factors (see for ex- ample, 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. Meg- alospheric 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 dimen- sions. In thin-sections, 242 transverse sections were mea- sured 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 ma- crospheric 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 assem- blage 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 porostro- mate algae and rare debris of an unknown tiny Clypeina spe- cies. 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 assem- blages 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 Ke- mal TASLI (Mercin) are thanked for their remarks that en- couraged us to better clarify some details of the manuscript. We would also like to thank Julie ROBSON (Open Univer- sity) for the language review. Figure 11: Test diameter (mm) of Robustoconus tisljari in transverse sections showing two separated maxima and their interpretation in terms of diffe- rent generations (A- or macrospheric and B- or microspheric). Number of measured specimens = 242. Further explanations in the text. Geologia Croatica 66/1Geologia Croatica 28 REFERENCES ALLEMANN, F. & SCHROEDER, R. 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DOI.org/10.1016/j.pal- aeo.2005.01.011 Manuscript received October 02, 2012 Revised manuscript accepted November 30, 2012 Available online February 28, 2013