Ceratobairdia? akhilleusi Forel

Forel, Marie-Béatrice & Grădinaru, Eugen, 2020, Rhaetian (Late Triassic) ostracods (Crustacea, Ostracoda) from the offshore prolongation of the North Dobrogean Orogen into the Romanian Black Sea shelf, European Journal of Taxonomy 727, pp. 1-83 : 28-29

Ceratobairdia? akhilleusi Forel sp. nov.

urn:lsid:zoobank.org:act:94F7B0AC-8359-4EEF-B620-8334E6DAB902

Fig. 7 G–S

Diagnosis

A species with rectangular carapace, dorsal overlap of left valve on right valve laterally compressed and postero-ventral border of right valve underlined by a ridge.

Etymology

From the ancient Greek Ἀχιλλεύς/Akhilleús referring to the legendary hero Achilles who has been long revered around the Black Sea.

Material examined

Holotype

ROMANIA • 1 LV; Black Sea , Romanian Continental Shelf , borehole 817LV, sample CM31B; Rhaetian, Upper Triassic; MNHN.F. F63224 .

Paratypes ROMANIA • 1 complete carapace; same locality as for holotype but sample CM31C; Rhaetian, Upper Triassic; MNHN.F. F63222 • 1 complete carapace; same locality as for holotype but sample CM31A ;

Rhaetian, Upper Triassic; MNHN.F. F63221 .

Other material

ROMANIA • 1 LV; same locality as for holotype but sample CM31C; Rhaetian, Upper Triassic; MNHN.F. F63220 • 1 RV; same locality as for holotype but sample CM31A; Rhaetian, Upper Triassic; MNHN.F. F63223 • 1 LV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63225 • 1 carapace, 2 LV, 1 RV; same locality as for holotype but samples CM31A, CM31B and CM31C; Rhaetian, Upper Triassic; MNHN.F. F63347 .

Dimensions

See Fig. 3D .

Description

CARAPACE. Large, rectangular in lateral view, with H max at mid-L, L max above mid-H; position of W max variable through ontogeny, posterior to mid-L in LV and anterior to mid-L in RV of immature specimens ( Fig. 7I ) and around mid-L at both valves in large specimens ( Fig. 7K ); LV larger that RV, overlapping it along VB and overreaching its dorsal margin with maximum at DB, overreach laterally compressed; surface smooth.

RV. Rectangular in lateral view, with H max along DB in large specimens ( Fig. 7L ) and at antero-dorsal angulation in immature ones ( Fig. 7H ); dorsal margin divided into subequal ADB, DB and PDB, with concave end of PDB more expressed in mature specimens, straight ADB and DB; dorsal angulations salient and compressed laterally; AB large, more truncate ventrally in mature specimens, with maximum located above mid-H; PB close to vertical in mature specimens, with maximum of convexity located above mid-H; ventral margin sinuous with oral concavity slightly in front of mid-L.

LV. Subtriangular in lateral view with H max at mid-L; dorsal margin evenly convex and laterally compressed, slightly tapered posteriorly in immature specimens ( Fig. 7H ); outlines of AVB and PVB similar to those of RV, VB straight; in inner view, PVB bordered by a shallow groove; calcified inner lamella from mid-ADB to mid-PDB, largest along AVB and PVB; vestibulum wide along AVB and PVB, nearly non-existent in oral area; selvage moderately expressed; hinge bar long, straight, apparently smooth, with conspicuous anterior and posterior triangular lists; AMS field only partly visible and shows a curved and compact posterior row of four scars and two subcentral scars ( Fig. 7O ).

Occurrence

Romanian Continental Shelf, Black Sea, Rhaetian, Upper Triassic (this paper).

Remarks

Ceratobairdia ? akhilleusi sp. nov. is attributed with doubt to Ceratobairdia because the postero-ventral ridge is only visible on RV and not well expressed in all specimens (e.g., Fig. 7L ). The laterally compressed overlap of Ceratobairdia ? akhilleusi sp. nov. is reminiscent of that in Petasobairdia Chen in Chen & Shi, 1982 described from the Late Permian of South China ( Chen & Shi 1982) and whose taxonomy and stratigraphical as well as geographical distributions are examined in the discussion chapter of this paper. However, the presence of a relatively flat posterior venter with regard to the lateral extension underlined by a ridge leads us to favour attribution to Ceratobairdia . Ceratobairdia ? akhilleusi sp. nov. is close to Bairdiacypris ? form G in Harloff & Jäger 1994 from the Sinemurian, Early Jurassic, of the Calcareous Alps ( Harloff & Jäger 1994), from which it differs by having a less upturned PB, a thicker and laterally compressed overlap along the dorsal margin and the presence of a ridge underlining the PVB at RV. The new species is also close to Bairdia michelseni michelseni Herrig, 1979 from the Late Sinemurian, Early Jurassic, of Th̹ringen ( Herrig 1979a), but the German species has a longer and more caudate PB at RV, an anterior maximum of curvature located higher, a pitted surface and lacks the laterally compressed overlap and the ridge along PVB of RV. It can also be compared to Bairdia michelseni arcuatocaudata Monostori, 1996 from the Pliensbachian, Early Jurassic of Hungary ( Monostori 1996), but this species also lacks the laterally compressed dorsal overlap and the postero-ventral ridge, which is replaced by a thin lateral compression. Ceratobairdia ? akhilleusi sp. nov. can finally be compared with Bairdia cf. michelseni in Honigstein et al. 2014 from the Early Jurassic of Austria ( Honigstein et al. 2014), but this species also lacks the distinctive laterally compressed overlap, has a lower PB with a more pronounced PDB concavity and lacks the angulation between DB and ADB on LV. The H/L diagram of measurable specimens ( Fig. 3D ) shows that the holotype is a sub-mature specimen. The largest specimen of the study, interpreted as an adult, is the second paratype. The ontogeny of Ceratobairdia ? akhilleusi sp. nov. is marked by an enlargement of the valves and an accentuation of the PV ridge and dorsal lamella.

Figures

Fig. 7 (opposite page). SEM micrographs of ostracods from borehole 817 Lebăda Vest, drill core CM31, western portion of Black Sea shelf, Rhaetian, Late Triassic.All specimens are housed in the collections of the Muséum national d’histoire naturelle, Paris, France (MNHN). A–B. Carinobairdia triassica triassica Kollmann, 1963. A. External view of a right valve, sample CM31B (MNHN.F.F63215). B. External view of a right valve, sample CM31C (MNHN.F.F63216). C–F. Carinobairdia alpina Kollmann, 1963 emend. Kristan-Tollmann 1970. C. External view of a right valve, sample CM31C (MNHN.F.F63217). D. External view of a broken left valve, sample CM31B (MNHN.F.F63218). E. External view of a left valve, sample CM31B (MNHN.F.F63219). F. Same specimen, inner view. G–S. Ceratobairdia? akhilleusi Forel sp. nov. G. External view of a left valve, sample CM31C (MNHN.F.F63220). H. Paratype 2, right lateral view of a carapace, sample CM31A (MNHN.F.F63221). I. Same specimen, dorsal view. J. Paratype 1, right lateral view of a carapace, sample CM31C (MNHN.F.F63222). K. Same specimen, dorsal view. L. External view of a right valve, sample CM31A (MNHN.F.F63223). M. Holotype, external view of a left valve, sample CM31B (MNHN.F.F63224). N. Same specimen, inner view. O. Detail of the muscle scar field area (topography mode). P. Detail of the posterior end of hingement. Q. Detail of the anterior end of hingement. R. External view of a left valve, sample CM31A (MNHN.F.F63225). S. Same specimen, inner view. T. Ceratobairdia sp., external view of a broken left valve, sample CM31A (MNHN.F.F63226). U. Fabalicypris cf. triassica Bolz, 1971, right lateral view of a carapace, sample CM31A (MNHN.F.F63227). Scale bars: 100 µm.

Fig. 3. Height/length scatter plots of species from borehole 817 Lebăda Vest, drill core CM31, western portion of Black Sea shelf, Rhaetian, Late Triassic. In all diagrams, the dimensions of right (stars) and left (circles) valves of complete carapaces are shown separately and circled or linked. P1 = paratype 1; P2 = paratype 2. A. Bairdiacypris multidentata Bolz, 1971. B. Bairdiacypris argonautaii Forel sp. nov. C. Carinobairdia alpina Kollmann, 1963. D. Ceratobairdia? akhilleusi Forel sp. nov. E. Histriabairdia pontuseuxinusensis Forel gen. et sp. nov. F. Isobythocypris atalantella Forel sp. nov. G. Petasobairdia amazonella Forel sp. nov. H. Paracypris ovidi Forel sp. nov.