Isobythocypris atalantella Forel, 2020

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 : 39-40

Isobythocypris atalantella Forel sp. nov.

urn:lsid:zoobank.org:act:E98F39D0-EF52-4BEC-A66B-439D4AD1D442

Fig. 9 B–I

Diagnosis

A new species of Isobythocypris with long and slightly arched dorsal margin, relatively large posterior border.

Etymology

From Άταλάντη / Átalántê, the only Augonaut woman.

Material examined

Holotype

ROMANIA • 1 complete carapace; Black Sea , Romanian Continental Shelf , borehole 817LV, sample CM31B; Rhaetian, Upper Triassic; MNHN.F. F63246 .

Paratype

ROMANIA • 1 complete carapace; same collection data as for holotype; Rhaetian, Upper Triassic; MNHN.F. F63247 .

Other material

ROMANIA • 1 complete carapace; same collection data as for holotype; Rhaetian, Upper Triassic; MNHN.F. F63248 • 1 complete carapace; same collection data as for holotype; Rhaetian, Upper Triassic; MNHN.F. F63249 • 1 complete carapace; same collection data as for holotype; Rhaetian, Upper Triassic; MNHN.F. F63251 • 1 complete carapace; same locality as for preceding but sample CM31C; Rhaetian, Upper Triassic; MNHN.F. F63250 • 9 complete carapaces; same locality as for preceding but samples CM31A, CM31B and CM31C; Rhaetian, Upper Triassic; MNHN.F. F63354 .

Dimensions

See Fig. 3F .

Description

Carapace of medium size, ovoid in lateral view, with H max around mid-L, L max slightly below mid-H, W max slightly posterior to mid-L; LV larger than RV, overlapping along dorsal margin and oral concavity, with interruption at AVB and PVB. Dorsal margin long and slightly convex, lacking angulations at LV, with weak angulations at RV resulting from tight folds of overlapping LV; antero-dorsal slope gentle in large specimens (± 10°), slightly steeper in smaller ones (± 20°); DB straight, somewhat sloping posteriorly (±25–35°); postero-dorsal slope short, steep, often poorly differentiated; anterior margin large, only slightly tapered ventrally, with poorly expressed bairdiid morphology and maximum of curvature at mid-H; ventral margin long and sinuous, with median oral concavity expressed on both valves, only slightly more in RV; AVB only slightly raised, PVB horizontal; posterior end narrowly rounded. Inner structures not observed. Surface smooth.

Occurrence

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

Remarks

Isobythocypris atalantella sp. nov. is relatively close to the species of Bythocypris illustrated in Bolz (1971b). Of them, Bythocypris sp. A is only shown as inner views ( Bolz 1971b: pl. 8, figs 118–119), which makes a comparison difficult. However, Isobythocypris atalantella sp. nov. differs from Bythocypris sp. A and Bythocypris sp. B from the Rhaetian of the Northern Calcareous Alps ( Bolz 1971b) by its narrower and uniformly convex posterior margin, which is close to vertical in both species of Bythocypris . Isobythocypris atalantella sp. nov. also differs from Bythocypris ubiquefrequens Bolz, 1971 from the Rhaetian of the Northern Calcareous Alps ( Bolz 1971b) in having a longer and less curved dorsal margin, a less marked bairdiid anterior margin and a larger posterior maximum of curvature. Isobythocypris atalantella sp. nov. is morphologically close to Isobythocypris ? sp. in Dreyer 1967 from the Early Domerian, Early Jurassic, of Germany ( Dreyer 1967) but the PB of Isobythocypris ? sp. is nearly vertical, with the maximum of curvature narrower and located more ventrally. It also differs from Isobythocypris elongata ( Blake, 1876) from the Early Jurassic of Yorkshire ( Blake 1876) by having a larger and not angulate posterior border. Isobythocypris atalantella sp. nov. also differs from Bythocypris faba Knitter, 1983 from the Toarcian, Early Jurassic, of Southern Germany ( Knitter 1983) by its more homogeneous convex dorsal margin, anterior maximum of convexity located higher and posterior border located lower and less bairdiid.

The H/L scatter plot of all measurable specimens of Isobythocypris atalantella sp. nov. ( Fig. 3F ) documents the occurrence of at least five ontogenetic stages, from A-4 to adult. The holotype ( Fig. 9 B–C) is the largest known adult. Conversely, the paratype ( Fig. 9D ) may correspond to an A-1 stage. The ontogeny of Isobythocypris atalantella sp. nov. is marked by the overall elongation of the carapace, narrowing of the PB and accentuation of the median oral concavity on both valves.

Figures

Fig. 9 (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. Isobythocypris sp., right lateral view of a carapace, sample CM31C (MNHN.F.F63245). B–I. Isobythocypris atalantella Forel sp. nov. B. Holotype, right lateral view of a carapace, sample CM31B (MNHN.F.F63246). C. Same specimen, ventral view. D. Paratype, right lateral view of a carapace, sample CM31B (MNHN.F.F63247). E. Right lateral view of a carapace, sample CM31B (MNHN.F.F63248). F. Right lateral view of a carapace, sample CM31B (MNHN.F.F63249). G. Right lateral view of a carapace, sample CM31C (MNHN.F.F63250). H. Right lateral view of a carapace, sample CM31B (MNHN.F.F63251). I. Same specimen, dorsal view. J–M. Lobobairdia salinara Kollmann, 1963. J. External view of a right valve, sample CM31B (MNHN.F.F63252).K. External view of a left valve, sample CM31B (MNHN.F.F63253). L. External view of a left valve, sample CM31B (MNHN.F.F63254). M. External view of a left valve, sample CM31C (MNHN.F.F63255). N. Lobobairdia? sp. 1, right lateral view of a carapace, sample CM31B (MNHN.F.F63256). O. Lobobairdia sp. 2, right lateral view of a carapace, sample CM31C (MNHN.F.F63257). P. Mirabairdia sp., external view of a left valve, sample CM31A (MNHN.F.F63258). Q–U. Petasobairdia amazonella Forel sp. nov. Q. Right lateral view of a carapace, sample CM31C (MNHN.F.F63259). R. Right lateral view of a carapace, sample CM31A (MNHN.F.F63260).S. Paratype, right lateral view of a carapace, sample CM31B (MNHN.F.F63261). T. Same specimen, dorsal view. U. Right lateral view of a carapace, sample CM31B (MNHN.F.F63262). 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.