Histriabairdia pontuseuxinusensis 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 : 35-37

Histriabairdia pontuseuxinusensis Forel gen. et sp. nov.

urn:lsid:zoobank.org:act:2A9EE0A7-227E-41AC-840C-3F3F93A26790

Fig. 8 G–O

Pustulobairdia subparallela – Harloff & Jäger 1994: pl. 2, figs 7–8. [not Kristan-Tollmann, 1970]

Diagnosis

Dorsal margin tripartite and lateral surface spinose except on the smooth, laterally compressed anteroventral and postero-ventral surfaces, anterior margin subvertical.

Etymology

From Pontus Euxinus , one of the Latin names for the Black Sea, where borehole 817LV was drilled.

Material examined

Holotype

ROMANIA • 1 complete carapace; Black Sea , Romanian Continental Shelf , borehole 817LV, sample CM31B; Rhaetian, Upper Triassic; MNHN.F. F63238 .

Paratype

ROMANIA • 1 LV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63239 .

Other material

ROMANIA • 1 LV; same locality as for holotype but sample CM31C; Rhaetian, Upper Triassic; MNHN.F. F63233 • 1 LV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63237 • 1 complete carapace; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63234 • 1 complete carapace; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63236 • 1 RV; same collection data as for preceding; Rhaetian, Upper Triassic; MNHN.F. F63235 • 5 complete carapaces; same locality as for preceding but samples CM31A, CM31B and CM31C; Rhaetian, Upper Triassic; MNHN.F. F63351 .

Dimensions

See Fig. 3E .

Description

Carapace subrectangular, large, inflated laterally except along compressed antero-ventral and posteroventral margins; carapace preplete with H max located at antero-dorsal angulation; L max located below mid-height; dorsal margin long and tripartite at both valves; LV larger than RV, overlapping it along dorsal margin with maximum along DB and ventral margin with maximum at oral concavity; DB long (43–55 % of L max), straight at RV and straight to slightly convex at LV, slightly inclined posteriorly (between 20° in small specimens and 5° in largest forms); ADB shorter (34–43 % of L max), concave at both valves, more strongly in adults that in younger instars; PDB short (8–20 % of L max), steeply bent posteriorly (from 60° to close to vertical in smallest instars) with terminal concavity more clearly expressed in adults; antero-dorsal and postero-dorsal angulations rounded; AB large and subvertical, with apex located in upper third of H max; AVB underlined by series of marginal denticles on both valves and bordered by a laterally compressed area; PB narrower, with maximum of convexity in lower third of H max on both valves, underlined by a narrow laterally compressed zone; 9 to 10 marginal denticles aligned along PVB, one located above PB; ventral margin long and sinuous with oral concavity located in front of mid-L, bordered by a laterally compressed zone that extends from anterior maximum of convexity to posterior one, attenuated at oral concavity; laterally inflated surface covered with spinose pustules arranged concentrically parallel to free margins except on smooth compressed zones along antero-ventral and postero-ventral margins; antero-ventral and postero-ventral margins bordered by row of sharp spines, length of which decreases ventrally, their number increasing through ontogeny; in inner view, calcified lamella large along anterior and posterior margins, weaker along oral concavity, vestibulum large; central muscle scar not visible; hinge not visible and bairdoppilate auxiliary structure absent.

Occurrence

Tyrol, Austria, Hettangian, Early Jurassic ( Harloff & Jäger 1994); Romanian Continental Shelf, Black Sea, Rhaetian, Upper Triassic (this paper).

Remarks

The short spines across the lateral surface of Histriabairdia pontuseuxinusensis gen. et sp. nov. are reminiscent of those in the Palaeozoic genus Pustulobairdia Sohn, 1960 . Pustulobairdia subparallella Kristan-Tollmann, 1970 was described from the Ladinian, Middle Triassic, of the Dolomites, Italy ( Kristan-Tollmann 1970) but lacks AD angulation, so that the DB and ADB are in line. On the contrary, specimens attributed to P. subparallella from the Hettangian, Early Jurassic, of Tyrol ( Harloff & Jäger 1994) have a well-expressed AD angulation on both valves, also expressed at young stages, similarly to in the Black Sea material. Therefore, this Jurassic material is not conspecific with P. subparallella ,which is restricted to the Ladinian of the Dolomites. Conversely, specimens from the Hettangian are attributed to Histriabairdia pontuseuxinusensis gen. et sp. nov. The surface ornamentation of H. pontuseuxinusensis gen. et sp. nov. is reminiscent of that in Anchistrocheles sp. 1 from the Carnian of Turkey ( Forel et al. 2019a), but this Carnian species is significantly smaller (L = 463 µm, H = 275 µm) and shorter (H/L = 0.6), with smaller and more abundant pustules covering the entire lateral surface. The very narrow calcified inner lamella of the only valve (LV) from the Carnian of Turkey implies that the specimen is a relatively young instar. Histriabairdia spinosa (Sheppard in Brand, 1990) gen. et comb. nov. from the Bathonian, Middle Jurassic, of France ( Brand 1990) is also defined by its pustulose surface, but it differs from H. pontuseuxinusensis gen. et sp. nov. by having much smaller spines.

The large distribution of the H and L scatter plot indicates the co-occurrence of several ontogenetic stages for Histriabairdia pontuseuxinusensis gen. et sp. nov. ( Fig. 3E ). However, the rarity of specimens of intermediate dimensions does not reasonably allow a discrimination of the stages. The holotype ( Fig. 8 L–M) and the paratype ( Fig. 8 N–O) are among the largest specimens and are thus considered as adults. The ontogenetic development of H. pontuseuxinusensis gen. et sp. nov. is marked by the progressive strengthening of the lateral spines, densification of the surface coverage, widening of the lateral compressions along AVB and PVB, an increase in the number of marginal spines and an upward migration of the PB. Jurassic specimens have a slightly higher H/L ratio for both valves ( Fig. 3E ), which may relate to distinct environmental conditions.

Figures

Fig. 8 (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. Fabalicypris cf. triassica Bolz, 1971, external view of a left valve, sample CM31A (MNHN.F.F63228). B. Hiatobairdia sp. 1, right lateral view of a carapace, sample CM31B (MNHN.F.F63229). C–E. Hiatobairdia sp. 2. C. Right lateral view of a carapace, sample CM31B (MNHN.F.F63230). D. Same specimen, dorsal view. E. External view of a left valve, sample CM31B (MNHN.F.F63231). F. Hiatobairdia sp. 3, right lateral view of a carapace, sample CM31B (MNHN.F.F63232).G–O. Histriabairdia pontuseuxinusensis Forel gen. et sp. nov. G. External view of a left valve, sample CM31C (MNHN.F.F63233). H. Right lateral view of a complete carapace, sample CM31C (MNHN.F.F63234). I. External view of a right valve, sample CM31C (MNHN.F.F63235). J. Right lateral view of a complete carapace, sample CM31C (MNHN.F.F63236). K. External view of a left valve, sample CM31C (MNHN.F.F63237). L. Holotype, right lateral view of a complete carapace, sample CM31B (MNHN.F.F63238). M. Same specimen, dorsal view. N. Paratype, external view of a left valve, sample CM31B (MNHN.F.F63239). O. Same specimen, inner view. P–R. Histriabairdia gen nov. sp. 1. P. External view of a right valve, sample CM31C (MNHN.F.F63240). Q. Right lateral view of a complete carapace, sample CM31C (MNHN.F.F63241). R. Same specimen, dorsal view. S–U. Histriabairdia gen. nov. sp. 2. S. Right lateral view of a carapace broken anteriorly and posteriorly, sample CM31C (MNHN.F.F63242). T. Right lateral view of a carapace broken anteriorly and posteriorly, sample CM31C (MNHN.F.F63243). U. External view of a broken right valve, sample CM31C (MNHN.F.F63244). 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.