Retroceramus (Retroceramus) everesti (Oppel, 1863)

Fuersich, Franz T., Alberti, Matthias, Pandey, Dhirendra K. & Ayoub-Hannaa, Wagih S., 2022, Jurassic bivalves from the Spiti area of the Himalayas, northern India, Zitteliana 96, pp. 153-178 : 153

Retroceramus (Retroceramus) everesti (Oppel, 1863)

Plate 2, figs 3, 4

1831 Ostrea ?, Inoceramus ? - Herbert: 272, pl. 17, fig. 7.

Inoceramus ?1833 Inoceramus (?) - Everest: 114, pl. 2, fig. 29.

Inoceramus Everesti *1863 Inoceramus Everesti Opp. - Oppel: 298.

Inoceramus hookeri ?1865 Inoceramus Hookeri -N.Sp. var. Inoceramus crenatulinus - Salter: 95, pl. 23, fig. 2.

1913 Inoceramus Everesti , Oppel - Holdhaus: 415, pl. 98, figs 12-14.

1913 Inoceramus gracilis sp. nov. - Holdhaus: 417, pl. 98, fig. 15.

1982 Retroceramus (Retroceramus) everesti (Oppel, 1865) - Crame: 576: pl. 58, figs 5-11, pl. 59, figs 1-11.

Material.

Internal mould of a left valve, locality and horizon unknown (SNSB-BSPG 2020 XCIX 66), and two internal moulds of right valves from the upper member near Demul (SNSB-BSPG 2020 XCIX 64-65). All specimens are fragmentary.

Description.

Specimens small- to large-sized (H of largest specimen>70 mm), posteroventrally elongated but of variable obliquity, umbonal area well-inflated. Hinge line straight, moderately long; anterior margin weakly convex to straight, posterior margin gently curved, ventral margin well-rounded. Umbonal area narrowly rounded in cross-section, umbo small, prosogyrate. Posterodorsal area nearly flat; angle between dorsal and anterior margin approximately 90°. Valves ornamented with commarginal folds, asymmetric in cross-section with steeper dorsal flank. Folds regular to irregular, generally increasing in strength and distance towards ventral margin. Hinge features not seen.

Remarks.

The species has been transferred to the genus and subgenus Inoceramus Retroceramus by Crame (1982). Crame (1982) regarded Inoceramus hookeri Salter, 1865 and I. gracilis Holdhaus, 1913 as junior synonyms, a view followed here. The species does not only occur on the northern shelf of the Indian craton but also in western Australia, New Zealand, Antarctica, and southern Chile ( Crame 1982). According to Crame (1882), R. (R.) everesti has a stratigraphic range from the Upper Tithonian to Lower Berriasian. In the Spiti area, it occurs in the Upper Tithonian.

Holdhaus (1913) described four species of Inoceramus , of which only R. everesti (Oppel, 1863) and R. haasti ((Hochstetter, 1863) are recorded here. I. gracilis Holdhaus, 1913 is regarded synonymous with R. everesti , and I. stoliczkai Holdhaus and has been included in the genus Anopaea by Crame (1982). Inoceramus cf. sularum Boehm, 1907 of Holdhaus (1913: 420, fig. on p. 421) is too poorly preserved to be positively identified (see also Crame 1982).

Order Ostreida Férussac, 1822

Figures

Plate 2. (1, 2) Retroceramus (Retroceramus) haasti (Hochstetter, 1863). 1. Fragment of internal mould of single valve with remains of shell from the upper member, road cut close to Demul Pass (SNSB-BSPG 2020 XCIX 62 a). 2. Fragment of external mould of single valve with remains of shell from the upper member, road cut close to Demul Pass (SNSB-BSPG 2020 XCIX 62 b). (3, 4) Retroceramus (Retroceramus) everesti (Oppel, 1863). Fragmented internal moulds from the upper member near Demul. 3. Right valve (SNSB-BSPG 2020 XCIX 65). 4. Right valve (SNSB-BSPG 2020 XCIX 64). (5, 6) Gryphaeidae indet. from the upper member close to the Demul Pass 5. Right valve (SNSB-BSPG 2020 XCIX 37). 6. Right valve (SNSB-BSPG 2020 XCIX 38).