Retroceramus (Retroceramus) haasti (Hochstetter, 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) haasti (Hochstetter, 1863)

Plate 2, figs 1, 2

1863 Inoceramus haasti sp. nov. - Hochstetter: 473, pl. 20, figs 1-2.

1982 Retroceramus (Retroceramus) haasti (Hochstetter, 1863) - Crame: 569, pl. 57, figs 1-4.

Material.

Numerous fragments of internal moulds of single valves with remains of shell from the upper member, road cut close to Demul Pass (SNSB-BSPG 2020 XCIX 60-63).

Description and remarks.

The specimens occur in loosely packed matrix-supported shell concentrations. The umbonal areas are not preserved, but the specimens are apparently quite large, the largest fragment measuring 135 mm in height. Preserved parts of moulds are more or less flat, with spaced commarginal folds, the distance between folds increasing ventrally. The folds are slightly asymmetric in cross-section, the ventral flank being steeper. Areas with preserved shell material show that the shells were very thin.

Despite the fragmentary nature of the material, the specimens can be safely referred to Retroceramus (R.) haasti , originally described by Hochstetter (1863) from Kimmeridgian strata of New Zealand, based on their large size, nearly flat shells, broad ventral area, and characteristic ornamentation. The species has been recorded in the past from the Kimmeridgian of New Zealand and Antarctica ( Hochstetter 1863; Crame 1982; Crampton 1988).

Inoceramus hookeri of Salter (1865: 95, pl. 23, fig. 1 only) exhibits a similar ornamentation but differs in outline being rounded-rectangular. Inoceramus haasti , Hochstetter, 1863 has been transferred to Retroceramus (Retroceramus) by Crame (1982). The latter author regarded the species as characteristic of the Kimmeridgian - Early Tithonian.

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).