Palaeonucula cuneiformis (J. de C. Sowerby, 1840)

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

Palaeonucula cuneiformis (J. de C. Sowerby, 1840)

Plate 1, figs 1-2

1831 Modiola - Herbert: 272, pl. 17, fig. 5.

1833 Modiola - Everest: 114: pl. 2, fig. 28a-c.

*1840 Nucula? cuneiformis - J. de C. Sowerby: pl. 22, fig. 4 and explanation.

1864 Nucula cuneiformis , Sowerby - Blanford: 135.

1908 Nucula cuneiformis , J. de C. Sowerby - Newton and Crick: 7, pl. 1, figs 5-7.

1913 Nucula spitiensis sp. nov. - Holdhaus: 428, pl. 95, figs 11-13.

1913 Nucula hyomorpha sp. nov. - Holdhaus: 430, pl. 95, figs 14-17.

1929 Nucula cuneiformis Newton - Weir: 5, pl. 4, figs 2-4.

?1930 Nucula cuneiformis Newton - Basse: 108. pl. 5, fig. 5.

?1939 Nucula cuneiformis Sow. - Stefanini: 219, pl. 24, fig. 3.

1940 Nucula (Palaeonucula) cuneiformis J. de C. Sowerby - Cox: 13, pl. 1, figs 5-10.

1940 Nucula (Palaeonucula) kaoraensis sp. nov. - Cox: 15, pl. 1, figs 11-14.

1956 Nucula (Palaeonucula) kaoraensis Cox - Agrawal: 51, pl. 7, fig. 3a.

1959 Nucula cuneiformis Sowerby - Jaboli: 46, pl. 6, fig. 3.

1980 Palaeonucula kaoraensis Cox - Kanjilal: 335, pl. 1, figs 8-10.

1980 Palaeonucula cuneiformis (J. de C. Sowerby) - Kanjilal: 334, pl. 1, figs 4-7.

1995 Palaeonucula cuneiformis (J. de C. Sowerby 1840) - Jaitly et al.: 155, pl. 1, figs 9-11, 13-17, text-figs 6-9 (pars). [non pl. 1, figs 8, 12, pl. 2, figs 1-2]

1998 Palaeonucula cuneiformis (J. de C. Sowerby 1840) - Kanjilal and Pathak: 30, pl. 1, fig. 1.

Material.

Nine articulated specimens and two left valves from the lower member at Langza (SNSB-BSPG 2020 XCIX 26), 30 articulated specimens, one right and two left valves from the lower member along the Kaza - Hikkim road (SNSB-BSPG 2020 XCIX 27), four articulated specimens from the lower member at Langza (road side exposure) (SNSB-BSPG 2020 XCIX 28), four articulated specimens from the lower member close to the pass to Tashigeng (SNSB-BSPG 2020 XCIX 29), three articulated specimens and one right valve from the lower member near Kibber (SNSB-BSPG 2020 XCIX 30), two articulated specimens from the Ferruginous Oolite Formation near Chichim (SNSB-BSPG 2020 XCIX 31), and three articulated specimens and one right and three left valves from the lower member at Langza locality 3 (SNSB-BSPG 2020 XCIX 32). The shells, several of which are fragmented, invariably suffered compactional and tectonic distortion.

Description.

Shell relatively large for genus (H: ~26 mm, L: 32.2 mm), thick-shelled, elongated-oval, strongly inequilateral, moderately inflated. Umbo broad, well-developed, terminal to subterminal, close to posterior end, opisthogyrate. Anterior end narrow to well-rounded, posterior end straight to slightly curved forming a blunt angle with the ventral margin, which describes a wide asymmetric curve with the ventral-most point of shell well anterior of mid-line. Anterodorsal margin straight, sloping; posterodorsal margin short, almost straight to slightly curved, passing gradually into posterior margin. Broad rounded ridge running from the umbo to the anterodorsal end, separating flank from flat, narrow anterodorsal part of shell. Lunule shallow, narrow, lanceolate. Second, equally rounded umbonal ridge running to the posteroventral corner of shell. Area heart-shaped slightly concave in articulated specimens. Surface of shell smooth except for growth lines, which vary in strength. Hinge poorly preserved, with characteristic taxodont teeth.

Remarks.

As nearly all specimens are crushed to some extent, the original outline and inflation only rarely can be observed. Everest (1833), Blanford (1864), and Stoliczka (1866) regarded the common nuculid in the lower member of the Spiti Shale Formation as the species figured by J. de C. Sowerby (1840) as Nucula? cuneiformis , a highly variable species occurring from the Bathonian to the Kimmeridgian in the Kachchh Basin. Subsequent workers, however, followed a much narrower species concept and regarded the forms from the Spiti Shale Formation as different species. Holdhaus (1913) placed the bivalves in two new species, Nucula spitiensis and N. hyomorpha . He realised that the specimens were generally distorted, but argued that N. hyomorpha was always dorso-ventrally compressed, whereas N. spitiensis was laterally compressed. In this way, he clearly related the diagnostic features of the two species to their preservational state. According to Holdhaus (1913), the two species also differ in the shape of their anterodorsal shell portion, which is, however, influenced by the compaction styles. In the case of laterally flattened shells, this part usually cannot be observed. In fact, the style of distortion depends on the final burial position of the shells; they either became dorso-ventrally shortened when buried in growth position, or laterally flattened when post-humously they were reoriented by burrowers or excavated by currents, post-mortem. All intermediate preservational stages occur. In the former case, the inflation was artificially increased and in the latter case the length-height ration was decreased. All intermediate preservational stages occur. Cox (1940) regarded N. spitiensis and N. hyomorpha as synonyms and as a "linear descendent of P. cuneiformis . He followed a narrow species concept recognising two further similar species, P. kaoraensis and P. blanfordi in the Kachchh Basin, which were regarded junior synonyms of P. cuneiformis by Pandey and Agrawal (1984) and Jaitly et al. (1995).

The species is widespread in the Ethiopian faunal province, occurring from Madagascar to the Arabian Peninsula and from the Kachchh Basin to Rajasthan and the Himalayan shelf of the Indian Craton.

Figures

Plate 1. (1, 2) Palaeonucula cuneiformis (J. de C. Sowerby, 1840). 1. Left valve from the lower member at Langza (SNSB-BSPG 2020 XCIX 26 a). 2. Left valve from the lower member at Langza (SNSB-BSPG 2020 XCIX 26 b). (3) Palaeonucula stoliczkai Cox, 1940. Laterally crushed articulated specimen, lower member at Langza (SNSB-BSPG 2020 XCIX 34); a, left valve view; b, right valve view. (4) Megacucullaea cf. kraussii (Tate, 1867). Internal mould of articulated specimen with remains of shell from the lower member near Chichim (SNSB-BSPG 2020 XCIX 1); a, right valve view; b, dorsal view. (5 - 7) Grammatodon (Indogrammatodon) egertonianus (Stoliczka, 1866). 5. Articulated specimen from the lower member near the pass to Tashigeng (SNSB-BSPG 2020 XCIX 4). a, left valve view; b, dorsal view. 6. Laterally crushed articulated specimen from the lower member along the Kaza-Hikkim road (SNSB-BSPG 2020 XCIX 3 b); right valve view. 7. Dorso-ventrally compacted articulated specimen from the lower member at Langza (SNSB-BSPG 2020 XCIX 6); right valve view. (8, 9) Grammatodon (Cosmetodon) sp. from the lower member along the Kaza-Hikkim road. 8. Right valve (SNSB-BSPG 2020 XCIX 2 a). 9. Right valve (SNSB-BSPG 2020 XCIX 2 b). a, lateral view; b, dorsal view. (10) Anopaea? Fragmented right valve from the upper? member at Langza (SNSB-BSPG 2020 XCIX 67).