Megacucullaea cf. kraussii (Tate, 1867)

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

Megacucullaea cf. kraussii (Tate, 1867)

Plate 1, fig. 4a, b

cf. 1850 Cucullaea cancellata - Krauss: 452, pl. 48, fig. 2a, b (non Phillips, 1829)

*cf. 1867 Cucullaea kraussii - Tate: 161.

cf. 1882 Cucullaea Kraussi Tate - Holub and Neumayr: 375, pl. 2, fig. 2a-c.

cf. 1940 Cucullaea (Megacucullaea) kraussii Tate - Cox: 57, pl. 4, figs 3-4.

cf. 1998 Megacucullaea kraussi (Tate, 1867) - Kanjilal and Pathak: 34, pl. 1, fig. 6.

Material.

A single poorly preserved articulated internal mould with remains of shell from the lower member of the Spiti Shale Formation (Oxfordian) near Chichim (SNSB- BSPG 2020 XCIX 1).

Description.

Specimen large, longer than high (L ~79 mm, H ~60 mm, I ~70), thick-shelled, strongly inflated. Maximum inflation at around one-third of shell height from umbo. Ventral margin faintly arched, anterior margin convex, forming an angle of ~90° with hinge margin. Posterior margin straight, oblique, posteroventral corner rounded. Umbones only partly preserved, prominent, straight, incurved, situated anterior of mid-line of shell. Posterodorsal carina distinct but rounded, posterodorsal area flat to slightly convex. Cardinal area large, largest width anterior of umbo, slightly concave. Ornamentation poorly preserved; there are at least four widely spaced, strong, rounded radial ribs with concave interstices. Hinge line straight, dentition and other internal features not seen.

Remarks.

The two valves are sligthly laterally sheared. Moreover, the specimen may be somewhat compressed dorso-ventrally. In this case, the inflation would be lower and the height greater than decribed above, and the point of maximum inflation would also be slightly off. The umbones are only partly preserved.

The various Jurassic species of Megacucullaea , i.e. M. eminens Cox (1940, p. 59, pl. 4, figs 5-7), M. kraussii ( Tate 1867, p. 161), and " M. sp. nov. aff. C. (M.) eminens Cox" of Agrawal (1956 p. 14, pl. 1, fig. 1) differ in outline and the number of radial ribs ( M. kraussii : 6-8; M. eminens : 16-19; Agrawal’s specimen: approximately 10). The present specimen is closest to M. kraussii , having a similar number of ribs, whereas in the other two species the number of ribs is distinctly higher. Due to the poor preservation of the specimen, a detailed comparison with M. kraussii is not possible. The material of Cox (1940, p. 57, pl. 4, figs 3-4) exhibits eight radial ribs, of which three are only faintly developed on one out of two specimens. Interestingly, M. eminens and M. kraussii co-occur in Upper Tithonian rocks of the Kachchh Basin. M. eminens Cox (1940, p. 59, pl. 4, figs 5-7) is shorter, carries about 16 radial ribs differing in strength (weak close to the umbonal carina and strong on the remainder of the shell), and the area posterior of the umbonal ridge is distinctly concave. " Cucullaea Kraussi Tate" of Barrabé (1929, p. 147, pl. 8, fig. 13) has more radial ribs and resembles M. eminens . The early Tithonian specimen figured as Megacucullaea kraussii by Kanjilal and Pathak (1998) has seven ribs.

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).