Grammatodon (Indogrammatodon) egertonianus (Stoliczka, 1866)

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

Grammatodon (Indogrammatodon) egertonianus (Stoliczka, 1866)

Plate 1 , figs 5-7; Fig. 3

1831 Arca sp. - Herbert: 272, pl. 17, fig. 6.

1833 Arca sp. - Everest: 114, pl. 2, fig. 27.

1864 Cucullaea virgata - Blanford: 136 (non J. de C. Sowerby, 1840).

Macrodon egertonianum *1866 Macrodon Egertonianum Stol. - Stoliczka: 89, pl. 8, fig. 7.

non 1908 Parallelodon egertonianus , Stoliczka - Newton and Crick: 5, pl. 1, figs 1-4.

1913 Arca (Cucullaea) egertoniana , Stoliczka - Holdhaus: 434, pl. 95, figs 1-10.

non 1935 Parallelodon egertonianus (Stoliczka) - Cox: 160.

non 1937 Grammatodon (Indogrammatodon) egertonianus (Stoliczka) - Cox: 196, pl. 15, figs 10-11. (= G. (I.) virgatus J. de C. Sowerby, 1840)

non 1939 Grammatodon egertonianus (Stol.) - Stefanini: 222, pl. 24, figs 7-10. (= G. (I.) virgatus J. de C. Sowerby, 1840)

non 1959 Grammatodon (Indogrammatodon) egertonianus (Stoliczka) - Jaboli: 48, pl. 5, fig. 12.

non 1976 Cucullaea egertoniana (Stoliczka) - Gu et al.:125, pl. 45, figs 1-6.

non 1983 Grammatodon (Indogrammatodon) egertonianus (Stoliczka) - Gou and Li: pl. 2, fig. 6.

non 1997 Grammatodon (Indogrammatodon) egertonianus (Stoliczka) - Gardner and Campbell: 495, fig. 5I.

1998 Indogrammatodon egertonianus (Stoliczka, 1865) - Kanjilal and Pathak: 34, pl. 1, fig. 3.

Material.

Six right and four left valves and 18 articulated specimens in shell preservation from the lower member of the Spiti Shale Formation at Langza (SNSB-BSPG 2020 XCIX 5, 6, 8, 9), Kazam-Hikkim road (SNSB-BSPG 2020 XCIX 3), Kibber (SNSB-BSPG 2020 XCIX 7), and near the pass to Tashigeng (SNSB-BSPG 2020 XCIX 4). The specimens are invariably distorted due to compaction and partly incomplete.

Dimension.

See Table 2 .

Description.

Shell rhomboidal to trapezoidal in outline, much longer than high (Fig. 3 ). Ventral margin straight to slightly sinuous, grading smoothly into curved anterior margin, which forms an angle of 80-90° with the hinge line. Posterior margin convex ventrally and straight, oblique dorsally, forming an angle of ~130° with hinge line. Hinge line straight. Posterodorsal carina pronounced near umbo, becoming rounded and indistinct towards posteroventral margin. Area posterior of umbonal ridge concave. Umbo situated one-third to two-fifths of hinge length from anterior end, forward directed and incurved. Ornamentation of right valve slightly differing from that of left valve. Number of radial ribs of right valve (16-19) larger than that of left valves (12-17). Radial ribs present only on flank and not posterior of the umbonal ridge, equally spaced except for the anteriormost part of shell where their distance is much wider. Rib density greatest at middle of flank. Ribs of left valve of equal strength, slightly stronger than those of right valve. No intercalatory ribs present. Ribs of right valve widely spaced, with intercalations of 1-2 weak secondaries towards posterior. Both valves with distinct commarginal growth rugae; at intersections of growth rugae and radial ribs small nodes are developed.

Ligament area long, relatively narrow, with numerous chevron-shaped grooves. Except for fragments of hinge no internal features seen. Hinge with 3-4 moderately long anterior teeth, arranged parallel to hinge margin anteriorly, converging towards ventral margin of hinge plate posteriorly. Posterior teeth longer, subparallel to dorsal margin of hinge. All teeth serrated. short central teeth not seen.

Remarks.

Nearly all specimens are distorted by compaction. Commonly, the dorso-ventral axis has been shortened suggesting that the individuals are preserved in growth position. Due to this distortion, the length-height ratio could not be calculated and the inflation could not be measured. G. (I.) egertonianus is closely related to G. (I.) virgatus (J. de C. Sowerby, 1840), which is widespread within the Ethiopian faunal province. It differs from the present species by being less elongated, having far more radial ribs on both valves, and the difference in ribbing between right and left valve is much more pronounced (see, for example, the specimens figured by Cox 1940, pl. 2, figs 22-30 from the Jurassic of Kachchh). These differences have already been noted by Stoliczka (1865). The specimens recorded as Parallelodon egertonianus Stol. by Basse (1930, p. 109, fig. 3a, b) from Ethiopia, Newton and Crick (1908, p. 5, pl. 1, figs 1-4) from the Arabian Peninsula, Stefanini (1925, p. 171, pl. 31, figs 6-7) from Somalia, and Weir (1930, p. 81, pl. 9, figs 12, 12a) from Kenya are generally much shorter and more densely ribbed and belong to G. (I.) virgatus or related forms. This is probably also true of the material listed by Cox (1935) from the Kimmeridgian-Tithonian of Somalia The specimen of P. egertonianus Stoliczka figured by Trechmann (1923, p. 263, pl. 14, fig. 8) from the Jurassic of New Zealand is too poorly preserved to warrant a precise identification.

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).

Figure 3. Length plotted against height of Grammatodon (Indogrammatodon) virgatus (J. de C. Sowerby, 1840) from the Callovian of Kachchh and G. (I.) egertonianus (Stoliczka, 1866) from the Spiti Shale Formation. Due to the distorted nature of nearly all specimens from the Spiti area, only two specimens yielded reasonable values.