Modiolidae Termier & Termier, 1950

Family Modiolidae Termier & Termier, 1950 View in CoL Genus Gigantidas Cosel & Marshall, 2003

Type species: Gigantidas gladius Cosel & Marshall, 2003 , by original designation; Recent, SW Pacific, off North Island , New Zealand, cold seeps and active hydrothermal vent sites .

Gigantidas cf. platifrons ( Hashimoto & Okutani, 1994) View in CoL

Fig. 15 View Fig .

Material.— One adult valve, SMF373100 About SMF ; one adult valve, SMF373094 About SMF ; one juvenile shell, SMF373099 About SMF ; one juvenile valve, SMF373096 About SMF ; two juvenile valves, SMF373098 About SMF ; one juvenile valve, SMF373101 About SMF ; one shell and three valves (all juveniles), SMF373097. Northern Indian Ocean, Krishna-Godavari Basin , Bay of Bengal, offshore eastern India, 15°51.6335’ N, 81°50.2274’ E, 1046 m water depth, 16.18– 16.78 mbsf level, core NGHP-01-12A; Upper Pleistocene. For additional material see Table 1. A single, small poorly preserved valve SMF373095 About SMF GoogleMaps from 42.01 mbsf level, core NGHP-01-10D may belong to this species.

Description.—Large bathymodiolid shell, piriform. Prodissconch, ovate with straightened dorsal margin and inflated umbo, length 470 μm. Dissoconch piriform with slightly raised umbo near anterior end on dorsal margin. Posterodorsal margin concave near umbo, convex posteriorly, straight ventral margin, maximum convexity at anterior end. Sculpture fine irregular growth lines. Maximum length 52 mm.

Remarks.—Our species is close in shape to Gigantidas platifrons Hashimoto & Okutani, 1994 , which was described from chemosynthetic environments off Japan. However, the umbones in the Japanese specimens are at the anterior end ( Hashimoto and Okutani 1994), whereas our shells have umbones protruding above the anterior-dorsal end. Gigantidas niobengalensis Oliver et al., 2024 , has been described from an active cold seep in the Krishna-Godavari Basin, depth 1750 m. This species has its umbo located anteriorly on the dorsal margin and has a convex anterior margin and a straightened posterior-dorsal margin ( Oliver et al. 2024: holotype figs. 3A–C). Our species has a terminal umbo at the pointed anterior end and its posterior-dorsal margin is convex. For these reasons we think our fossil specimens do not belong to Gigantidas niobengalensis , although we note there is quite a lot of morphological variation in bathymodiolin species. The type species of Gigantidas has large narrowly elongated shells with protruding umbones on the dorsal margin ( Cosel and Marshall 2003). Currently, the genus contains 11 species (MolluscaBase 2024) with a wide morphological variation; ranging from typical modioliform shape towards the elongated shape of the type species. The juvenile shell ( Fig 13C View Fig 1 View Fig ) of the fossil Gigantidas cf. platifrons specimens has a typical modiolid outline and the large prodissoconch (length 470 μm) confiming a planktonic stage.

Order Pectinida Gray, 1854 View in CoL

Superfamily Pectinoidea Rafinesque, 1815 View in CoL

Family Propeamussiidae Abbott, 1954 View in CoL

Genus Parvamussium Sacco, 1897 View in CoL

Type species: Pecten duodecimlamellatus Bronn, 1831 , by original designation; Cenozoic , Italy .