Conchocele townsendi ( White, 1890 )

Hryniewicz, Krzysztof, Amano, Kazutaka, Jenkins, Robert G. & Kiel, Steffen, 2017, Thyasirid bivalves from Cretaceous and Paleogene cold seeps, Acta Palaeontologica Polonica 62 (4), pp. 705-728 : 709-710

Conchocele townsendi ( White, 1890)

Fig. 4 .

1890 Lucina? townsendi sp. nov.; White 1890: 14, pl. 3: 1, 2. 1903 Lucina? townsendi White, 1890 ; Weller 1903: 67, pl. 11: 2, 3. 1910 Thyasira townsendi ( White, 1890) ; Wilckens 1910: 53–56, pl. 2:

31, 32, pl. 3: 1. 1988 Thyasira townsendi (White, 1990) (sic!): Zinsmeister and Macellari 1988: 273, 276, figs. 9.7, 9.8. 2008 “ Thyasira ” townsendi ( White, 1890) ; Kiel et al. 2008: 535. 2015 “ Thyasira ” townsendi ( White, 1890) ; Little et al. 2015: fig. 5G–I.

Type material: Holotype unknown; hypotypes NRM Mo 1560; USNM 405773 ( Zinsmeister and Macellari 1988).

Type locality: Saint Paul’s and Saint Peter’s islands, Magellan Strait .

Type horizon: Cretaceous (details unknown).

Material.— Two specimens ( NRM Mo 1552, 1560) from Maastrichtian cold seep carbonates of Seymour Island, Antarctica .

Remarks.—The species was initially described as a possible species of Lucina by White (1890) from St. Paul’s and St. Peter’s islands in the Straits of Magellan, both of which we could not locate. It was thereafter noted from Antarctic strata by Weller (1903), who mentioned it from outcrops of the Admiralty Sound area, Weddell Sea. The first precise locality information comes from the work of Wilckens (1910), who mentions numerous occurrences on Snow Hill and Seymour Islands, western Weddell Sea. Wilckens (1910) formally transferred the species to Thyasira . This interpretation was followed in a description of Zinsmeister and Macellari (1988), who noticed that the species is similar to Conchocele disjuncta Gabb, 1869 , from the Pliocene of California. However, they considered a closer relationship between the two species unlikely due to the significant temporal and spatial distance between them.

Doubts about including “ Thyasira ” townsendi in Thyasira were expressed by Kiel et al. (2008) and Little et al. (2015). Here we formally transfer the species to Conchocele based on the beaks located close to the anterior of the shell, a steeply sloping anterior margin bound by a keel, the lack of a distinct submarginal and deep posterior sulcus, and a posterior margin indented by a deep sinus. This is the only Cretaceous thyasirid which we can confidently place in the genus Conchocele . The Coniacian “ Thyasira ” cretacea Whiteaves, 1874, from Enos Canyon, California ( Anderson 1958) is not figured well enough for a more detailed comparison. The same applies to “ Thyasira ” cretacea from the Late Cretaceous of Vancouver Island ( Whiteaves 1874), and to “ Thyasira ” collignoni from Campanian of New Caledonia Freneix 1980), the figured specimens of which lack a well preserved posterior margin. “ Aphrodina ” hataii Katto and Hattori, 1965, from the Campanian–Maastrichtian Sada Limestone seep deposit in Shikoku, Japan ( Nobuhara et al. 2008) has a deep posterior sulcus but lacks a characteristic posterior sinus (KH personal observation) as well as a truncated anterior margin and should not be included in Conchocele .

Stratigraphic and geographic range. —Maastrichtian of the James Ross Basin and possibly the Magellan Strait area.

Figures

Fig. 4. Thyasirid bivalve Conchocele townsendi (White, 1890) from Maastrichtian cold seep carbonates of Seymour Island, James Ross Basin, Antarctica. A. NRM Mo 1560; a complete shell in right (A1) and left (A2) lateral view, showing outline and fine commarginal ornament; in anterior view (A3), showing two ridges running from the umbo towards the anteroventral angle; in dorsal view (A4), showing narrow and sharp posterior sulcus and posterior fold and weak furrow possibly representing accessorial ligament attachment surface. B. NRM Mo 1552; an internal mold in left lateral view with fragments of the shell adhering to the anterior and umbonal area (B1) and with outline of the anterior adductor muscle scar visible (B2).