Acila (Truncacila) himenourensis Tashiro, 1992

Kiel, Steffen, Amano, Kazutaka & Jenkins, Robert G., 2008, Bivalves from Cretaceous cold-seep deposits on Hokkaido, Japan, Acta Palaeontologica Polonica 53 (3), pp. 525-537 : 530

Acila (Truncacila) himenourensis Tashiro, 1992

Fig. 4B .

1958 Acila (Truncacila) hokkaidoensis (Nagao) ; Ichikawa and Maeda 1958: 79–80, pl. 3: 9–11, 14.

1962 Acila (Truncacila) hokkaidoensis (Nagao) ; Saito 1962: 59–60, pl. 1: 1.

1976 Acila (Truncacila) hokkaidoensis (Nagao) ; Tashiro 1976: pl. 1: 12–16.

1985 Acila (Truncacila) himenourensis sp. nov. (in MS); Tashiro 1985: 56.

1992 Acila (Truncacila) himenourensis Tashiro ; Tashiro 1992: 26, pl. 3: 4.

2007 Acila hokkaidoensis Nagao ; Tsujino and Maeda 2007: fig. 6C, D.

Material.— One specimen from the Kanajirisawa seep site,

three from the Yasukawa seep site (figured is UMUT MM

29532). Original description.—“Shell ovate with short posterior margin and strongly arched ventral margin. Ribs on surface fine, but strong. Lines connecting tops of v−shaped ribs gently sloping anteriorly, attaining to slightly anterior part to center of ventral margin.” (translated from the Japanese by KA).

Discussion.— Acila (Truncacila) himenourensis has a narrower umbonal angle than A. (T.) hokkaidoensis , and the line from which the radial ribs diverge is not as far anterior as in the specimen of A. (T.) hokkaidoensis figured by Nagao (1932: pl. 5: 17, 18). Acila ( T.) picturata Yokoyama, 1890 from the late Eocene Poronai Formation differs from A. (T.) himenourensis by having a broader apical angle and finer, and more numerous ribs ( Yokoyama 1890; own observations). Also Acila brevis Nagao and Huzioka, 1941 from the Miocene of Hokkaido has a broader apical angle and finer ribs ( Kanno and Ogawa 1964). A similar species from the North American Pacific Coast Cretaceous is A. ( T.) grahami Squires and Saul, 2006, which has usually a more elongate shape and thicker ribs with narrower interspaces than A. (T.) himenourensis ( Squires and Saul 2006: 95, figs. 27–38).

Stratigraphic and geographic range.— Japan; Cenomanian to Campanian of the Yezo Group, Hokkaido; Coniacian to Santonian of the Futaba Group, northeast Honshu; Campanian part of the Izumi Group, Kinki District of Honshu; Coniacian to Maastrichtian of the Himenoura Group, western Kyushu.

Figures

Fig. 4. Acila (Truncacila) from the Campanian Yasukawa seep site on Hokkaido, Japan. A. Acila (Truncacila) hokkaidoensis Nagao, 1932 (UMUT MM 29531), lateral view on left valve (A1) and dorsal view (A2). B. Acila (Truncacila) himenourensis Tashiro, 1985 (UMUT MM 29532), lateral view on right valve (B1), dorsal view showing escutcheon (B2), and dorsal view showing lunule (B3).