Thyasira tanabei, Kiel & Amano & Jenkins, 2008

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 : 530-532

publication ID	https://doi.org/ 10.4202/app.2008.0310
persistent identifier	https://treatment.plazi.org/id/497287A1-F62B-7B24-9C3B-FB2AFA8BF45D
treatment provided by	Felipe
scientific name	Thyasira tanabei
status	sp. nov.

Treatment

Thyasira tanabei View in CoL sp. nov.

Figs. 5A–G View Fig , 6A View Fig .

1993 Conchocele ? sp.; Kanie et al. 1993: fig.2.

2008 Thyasirid bivalve; Kiel, Amano et al. in press: fig. 3L, M.

Etymology: For Prof. Kazushige Tanabe (The University of Tokyo), a specialist on ammonoids from Hokkaido, who also brought our attention to several cold seep deposits there.

Type material: Holotype ( UMUT MM 29533 : articulated specimen, length 10.4 mm, height 10.3 mm, width 6.1 mm) and paratypes ( UMUT MM 29534 , articulated specimen, length 10 mm, from the Kanajirisawa site; UMUT MM 29535 : articulated specimen, length 9.2 mm; UMUT MM 29536 : specimen with preserved shell material from the Omagari site, height 11.2 mm; UMUT MM 29537 : specimen with preserved hinge; UMUT MM 29539 : articulated specimen from the Ponbetsu site, length 7.1 mm).

Type locality: Yasukawa seep site in Nakagawa Town , Hokkaido .

Type horizon: Early Campanian, Omagari Formation, Yezo Group.

Material.— 29 specimens from the type locality, five from the Kanajirisawa seep site, nine from Ponbetsu River .

Diagnosis.—Medium−sized Thyasira with narrow posterior area and sharp 1 st posterior fold.

Description.—Shell moderate size attaining 12.9 mm in length, thin−walled, subcircular, equivalve and inequilateral. Convexity of shell variable, from moderately inflated ( Fig. 5A View Fig 1, G 1 View Fig ) to strongly inflated ( Fig. 5C View Fig 3 View Fig ). Beak projecting above dorsal margin, prosogyrate, situated at anterior one−third of shell length. Antero−dorsal margin concave, making right angle with ventral margin at anterior end; ventral margin well rounded. Posterior area narrow; posterior sulcus rather deep, but posterior sinus shallow; 1 st posterior fold elevated and sharp, rib−like; submarginal sulcus relatively shallow with very shallow marginal sinus; auricle narrow. Lunule lanceolate, concave, bounded by sharp ridge. Surface sculptured only by growth lines or irregular growth ridges in some specimens. In right valve, no evidence of cardinal tooth ( Fig. 5E View Fig ). Pallial line entire. Anterior muscle scar large, elongate ovate, weakly impressed, separated from pallial line ventrally ( Figs. 5A View Fig 2 View Fig , 6A View Fig ); posterior muscle scar small, subquadrate, in front of 2 nd posterior fold ( Figs. 5D View Fig , 6A View Fig ). Radiating interior striae distinct.

Discussion.— Kanie et al. (1993: fig. 2) illustrated a shell silhouette as Conchocele ? sp. from the Albian Ponbetsu seep site in Mikasa City, central Hokkaido. Their figure of Conchocele ? sp. does not show a posterior fold and an auricle. However, these features can be seen in several specimens that we collected at this site. Thus we assume that Kanie et al. (1993) overlooked these features and that their Conchocele ? sp. is in fact Thyasira tanabei described herein.

Thyasira tanabei View in CoL sp. nov. resembles Thyasira View in CoL sp. nov. described by Tashiro (2004) from the Albian to Cenomanian sediments of the Yezo Group in Horokanai Town in its general outline and size. But judging from the figure ( Tashiro 2004: pl. 1: 3, 4) the Horokanai specimen seems to lack a posterior fold and auricle. Thyasira tanabei View in CoL also resembles T. nakazawai Matsumoto, 1971 View in CoL from the early Miocene Wappazawa Formation in Shizuoka Prefecture, central Honshu, by having a sharp rib−like 1 st posterior fold. However, T. tanabei View in CoL reaches twice the size of T. nakazawai View in CoL (maximum length = 21.0 mm) and the wide posterior part of T. nakazawai View in CoL enables us to separate it from T. tanabei View in CoL . The thyasirids from the Campanian Western Interior Seaway of the USA are different from T. tanabei View in CoL by having a wider posterior area, a shallowly depressed lunule, and a posterior adductor scar astride the posterior sulcus ( Kauffman 1967). Thyasira tanabei View in CoL can be distinguished from Recent Thyasira species by its narrow posterior area, the posterior adductor scar in front of the posterior sulcus, and the sharp rib−like 1 st posterior fold.

Hikida et al. (2003) illustrated two fragmentary specimens from the Campanian Omagari seep site as Thyasira sp. Because the type locality and formation of the new T. tanabei View in CoL is the Omagari Formation near their site, these specimens may be identical with T. tanabei View in CoL , but better preserved material from the Omagari seep site is necessary to confirm this suggestion.

Measurements.—See Table 2. Stratigraphic and geographic range.—Albian to Campanian,

Yezo Group , Hokkaido Japan; so far found at the Ponbetsu site (Albian), the Kanajirisawa seep site (Cenomanian), and the type locality (Campanian, Omagari Formation), as well as associated with Coniacian sunken wood in the Nishichirashinai Formation (Kiel, Amano et al. in press) .