Thyasira tanabei Kiel, Amano, and Jenkins, 2008

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 : 720-721

Thyasira tanabei Kiel, Amano, and Jenkins, 2008

Fig. 15 .

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

2008 Thyasira tanabei sp. nov.; Kiel et al. 2008: 530 , fig. 5A–G, fig. 6A.

?2009 Thyasirid bivalve; Kiel et al. 2009: fig. 3L, M.

2017 Thyasira tanabei Kiel, Amano, and Jenkins, 2008 ; Jenkins et al. 2017: fig. 3H, I.

Type material: Holotype ( UMUT MM 29533 ) and paratypes ( UMUT

MM 29534–29537, 29539) ( Kiel et al. 2008).

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

Type horizon: Omagari Formation , Yezo Group, lower Campanian ,

Upper Cretaceous.

Material.— Three illustrated specimens ( ZPAL L.16/14–17; Fig. 15 ) and 3 unnumbered from Santonian cold seep carbonates from Maeshima, Kyushu, Japan.

Remarks.—The specimens from Santonian cold seep carbonates of Himenoura Group, Maeshima, Kyushu, are similar in their general appearance to Thyasira tanabei specimens from Cretaceous cold seep, sunken leatherback turtle carcass and possibly sunken wood associations from Yezo Group, Hokkaido, previously described and figured by Kiel et al. (2008, 2009) and Jenkins et al. (2017). The specimens from Maeshima (max. 5 mm in length) are smaller than their conspecifics from cold seeps of Hokkaido, which approach 12 mm in length ( Kiel et al. 2008: table 2), and are also less inflated and have more pronounced posterior folds. In shape, the Maeshima specimens resemble an undescribed thyasirid species from the Akita River wood fall in Hokkaido, which are in the size range of the Maeshima specimens (4 mm in length; Kiel et al. 2009: 78, fig. 3L, M) and are also only slightly inflated. However, due to poor preservation of the material from the Akita River wood fall, a more detailed comparison between the specimens of the two sites is currently not possible.

Stratigraphic and geographic range. — Cretaceous of Japan; known from Albian–Campanian cold seeps of Hokkaido (the Ponbetsu, Kanajirisawa, and Yasukawa sites; Kiel et al. 2008), Santonian cold seeps of Maeshima, Kyushu, Campanian turtle fall from Nio Creek, Hokkaido, and questionably the Coniacian Akita River wood-fall on Hokkaido ( Kiel et al. 2009).

Figures

Fig. 15. Thyasirid bivalve Thyasira tanabei Kiel, Amano, and Jenkins, 2008, from Santonian seep carbonates from the Himenoura Group, Maeshima, Goshoura Island, Kumamoto Prefecture, southern Kyushu, Japan. A. ZPAL L.16/14, a partial internal mold in oblique right lateral view (A1); detail with indicated radial striae and anterior adductor muscle scar (A2, A3). B. ZPAL L.16/15, a partial internal mold in oblique right lateral view (B1); probably anterior pedal retractor muscle scar (B2). C. ZPAL L.16/16, a specimen in oblique dorsal view (C1); a very weak posterior sulcus and posterior fold, with minute auricle visible (C2). D. ZPAL L.16/17, a specimen in oblique posterodorsal view (D1); posterodorsal shell margin with posterior sulcus, posterior fold, submarginal sulcus and auricles indicated (D2).