Nipponothracia yezoensis ( Kanie and Kuramochi, 1996 )

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 : 533

Nipponothracia yezoensis ( Kanie and Kuramochi, 1996)

Figs. 6B , 7A, B .

1996 Thracia yezoensis sp. nov.; Kanie and Kuramochi 1996: 64, fig. 1. Material.—Several specimens from the type locality at Obira. Supplementary description.—Hinge plate narrow and smooth; ligament rapidly widening posterior of umbo, supported by narrow nymph. Internal radial striations well developed; pallial line distant from shell margin, marked by a groove; anterior adductor muscle scar elongate, about 1/3 of shell length, tongue−shaped, marked by a groove, ventrally detached from pallial line by a thin margin for about 4/5 of its length; posterior adductor muscle scar less distinct, apparently subcircular with a short ventral protrusion.

Discussion.— Kase et al. (2007) reported that a specimen of Nipponothracia yezoensis from the type locality stored in the National Museum of Nature and Science, Tokyo, has “an elongate anterior muscle that is more elongate than the posterior one and seemingly detached from the pallial line”. This observation is here confirmed. The anterior muscle scar of N. yezoensis is more elongate than that of N. gigantea , the posterior muscle scar is more rounded, and the pallial line is more distant from the shell margin than in N. gigantea ( Fig. 6B ). In addition, the hinge plate of N. gigantea is deeply excavated below the umbo, a feature not seen in N. yezoensis . Thus N. yezoensis can clearly be distinguished from the type species. Because no internal features except radial striations are known for N. ponbetsensis (see below), the question whether N. ponbetsensis and N. yezoensis are synonymous remains unresolved.

The Indonesian Nipponothracia hetzeli (Martin, 1933) from a late Miocene asphalt deposit at Buton, Indonesia, also has an excavated hinge plate, a more elongate ligament, and a broader nymph than N. yezoensis ( Beets 1942; Kase et al. 2007; SK, own observations). The same applies to Cryptolucina elassodyseides Saul, Squires, and Goedert, 1996 from the Eocene of Washington State, USA. Species of the Recent Anodontia clade generally have a long and narrow ligament that slowly tapers toward the umbo ( Taylor and Glover 2005). This is in contrast to the ligament of N. yezoensis which appears to attain its maximum width just posterior to the umbo.

Stratigraphic and geographic range.— Only known from the Cenomanian type locality on Hokkaido, Japan .

Nipponothracia ponbetsensis Kanie and Sakai, 1997 Fig. 7C .

1993 Thraciidae gen. et sp. indet.; Kanie et al. 1993: 33, fig. 4.

1997 Nipponothracia ponbetsensis Kanie and Sakai, 1997: 214 , figs. 6, 7.

1997 Yoldia (Megayoldia) cf. thraciaeformis Kanie and Sakai, 1997: 210 , fig. 4.

Material.—Several specimens from the Ponbetsu site.

Discussion.— Kase et al. (2007) indicated that Nipponothracia ponbetsensis has a long ligamental nymph posterior to the beaks and that it appears to belong to the Lucinidae . Here we document radial internal striations in this species that suggest its position within Lucinidae . The specimen that Kanie and Sakai (1997) figured as Yoldia (Megayoldia) cf. thraciaeformis is clearly a deformed specimen of N. ponbetsensis ; we have several specimens from the Ponbetsu site that experienced a similar kind of deformation.

Stratigraphic and geographic range.— Only known from the Albian type locality on Hokkaido, Japan .

Lucinidae gen. et sp. indet.

Fig. 7D, E .

Material.— Three specimens from the Kanajirisawa seep site.

Description.—Shell oval in outline, compressed, slightly wider than high, becoming more circular during ontogeny; sculpture of irregular, fine commarginal ribs, fine radial ribs in larger specimens; radial striations on inner side of shell, one anterior lateral tooth; anterior adductor muscle scar very weakly impressed, elongate, tongue−shaped, apparently detached from the pallial line. The smaller of the two figured specimens is 26 mm wide and 21 mm high, the larger one is 43 mm wide, 40 mm high, and a single valve is 8 mm thick.

Discussion.—Fossil bivalves with circular, compressed shells like the one described here have frequently been placed in the genus Miltha H. Adams and A. Adams, 1857 , without knowledge of internal features like hinge dentition and muscle attachment scars. Chavan (1969), Vokes (1969), and Bretsky (1976) diagnosed that lateral teeth are absent from Miltha , hence the Kanajirisawa species can not be placed in this genus. Because other internal features such as the cardinal hinge dentition and the shape of the ligament are not preserved in the available specimens, they are described here only in open nomenclature.

Figures

Fig. 6. Outline drawings showing internal features. A. Thyasira tanabei sp. nov.B. Nipponothracia yezoensis (Kanie and Kuramochi, 1996).Not to scale.

Fig. 7. Lucinidae from Cretaceous seep carbonates on Hokkaido, Japan. A, B. Nipponothracia yezoensis Kanie and Kuramochi, 1996 (Lucinidae) from the Cenomanian Kanajirisawa seep site on Hokkaido, Japan. A. Silicified specimen (UMUT MM 29541) showing the broad ligament (A1) and the edentulous hinge (A2). B. Internal mold (UMUT MM 29542) showing internal features, right and left valves (B1, B2), anterior part showing the elongate muscle scar (B3, arrow), posterior part with muscle scar and pallial line (B4). C. Nipponothracia ponbetsensis Kanie and Sakai, 1997, from the Albian Ponbetsu site in Mikasa City; specimen (UMUT MM 29543) showing the radial internal ribs (arrow). D, E. Indetermined lucinid from the Cenomanian Kanajirisawa seep site. D. Large specimen (UMUT MM 29544) with nearly circular outline. E. Internal mold (UMUT MM 29545) showing faint radial sculpture, arrow indicates impression of lateral tooth.