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Journal of Paleontology 92(4):596–610
Formation,Yezo Group, and dated asCampanian (Takahashi et al., 2003, 2007; Jenkins et al., 2007). It was identified as an ancient methane-seep deposit using stable carbon isotope, petrographic, and biomarker evidence (Hikida et al., 2003; Ogihara, 2005; Jenkins and Hikida, 2011).Asingle large bivalve from this deposit was previously identified as Calyptogena sp. (Hikida et al., 2003) and is here described as Caspiconcha lastsamurai n. sp.
Materials and methods
Most of the specimens described here were collected by the authors, and selected specimens were mechanically prepared. The comparative figured material of C. whithami Kelly in Kelly et al., 2000 and C. major (Gabb, 1869) is housed at the Sedg- wick Museum, University of Cambridge, UK (SMUC) and the California Academy of Science (CAS), respectively.
Repositories and institutional abbreviations.—NMM, Nakagawa Museum of Natural History, Nakagawa Town, Hokkaido, Japan; NMNS, National Museum of Nature and Science, Tsukuba, Japan; NRM, Swedish Museum of Natural History, Department of Palaeobiology, Stockholm, Sweden; GNS, Geological and Nuclear Science, Lower Hutt, New Zealand; UMUT, University Museum, the University of Tokyo.
Systematic paleontology Class Bivalvia Linnaeus, 1758
Subclass Autobranchia Grobben, 1894 Superorder Heteroconchia Gray, 1854 Order Carditida Dall, 1889
Superfamily Kalenteroidea Marwick, 1953 Family Kalenteridae Marwick, 1953
Remarks.—Kalenterids were previously classified within the Pholadomyoida (e.g., Waller, 1990; Fang and Morris, 1997; Kelly et al., 2000; Griffin and Pastorino, 2006), Palaeoheter- odonta (Cox et al., 1969; Hautmann, 2001, 2008), or Carditida (e.g., Chavan, 1954, 1969; Newell, 1957, 1969; Nevesskaja, 2009). Here we follow the interpretation of Kaimand Schneider (2012) who placed the kalenterids among the Carditida based on the presence of complex cross-lamellar shell microstructure and the lack of a nacreous layer in two genera of kalenterids: Caspiconcha (Jenkins et al., 2013) and Myoconcha J. Sowerby, 1824 (Kaim and Schneider, 2012).
Genus Caspiconcha Kelly in Kelly et al., 2000
Type species.—Caspiconcha whithami Kelly in Kelly et al., 2000; Early Cretaceous (late Barremian), NE Greenland.
Occurrence.—Tithonian (Late Jurassic) to Campanian (Late Cretaceous). Barremian of Greenland, Tithonian to Albian of California, Hauterivian of Crimea and possibly Czech Republic, Albian of Basque Country, Spain, Albian–Cenomanian of New Zealand, and Albian and Campanian of Hokkaido, Japan.
Remarks.—Due to its similarity to Myoconcha regarding the modioliform shape, deeply set anterior adductor muscle scars
with myophoric buttress, and extreme reduction of the anterior side of the shell, Kelly et al. (2000) placed Caspiconcha among the Modiomorphidae, subfamily Myoconchidae. Later, Myoconchidae/Myoconchinae have been synonymized with Kalenteridae (Damborenea, 2004; Bouchet et al., 2010; Jenkins et al., 2013), and subsequently Caspiconcha is here classified in Kalenteridae. Caspiconcha differs from Myoconcha by lacking external radial ornament and by possessing an edentulous hinge and a caspiconchiid process. The nominal species are Caspiconcha whithami Kelly in
Kelly et al., 2000 from the Barremian of Greenland (Kelly et al., 2000), C. rubani Kiel, Campbell, and Gaillard, 2010 from the late Berriasian of the Crimean Peninsula (Kiel and Peckmann, 2008; Kiel et al., 2010), C. major (Gabb, 1869) from Tithonian to Albian localities in the Great Valley Group in California (Stanton, 1895; Kiel et al., 2008; Jenkins et al., 2013), and the new species C. basquensis n. sp. from the late Albian of northern Spain, C. yubariensis n. sp. from the late Albian of Utagoesawa seep, central Hokkaido, Japan, C. raukumaraensis n. sp. from the late Albian to mid-Cenomanian of New Zealand, and C. lastsamurai n. sp. from the Campanian of the Omagari seep, northern Hokkaido. Illustrations with indication of selected key characters of all nominal species, including the four new species described herein, are in Figure 2. In addition, ‘Myoconcha aff. transatlantica’ from the Early Cretaceous (Hauterivian) of the Czech Carpathians (Ascher, 1906) is most likely a species of Caspiconcha (Kiel and Peckmann, 2008; Kaim, et al., 2013). A specimen possibly belonging to
Caspiconcha was found in a Santonian (Late Cretaceous) debris flow deposit in Amakusa, southwestern Japan (N. Kikuchi and Y. Kondo, Kochi University, personal communication, 2012): it reaches nearly 300mm in length and has a broad, modioliform outline, but no internal shell features were observed.
Caspiconcha basquensis new species Figure 3
2013 Caspiconcha sp. Agirrezabala et al., p. 102, fig. 16C, D. Holotype.—NRM Mo 182801.
Paratypes.—NRM Mo 182802, 182803, and six unnumbered, fragmentary specimens.
Diagnosis.—Modioliform shell with strong anteroposterior elongation; nearly straight dorsal and ventral margin, broad ridge from the umbo to the posteroventral margin; no triangular depression runs below the ridge; moderately deep-set anterior adductor muscle scar.
Occurrence.—Seep carbonates in Ubidepea mudstone (infor- mal subunit; late Albian), Ogalla unit, Black Flysch Group exposed along the shore between the villages Ea and Ispaster, west of Lekaitio, Basque country, northern Spain.
Description.—Shells elongate modioliform, up to 210mmlong, dorsal and ventral sides nearly straight; umbo subterminal, prosogyrate, very low, pointed; inflation moderate, greatest along a broad ridge from the umbo to the posteroventral margin,
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