606
Journal of Paleontology 92(4):596–610
Figure 8. Schematic illustration of the right valve internal features of Caspiconcha lastsamurai new species from Omagari seep, northern Japan.
invisible; anterior adductor muscle scar elliptically rounded with pedal elevator muscle scars located dorsally; posterior adductor muscle scar is trapezoidal with dorsally located narrow posterior pedal retractor muscle scar projected anteriorly; connection between posterior pedal retractor muscle scar and posterior adductor muscle scar forms smoothly curved line (Figs. 7, 8); weak internal ridge running from anterior pedal elevator muscle scar toward ventral margin of the posterior adductor muscle scar; myophoric buttress moderately steep. Hinge area not per- fectly preserved, apparently edentulous; caspiconchiid process present; nymph and ligament groove very long and straight.
Etymology.—The species is the last known occurrence of the genus and it was found in Japan. We coined the species name after Hollywood motion picture ‘The Last Samurai,’ 2003 © Warner Brothers.
Remarks.—Caspiconcha lastsamurai n. sp. was first reported as Calyptogena sp. based on its general shell shape (Hikida et al., 2003). This identification was subsequently questioned and instead affinities to Caspiconcha were suggested (Amano and Kiel, 2007; Kiel and Peckmann, 2008; Kiel et al., 2010; Jenkins et al., 2013). Caspiconcha lastsamurai differs from other Caspiconcha species (i.e., C. whithami, C. rubani, C. basquensis, and C. major) in having a strongly recurved shell, trapezoidal posterior adductor muscle scar, and smoothly curved connection line between posterior pedal retractor muscle scar and the posterior adductor muscle scar (Fig. 9). Caspiconcha lastsamurai n. sp. differs from Caspiconcha raukumaraensis n. sp. from New Zealand (Kiel et al., 2013) by
having a less recurved shell shape and an almost straight dorsal margin. Caspiconcha lastsamurai differs from C. basquensis n. sp. from Spain (Agirrezabala et al., 2013) by its strongly curved shell and its deeper anterior adductor muscle scar.
Discussion
History of Caspiconcha.—We demonstrate the currently known stratigraphic range of Caspiconcha from Tithonian to Campa- nian in Figure 10. The fossil record of Caspiconcha consists of seven nominal species and one uncertain species from Santo- nian (Upper Cretaceous) deposit of Amakusa area in Kyushu, Japan. Caspiconcha first appeared with C. major in the latest Jurassic (Tithonian) in the eastern Pacific, and shortly afterward the first Tethyan species, C. rubani, appeared in the earliest Cretaceous (Berriasian). Two further species appeared in the Barremian and Hauterivian. The diversity reached a peak in the Albian with four species distributed worldwide. Afterward, the genus seems to have declined in diversity and local abun- dance. The mid-Cenomanian seep site at Port Awanui, New Zealand, was the last seep deposit dominated by a species of Caspiconcha (C. raukumaraenis); the remaining Late Cretaceous records are single occurrences, including the youngest species, the Campanian C. lastsamurai. This fading abundance of Caspiconcha seems to be a real phenomenon rather than sampling bias because several well-investigated Late Cretaceous seep deposits exist and no additional specimens of Caspiconcha have been recovered up to now. Although Maastrichtian (Late Cretaceous) seeps are extremely rare at this moment, there is a newly found Campanian-Maastrichtian seep in Alaska (unpublished data, RGJ and AK, 2007) and there are
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