Yabepecten okiensis Matsubara, Noro, Matsuura & Irizuki, 2014
MATSUBARA, T., I. NORO, Y. MATSUURA & T. IRIZUKI. 2014. Miocene Mollusca from the Ichibu Formation on Nishinoshima Island, Oki Islands, Southwest Japan. Paleontological Research, 18 (1): 6-32, figs. 1-11. [p. 21, figs. 8.1a–8.4b, 9.1, 9.4, 9.5a–b]
2014 Yabepecten okiensis Matsubara, Noro, Matsuura & Irizuki, 2014
T. Matsubara, I. Noro, Y. Matsuura & T. Irizuki, 2014, figures 8 and 9.
|
«?Patinopecten tokunagai (Yokoyama). Naemura and Shimada, 1984, p. 159. [non Yokoyama, 1911]
Type material.— Holotype: OKNM NM0001. Paratypes: OKNM NM0002, NM0003, DGSU T.3575–T.3578, T.3581–T.3583.
Type locality.— Loc. 2: A small seaside cliff 50 m west-northwest of the Oki Seaside Hotel “Tsurumaru”, Mita, Nishinoshima Town, Oki County, Shimane Prefecture (36°05'35''N; 133°00'34.3''E). Diagnosis.— A Yabepecten with nearly flat to weakly inflated left valve sculptured by ca. 16, fine, narrow, roof-topped radial ribs and strong shagreen microsculpture, and weakly inflated right valve sculptured by 13–15 low, irregular, round-topped radial ribs. Etymology.— Named after the type locality, the Oki Islands. Description.— Shell subcircular, rather large; shell length to 125 mm; umbonal angle ca 116°. Auricles moderate in size, subequal between anterior and posterior ones except for byssal notch in anterior auricle in right valve. Anterior auricles with a few fine radial ribs. Posterior auricles may have a few fine radial ribs. Resilifer moderate in size. Left valve nearly flat to weakly inflated, with 14–18 (average: 16), low, narrow, rooftopped radial ribs; shagreen microsculpture distinct. Infradorsal teeth weak. Auricular crura with a low distal denticle. Right valve weakly inflated, with 13–15, low, broad, rather irregular radial ribs. Resilifer teeth and dorsal teeth moderately strong. Internal rib carinae absent. Discussion.— Naemura and Shimada (1984) reported “Patinopecten tokunagai (Yokoyama)” [=Yabepecten tokunagai (Yokoyama, 1911)] from the Ichibu Formation. This species is known as one of the representatives of the Omma-Manganzi Fauna (Otuka, 1939a) of Pliocene–early Pleistocene age. Although Yamauchi et al. (2000) doubted this occurrence, they could not reexamine the specimens referred by Naemura and Shimada (1984). Yabepecten okiensis sp. nov. resembles Y. tokunagai, but differs in having a larger umbonal angle, broader and fewer nondichotomous radial ribs in the right valve, and broader, roof-topped radial ribs, which do not tend to become obsolete with shell growth, in the left valve.
Yabepecten ogasawarai Matsubara, 2003 from the upper Miocene Hongô Formation in Yamagata Prefecture, Northeast Japan, is another allied species, especially in the left valve sculpture in the younger stage. However, Y. ogasawarai is distinguished from Y. okiensis sp. nov. by having a narrower umbonal angle, bi- or trifurcated radial ribs in the left valve, flat-topped, more regular radial ribs in the right valve, and stronger hinge teeth. “Mizuhopecten paraplebejus murataensis Masuda and Takegawa” of Kakudate (1988, pl. 2, figs. 5, 6) and “Mizuhopecten cf. paraplebejus Masuda and Takegawa” of Sato (2002, p. 6, pl. 1, figs. 1, 2) from the upper Miocene Tsuma Formation on Dôgo Island is referred to Y. ogasawarai. Because the geologic age of Y. okiensis sp. nov. (ca. 12–11.5 Ma) is older than Yabepecten condoni (Hertlein) from the upper Miocene Montesano Formation of Washington (ca. 9.5–9.0 Ma) and Y. ogasawarai (between 9.2–7.3 Ma) (Matsubara, 2003), Y. okiensis sp. nov. is the oldest member of the genus Yabepecten Masuda, 1963. This fact indicates that Yabepecten in the Northwest Pacific is not a descendant of Y. condoni, and that Y. okiensis sp. nov. was probably derived from a species of Patinopecten Dall, 1898 in the Northeast Pacific by westward migration in the late middle Miocene. Yabepecten okiensis sp. nov. is separated from Mizuhopecten kitamiensis Uozumi, Fujie and Matsui, 1966 from the upper middle Miocene Ainonai Formation in the Kitami area, Hokkaidô, Northeast Japan, by having lower, finer radial ribs in the left valve and weaker, nondichotomous radial ribs in the right valve. Y. okiensis sp. nov. is easily distinguished from Mizuhopecten paraplebejus (Nomura and Hatai, 1936), one of the characteristic species of the Shiobara-Yama Fauna (Chinzei, 1963; Iwasaki, 1970a; Ogasawara and Sato, 1986; Matsubara, 1996; Kurihara, 2010), by having smaller auricles, a greater number of radial ribs, less inflated right valve, shagreen microsculpture on the whole of the left valve surface, and auricular crura with a denticle on the left valve. In addition, the radial ribs of M. paraplebejus tend to become obsolete with shell growth. Yamauchi et al. (2000) and Sakanoué et al. (2000) described and illustrated “Mizuhopecten cf. nakatombetsuensis (Akiyama)” from the upper part of the Ichibu Formation. It is easily distinguished from Y. okiensis sp. nov. by having a much larger shell and a greater number (19–22) of weakly bifurcated radial ribs in the right valve. In my opinion, it closely resembles Mizuhopecten tryblium (Yokoyama, 1925) from the lower Pliocene Shigarami Formation in Nagano Prefecture, central Japan, rather than M. nakatombetsuensis (Akiyama, 1962). Measurements.— Table 3. Distribution.— Known only from the Ichibu Formation. Subfamily Palliolinae Korobkov in Eberzin, 1960» TAKASHI MATSUBARA, IKKEI NORO, YASUTAKA MATSUURA & TOSHIAKI IRIZUKI, 2014
|