Yabepecten ogasawarai Matsubara 2003
MATSUBARA, T. 2003. A new Miocene Yabepecten (Bivalvia: Pectinidae) from the Hongô Formation in northeast Japan. Paleontological Research, 7 (2): 167-179 [p. 169, figs. 2, 3, 4.2, 4.4, 4.6a-b]
2003 Yabepecten ogasawarai Matsubara 2003
Figure 3. Yabepecten ogasawarai sp. nov. All figures natural size. 1a–c. Right valve. IGPS 98911-1 (holotype). 1a. Internal view. 1b. External view. 1c. Apical view. 2a–c. Left valve. IGPS 98911-2 (paratype). 2a. Internal view. 2b. External view. 2c. Apical view.
Figure 4. Mizuhopecten paraplebejus murataensis Masuda and Takegawa, 1965 and Yabepecten ogasawarai sp. nov. All figures natural size. 1, 3, 5, 7a–c, 8a–c. Mizuhopecten paraplebejus murataensis Masuda and Takegawa, 1965. 1. Right valve. IGPS 90826-4 (paratype). 3. Left valve. IGPS 90826-3 (paratype). 5. Left valve. IGPS 90826-5 (paratype). 7a–c. Right valve. IGPS 90826-1 (holotype). 7a. Apical view. 7b. Internal view. 7c. External view. 8a–c. Right valve. IGPS 90826-2 (paratype). 8a. Apical view. 8b. External view. 8c. Internal view. Shown for comparison. 2, 4, 6a–b. Yabepecten ogasawarai sp. nov. 2. Right valve. IGPS 98911-5 (paratype). 4. Left valve. IGPS 98911-4 (paratype). 6a–c. Left valve. IGPS 98911-3 (paratype). 6a. External view. 6b. Internal view.
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«?Mizuhopecten paraplebejus murataensis Masuda and Takegawa. Ogasawara, 1983, p. 61-62 , pl . 6-1, figs 1, 2.
Mizuhopecten paraplebejus murataensis Masuda and Takegawa. Ogasawara et al., 1985, p. 31, pl. 2, figs. 5, 7, pl. 3, figs ,1, 5. [not of Masuda and Takegawa, 1965]. Type specimens.— Holotype: IGPS 98911-1; paratypes: IGPS 98911-2, 98911-3, 98911-4 and 98911-5. These are specimens of Mizuhopecten paraplebejus murataensis figured by Ogasawara et al. (1985). There should be another figured specimen in the IGPS collection (Ogasawara et al., 1985, pl. 2, fig. 5: IGPS 98911), but it was not found when I visited the Natural History Museum, Tohoku University.
Type locality, formation and age.— "Bed of the Sagae River, about 250 m downstream of 'Uwano O-hashi' ['Uwano big bridge'] over the Sagae River and about 500 m south-southwest of the hamlet of Uwano, Sagae City, Yamagata Prefecture" (Ogasawara et al., 1985, p. 7), Ôya Tuffaceous Sandstone Member of the Hongô Formation, late Miocene. Diagnosis.— Yabepecten with moderate-size, mederately thick shell; LV rather inflated auricles small; byssal fasciole weakly flexed rather broad; RV radial costae 18, low, broad, flat-topped round-edged; LV radial costae 18-20, fine tending to become weakly bi- or tripartite with shell growth; dorsal and inner dorsal teeth strong.
Description.— Shell moderate in size, moderately thick, circular, slightly longer than high, compressed, nearly equilateral except for auricles; apical angle about 120°; both antero- and postero-dorsal margins gape. RV rather inflated compared to LV; radial costae 18, low, flat-topped rather irregular indistinctly defined from interspaces rarely dichotomous owing to a very shallow median groove; interspaces shallow, somewhat narrower than costae; commarginal growth lines rather distinct, fine to coarse, irregular; auricles small; byssal fasciole broad, very weakly flexed, sculptured by fine to coarse, irregular growth lines; byssal notch very shallow; hinge line very bluntly v-shaped; resilifer moderate in size, moderately concave; resilifer teeth rather strong, with anterior tooth stronger than posterior tooth; dorsal teeth strong; anterior auricular crus indistinct; posterior auricular crus weakly elevated; inner dorsal flexures distinct; thin, foliated calcite laye irnside of pallial line; adductor muscle scar indistinct except for dorsal part, reentered by foliated calcite layer; internal disc very weakly folded in concert with radial costae. LV weakly inflated radial costae 18-20, fine, tending to become weakly bi- or tripartite with shell growth faint costae rarely present in interspaces; shagreen microsculpture on entire external shell; commarginal growth lines indistinct; interspace of costae shallow, slightly round-bottomed; auricles small, sculptured by irregular, rather widely spaced growth lines and fine, low, radial costae; hinge line nearly straight; resilifer same as that of RV; sockets of resilifer teeth distinct, especially the anterior one; infradorsal teeth strong; supradorsal teeth very narrow; sockets of dorsal teeth rather deeply concave; anterior auricular crus distinct, with a low distal denticle; posterior auricular crus indistinct, but with a distal denticle; dorsal flexures distinct; foliated calcite layer inside of pallial line, very thin; other internal features same as RV. Etymology.—This species is named in honor of Prof. Kenshiro Ogasawara of the Institute of Geoscience, the Universi ty of Tsukuba.
Discussion.— The type specimens of this new species were once referred to as Mizuhopecten paraplebejus murataensis Masuda and Takegawa, 1965, which was originally described from the upper Miocene Fukuda Formation in Miyagi Prefecture, northeastern Japan (Ogasawara et al., 1985). The most significant difference between the two taxa is the mode of the auricular crura. The present new species has auricular crura with rather distinct denticles on the distal end, whereas M. paraplebejus murataensis has less developed crura that lack denticles, as seen in other members of Mizuhopecten. In addition, M. paraplebejus murataensis has much large aruricles, a more inflated RV with more distinct radial costae, and an LV with stouter radial costae and less developed shagreen microsculpture (Fi g. 4.2, 4.4, 4.6a-c). The other significant difference is the mode of development of the inner dorsal flexures. The flexures are well developed in the LV of the Hongô specimens, whereas they are indistintinct M. paraplebejus murataensis. Taking account of these characters, the Hongô specimens are assigned to Yabepecten rather than Mizuhopecten. The specimens illustrated by Ogasawara (1983) as M. paraplebejus murataensis from the Hashigami Sandstone Member of the Hongô Formation is probably referrable to the present new species, although a definite assignment cannot be made because the specimens are not preserved well. The present new species closely resembles Yabepecten condoni (Hertlein, 1925), from the lower upper Miocene Montesano Formation of Washington, in having a weakly inflated LV. However, it differs from that species by having a larger shell with smaller auricles and stronger shagreen microsculpture. Y. ogasawarai sp. nov. is also similar to the "smooth form" of Patinopecten (Patinopecten) healeyi (Arnold, 1906) of Moore (1979), from Pliocene strata in California, but the latter species has a nearly smooth RV shell surface near the umbo. Yabepecten tokunagai (Yokoyama, 1911) clearly differs from the present new species by having a larger shell with an almost flat or weakly concave LV sculptured by much finer and lower radial costae. Miyagipecten alaskensis MacNeil, 1967, from the upper Miocene upper part of the Yakataga Formation in south-central Alaska differs in having a smaller shell with more irregular distinct LV intercalary costae. Masuda and Addicott (1970) tentatively synonymized this species with Yabepecten condoni (Hertlein). The nearly smooth RV shell surface and the LV sculptured by fine, stringy radial costae and shagreen microsculpture indicate that M. alaskensis probably belongs in Yabepecten, as was done by Masuda and Addicot (1970), Uozumi et al. (1986a) and Matsubara (1996). The holotype and paratype of M. alaskensis are too fragmentary to evaluate thoroughly, and additional material is needed to determine its precise taxonomic position, Miyagipecten saromensis Hasimoto and Kanno, 1958, from the Miocene Chirai Formation of northern Hokkaidô is, easily distinguished from Y. ogasawarai sp, nov. by a narrower umbonal angle, stouter and very irregular radial costae on the LV, and less distinct radial costae on the RV. Amano and Karasawa (1988) tentatively referred this species to Yabepecten, based on the characters of the auricles, umbonal angle and RV sculpture. Unfortunately, the precise generic position of M. saromensis is uncertain, because its detailed LV microsculpture and cardinal properties are unknown. Measurements.— Table 1, Distribution.— Ôya Tuffaceous Sandstone Member of the Hongô Formation, early late or middle late Miocene; ?Hashigami Sandstone Member of the Hongô Formation, early late or middle late Miocene.»
TAKASHI MATSUBARA, 2003
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