Cryptopecten spinosus Hayami, 1984
HAYAMI, I. 1984. Natural history and evolution of Cryptopecten (A Cenozoic-Recent Pectinid Genus). The University Museum, The University of Tokyo, Bulletin, 24: 1-149, pls. 1-13. [p. 111, pl. 8, figs. 1-5; pl. 12, figs. 3-5]
1984 Cryptopecten spinosus Hayami, 1984
I. Hayami, 1984, plate 8.
I. Hayami, 1984, plate 12.
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«1934. Pecten (Aequipecten) vesiculosus Dunker: Nonrura and Zinbo, Sci. Rep. Tohoku Imp. Univ., ser. 2, vol. 16, no. 2, p. 117. [non Pecten vesiculosus Dunker, 1877]
Type.— The holotype is a nearly complete right valve (UMUT CM16170c) from the coral sand of the Late Pleistocene Wan Formation of the Ryukyu Group at about 500 m north of Kamikatetsu, Kikai Island (Kikai Town), Kagoshima Prefecture, south Japan (28°17.0'N, 129°56.8'E). 25.7 mm long, 25.1 mm high, 6.1 mm thick. Material (Paratypes).— In addition to the holotype, 189 specimens of the sample Kk (S) from the type locality are concerned with the following description and discussions. Diagnosis.— Medium-sized species of Cryptopecten, characterized by relatively weak convexity of left valve, 12-15 radial ribs, a narrow and sharp central ridge which is spinously projected beyong ventral margin, somewhat coarse and oppositely disposed imbricated scales covering tubular hollow parts, and conspicuous scales on byssal wing. Description.— Shell rarely exceeding 26 mm in length and height, highly inequivalve, subequilateral, nearly acline throughout growth, right-convex except for very early stages of dissoconch; height commonly exceeding length in young stages, but negatively allometric and becoming subequal to length in adult stages; thickness positively allometric to length in right valve but isometric or rather negatively allometric in left valve; form ratio T/L as small as 0.23-0.27 in adult right valve and 0.17-0.19 in adult left valve; hinge line comparatively short, negatively allometric to overall length; test solid, moderate in thickness; wings highly unequal in size; pre-umbonal dorsal margin of right valve elevated above hinge axis; posterodorsal margin of disk slightly concave, a little longer than anterodorsal; apical angle about 90-95 degrees; byssal notch considerably deep, provided with three or four exposed denticles of ctenolium and relatively narrow fasciole area. Outer surface of disk ornamented with 12-15 highly elevated and strong radial ribs; each rib reinforced by a narrow, flat-topped and scaly central solid ridge, the sides of which are steep and remarkably excavated; imbricated scales oppositely disposed, coarse (about four per 1 mm), strongly convex and granular in appearance; consequently, hollow space covered with imbricated scales comparatively wide and tubular; terminal of each central ridge spinously projected beyond ventral margin and observable also from inside of shell; rib interspace moderate in width, marked with erect scales which are quite independent from and more distantly spaced than imbricated scales on hollow parts; fine intercostal threads occurring only in adult stages; byssal wing ornamented with four (occasionally five) radial riblets on which several finny scales are developed; other wings marked with weaker riblets; stepwise growth rings sometimes observed on adult shell. Color pattern recognizable, though obscurely, on external and internal surface especially in left valve, characterized by irregular bands and spots on darkly pigmented ground. Internal surface covered with thin inner shell layer with clearly impressed radial sculpture except for umbonal area; cardinal crura relatively weak; muscle scars not much different from those of. C. vesiculosus vesiculosus in size and shape. Remarks.—The present species, as interpreted by Nomura and Zinbo (1934), resembles C. vesiculosus, but is clearly distinguishable from that species by the weaker convexity of both valves (see Figs. 15 and 16), significantly fewer radial ribs (Table 10) and central ridge projected beyond ventral margin. One of the most distinctive characters is the shape of the imbricated scales on the hollow parts of the ribs. In Phenotype Q of. C. vesiculosus the slopes of the central solid ridge are never so deeply excavated, and the imbricated scales are not so strongly convex. Consequently, in the transverse section and also in ventral view, the hollow space is lenticular in Phenotype Q of. C. vesiculosus but nearly circular in the present species. In short, the sculpture of C. spinosus is more prominent and decorative than that of. C. vesiculosus and other species of Cryptopecten. The present new species is known only from raised reef sediments of the Late Pleistocene age, and appears to have soon become extinct. Its reproductive isolation from C. vesiculosus before this stage is suggested by the absence of dimorphism in the sample Kk (S). I presume that C. spinosus represents an incipient species which was derived from the main stock of C. vesiculosus. Distribution.— The present species is known only from the type locality. According to Omura (1983), this fossil bed represents grainstone facies deposited at a depth of 40-100 meters, and its 230Th/234U age is 82,000±2,000 years B. P. as determined from three species of ahermatypic corals. Late Pleistocene.» ITARU HAYAMI, 1984
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