Radulonectites japonicus Hayami, 1957
HAYAMI, I. 1957. Radulonectites, a new pectinid genus, from the Liassic Kuruma group in central Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 27: 89-93, pI. 16. [p. 90, pl. 16, figs. 1-7]
1957 Radulonectites japonicus Hayami, 1957
1957 Radulonectites japonicus var. convexus Hayami, 1957
1968 Radulonectites hayamii Polubotko, 1968
1957 Radulonectites japonicus var. convexus Hayami, 1957
1968 Radulonectites hayamii Polubotko, 1968
I. Hayami, 1957, plate 16.
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«Description:– Shell highly inequivalve, inequilateral especially in right valve, slightly opisthocline, higher than long, Pleuronectites-like in outline; posterior area develops more than anterior; left valve moderately convex but the convexity of right one is weak; hinge-margin straight; in right valve antero-dorsal margin of shell-body remarkably sigmoidal but not so much in left; postero-dorsal one almost straight in each valve; ventral one gently arcuate, passing into postero-dorsal one, but incurving somewhat abruptly at the junction with antero-dorsal one in right valve; umbo fairly pointed, located slightly posteriorly and scarcely rising above hinge-margin even in left valve; apical angle about 110 degrees in adult stage, but it is much smaller in juvenalia; auricles large, well defined; right anterior one protruded forwards, linguiform, supported by a triangular auricular sulcus below; left anterior one fairly convex, slightly undulated, truncated subvertically; posterior one depressed, triangular, truncated with an obtuse angle of 120 degrees or more at the extremity in each valve. but its posterior margin slightly concave in left valve and feebly convex in right; both valves similarly ornamented with 65 to 70 almost straight radial grooves in maturity, although umbonal region is smooth except for faint concentric lines of growth; radial grooves unbranching, curving slightly out wards in anterior part, broadened gradually, almost as wide as their interspaces near ventral periphery; auricles sculptured also by several weak radials in adults except for right anterior one; ventral side and auricles marked with strong concentric lines of growth; ctenolium, resilifer and muscle system unknown.
Measurement in mm. Length Height Thickness Apical angle* Grooves**
Holotype (lMM 2689) bivalved 45.5 50.0 8.8 110° 67 Paratype (MM 2690) right valve 42.0 47.0 5.0 105° 66 Paratype (MM 2!l91) right valve 30.5 34.0 4.5 95° 68 (MM 2692) juvenile right valve 17.0 34.0 2.5 95° 12 Observation:– Many right valves and three bivalved specimens are at hand. The holotype (Figs. 2a. 2b, 2d) and adult paratype (Fig. 1a) are more or less compressed secondarily, but they show the outline and ornamentation in adult stage of this species. The aspects of the middle and early stages are represented respectively by a paratype (Fig. 6a) and a small right valve (Figs. 3a, 3b).
The juvenile shell is acline to slightly prosocline and somewhat Chlamys-like in outline. Its apical angle is comparatively small due to the sinuation of the antero-dorsal margin of the shellbody. The byssal auricle is fairly large, if compared with those of adults, and ornamented with two or three radial ribs which become obsolete later. The surface of shell-body is almost smooth, and radial grooves are, if present, very narrow, small in number and restricted to the antero-ventral side. It shows that radial grooves appear earlier in the antero-ventral than in the other part. The shell-surface is liable to exfoliate, but the characteristic ornaments are completely preserved on the external moulds of three type specimens (Figs. 1b, 2c. 6b). None of artificial internal moulds*** reveals either resilifer or muscle system. It may be primarily due to the complete erosion of inner aragonite layers on which those internal organs adhered. Comparison:– The holotype left valve (Fig. 2b) is very similar to that of Pleuronectites laterestriatus (PHILIPPI) (1899: SCHMIDT, 1928) in outline and radial grooves, but the radial striations are much finer and more partial in laterestriatus than in this species. That species is possibly a collateral ancestor to this species, although the right valve is fairly different in outline. Pecten (Camptonectes) lens LEANZA (1942) (non SOWERBY) is another intimate species to this, judging from its ornamentation. But that species is different from this in the more Chlamys-like outline i. e. less sinuated antero-dorsal margin of shell-body and denser radial striae. Chlamys sp. by DECHASEAUX (1936. pl. 4. fig. 6) has a similar outline to the smaller paratype (Fig. 6a). although radial ornaments are entirely absent in that form. If it is a juvenalium, the French form may be more or less related to this. Occurrence:– Common in the Liassic Kuruma group at Kuruma and Tsuchizawa in Kitaotari-mura. Kitaazumi-gun, Nagano Prefecture (Province of Shinano).» * Maximum umbonal angle between antero-dorsal and postero·dorsal margins of shell-body.
** Number of radial grooves on the shell·body exclusive of auricles. *** NEWELL (1937) calls such a specimen a subinternal mould. ITARU HAYAMI, 1957
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«Remarks. Radulonectites sosneadoensis is known from Argentina (e.g. WEAVER 1931, DAMBORENEA 1993a), and Late Pliensbachian specimens from Chile have been discussed in some detail by ABERHAN (1994). The Canadian specimens correspond well to the South American forms.
Closely related species are Radulonectites japonicus HAYAMI (1957b: 90, pl. 16, figs. 1-8; 1975: pl. 3, figs. 9-10) from the Upper Pliensbachian of Japan and its junior synonym R. hayamii POLUBOTKO (in EFIMOVA et al. 1968: 82, pl. 24, figs. 1-5; MILOVA 1988: 88, pl. 12, figs. 7-9, pl. 13, figs. 1-6; SHURYGIN & LUTIKOV 1991: 71, pl. 14, figs. 8-10; HAYAMI in SATO & WESTERMANN 1991) from the Upper Pliensbachian of eastern Siberia. The observed range in the number of riblets is 70 to around 90 in R. sosneadoensis and 65 to 85 in R. japonicus. Consequently, the two species cannot be separated on this basis. The western Pacific form, however, can be distinguished from R. sosneadoensis by a sub-orbicular outline, and a right valve, which is distinctly inequilateral and exhibits a more strongly concave antero-dorsal shell margin. Furthermore, the umbonal part is devoid of a radial sculpture in R. japonicus, whilst in unabraded R. sosneadoensis it is covered by fine radial striae extending to the beak.» ABERHAN, M. 1998. Early Jurassic Bivalvia of western Canada. Part I. Subclasses Palaeotaxodonta, Pteriomorphia and Isofilibranchia. Beringeria, 21: 57-150, text-figs. 1-14, pls. 1-19. [p. 119]
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