Camptonectes inexpectatus Hayami, 1959
HAYAMI, I. 1959. Pelecypods of the Mizunuma Jurassic in Miyagi Prefecture, with some stratigraphical remarks. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 34: 66-77, pI. 7. [p. 70, pl. 7, figs. 4, 5]
1959 Camptonectes inexpectatus Hayami, 1959
I. Hayami, 1959, plate 7.
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«Description: — Shell medium to small for genus, inequivalve. fairly inequilateral, nearly acline, not strongly inflated; antero-dorsal margin of main body slightly sinuated in each valve, while postero-dorsal one is nearly straight: umbo lying near mid-point of shell length but at about two-thirds of hingeline from front, very slightly prosogyrous; apical angle probably about 115 degrees, though the value is with no accuracy because of deformed material, and may be more or less smaller in early stage; auricles of two valves very unequal in size and shape; right anterior one, i.e. byssal auricle. protruded forwards, of Chlamys-type, about twice as long as posterior one, defined from main body by an auricular sulcus, deeply excavated to form a byssal notch; ctenolia distinct, five or more in number; left anterior auricle very large, subtrigonal with a slightly convex anterior margin, truncated subvertically at corner, ill-defined from main body; posterior auricle of each valve comparatively small, trigonal, truncated obliquely, well defined; surface of main body and auricles but for byssal one ornamented with numerous fine flabellate characteristic Camptonectes- striations which are never punctate, often bifurcated near ventral periphery of left valve and strongly bent up in both lateral areas: umbonal region almost smooth; concentric lamellae somewhat regularly spaced but restricted to anterior part of each valve; on left anterior auricle more than ten erect subvertical lamellae countable already from early stage and abruptly strengthened in middle stage; in later stage some of them continuous to anterior part of main body and fading away downwards from inner side; byssal auricle marked with numerous concentric lamellae of growth. while they are very weak on posterior one of each valve; resilifer triangular, completely internal; hinge and crural teeth absent; musculature unknown.
Observation and Comparison: — Four specimens are more or less broken or deformed. The holotype (Figs. 4a-c) shows nearly complete antero-dorsal part of left valve, ill-defined anterior large auricle, typical Camptonectes-striations and prominent concentric lamellae. The paratype (Fig. 5) reveals almost complete outline of right valve, resilifer and distinct ctenolia, though more or less compressed in dorso-ventral direction. The mode of surface-markings may be somewhat different between two valves. In right valve Camptonectes-striations are comparatively weak and increase their number by insertion, but mainly by bifurcation in left valve.
Several species with Camptonectes-striations appeared already in the Permian of North America (NEWELL, 1931) and again in the Upper Triassic of Japan (NAKAZAWA, 1952), but this genus flourished most extensively in Aalenian and later times. So far as I am aware, there are few representatives of true Camptonectes in the Lias (exclusive of Aalenian) except for Camptonectes psilonoti STAESCHE (1926) and Camptonectes sp. in DECHASEAUX (1936. p. 29, pl. 4, figs. 1-8). STAESCHE and DECHASEAUX considered that Pecten punctassimus QUENSTEDT (1856; TERQUEM and PIETTE, 1868) from the Hettangian of Lorraine and some other areas is intermediate between Chlamys and Camptonectes, but COX (1952) did not accept STAESCHE's phylogenetical concept from finely ribbed Chlamys to Camptonectes during Liassic times. The present species is a lower Liassic one belonging to Camptonectes (s. s.) defined by COX (1952), and therefore suggests the presence of a lineage independent from other pectinid genera, which was originated probably in such older species as mentioned above and persistent throughout the Lias. Another problem exists in the relationship between Camptonectes and Entolium. Similarly tlabellate surface striae are actually seen in several species of entoliids, Pecten (Syncyclonema) quotidianus HEALEY (1908) for example. But such forms have typical hinge-characters of the Amusiidae, and the resemblance of surface ornaments may be superficial. This species is somewhat similar to Camptonectes lens (SOWERBY) (1818), type of the genus. The species and its close allies flourished world-widely from Aalenian to Oxfordian. Though the synonymy among them was discussed repeatedly by many authors, this species differs from any hitherto described as Camptonectes lens, C. auritus, C. arcuatus and C. aratus from Europe, India, and South America in having non-punctate surface, more prominent concentric lamellae on large left anterior auricle and anterior part of main body. In these respects this may be more related to Camptonectes laminatus SOWERBY (1821) from the Dogger of England, New Zealand and Argentina. According to ARKELL (1930b), C. laminatus is distinguishable from lens by the large left anterior auricle which bears some ten conspicuous raised verticle lamellae (12 to 20 in full-grown specimens). This species is, however, separable from laminatus by the absence of strong concentrics on posterior auricle of each valve and longer hinge line. Judging from SOWERBY's and ARKELL's figures, that species has probably more slender outline with a smaller apical angle than this. Camptonectes albertensis WARREN (1932) from the Dogger of Alberta seems also related to this in the outline and the inequality of auricles of left valve. The surface of the Canadian specimen may be exfoliated as noted by himself, and further comparison is difficult. But this is presumably different from it because neither radial nor concentric ornaments are discernible on left anterior auricle of that species. Camptonectes sp. in DECHASEAUX (1936. loc. cit.) from the Pliensbachian of Paris basin and Camptonectes psilonoti STAESCHE (1926) from the middle Lias of Swabia may be other intimate forms, but their concentric lamellae are probably much weaker. This is easily distinguishable from several Jurassic camptonectids described in Japan (KIMURA, 1951; HAYAMI, 1957c), since those species have no surface striae of Camptonectes-type. They should be rather excluded from Camptonectes, though no suitable genus for them is not yet found in any foreign literature. Occurrence: — Rare at Futamataji. This is the first pectinid from so-called "cyrenoid beds'' of the Shizukawa faunal province.» ITARU HAYAMI, 1959
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