Camptochlamys Arkell, 1930
ARKELL, W. J. 1930. A monograph of British Corallian Lamellibranchs, Part II. Monographs of the Palaeontographical Society, 82 (374): 73-104, pls. 5-8. [p. 102]
«The right valve is flat and the left valve convex, and the principal external ornament consists of coarse annular growth-lamellae over the surface of both valves. There are, in addition, numerous fine radial ribs on both valves, but these are of low elevation, more like the ribs of Velata (olim Velopecten) than of Chlamys, and in some species (e.g., C. obscurus = annulatus) they are only developed on the umbonal region, giving place over the rest of the shell to fine striae as in Camptonectes.»
WILLIAM JOSCELYN ARKELL, 1930
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Pecten intertextus Nob.; F. A. Roemer, 1839, Die Versteinerungen des Norddeutschen Oolithen-Gebirges. Ein Nachtrag, plate 18, figure 23.
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Camptochlamys alaskensis n. sp.; T. R. Waller & L. Marincovich Jr., 1992, New species of Camptochlamys and Chlamys (Mollusca: Bivalvia: Pectinidae) from near the Cretaceous/Tertiary boundary at Ocean Point, North Slope, Alaska, figures 4.1-4.16.
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«CAMPTOCHLAMYS Arkell, 1930, new status
Type species.— By original designation (Arkell, 1930, p. 102), Pecten intertextus Roemer, 1839, Oxfordian, N. Germany, considered to be a subjective junior synonym of Pecten clathratus Roemer, 1836, by Johnson (1984, p. 147).
Original diagnosis (Arkell, 1930).— "The right valve is flat and the left valve convex, and the principal external ornament consists of coarse annular growth-lamellae over the surface of both valves. There are, in addition, numerous fine radial ribs on both valves, but these are of low elevation, more like the ribs of Velata (olim Velopecten) than of Chlamys, and in some species (e.g., C. obscurus = annulatus) they are only developed on the umbonal region, giving place over the rest of the shell to fine striae as in Camptonectes."
Emended diagnosis.— Camptonectinae with left valve convex and right valve nearly flat; external sculpture consisting of radial costae across entire disk at least in early ontogeny of both valves; projecting commarginal lamellae more prominent on left valve than on right and crossing over radial costae.
Stratigraphic range.— Jurassic: Aalenian through Tithonian (Johnson, 1984), extended herein to the uppermost Cretaceous (upper Maastrichtian) or lower Paleocene (Danian).
Geographic range.— Northern Europe and possibly Asia and Africa (Johnson, 1984); northern Alaska (range extension dis- cussed below).
Discussion.— Arkell (1930) introduced Camptochlamys as a subgenus of Chlamys, but Cox (1952), upon finding divaricate (i.e., antimarginal) striae, transferred the subgenus to Camptonectes. In fact, the presence of antimarginal microsculpture is plesiomorphic for the Pectinidae, and there are more satisfactory reasons for considering Camptochlamys to be allied to Camptonectes. Specifically, Camptochlamys shares with Camptonectes an opisthocline prosogyrate shell, anterior disk flanks that are distinctly concave in lateral view, and the common presence of strongly projecting commarginal flanges. As noted by Johnson (1984), antimarginal striae are obscure or absent in some Camptochlamys, e.g., C. clathrata, but prominent in others, e.g., C. obscura. For reasons stated above, Camptochlamys is elevated herein to the genus rank within the subfamily Camptonectinae, as are the other genus-group names within the subfamily.
Johnson (1984) reduced 27 nominal Jurassic Camptochlamys species to only two that he considered to be valid. One of these, Camptochlamys clathrata (Roemer, 1836), has radial costae that persist throughout ontogeny, reaching the ventral margin even in the largest specimens. The other species, C. obscura (J. Sowerby, 1818), has radial costae that fade out within at most a few centimeters of the umbo. Both of these species are limited to the Jurassic, C. clathrata ranging from the Bajocian into the Kimmeridgian and C. obscura ranging from the Aalenian into the Tithonian (Johnson, 1984, p. 140, 147). The geographic ranges of the two species, and presumably their habitat preferences, differ. Camptochlamys obscura, according to Johnson (1984, p. 140), "was essentially a Boreal species," which at first was centered in northern Europe and later spread into Britain and possibly into Asia and Australia. The geographic distribution of C. clathrata broadly overlaps that of C. obscura, but according to Johnson (1984, p. 148), C. clathrata was more environmentally constrained, its occurrence correlated with accumulations of coral debris suggesting nearby coral reefs. Kanjilal (1979) introduced the subgeneric name Camptonectes (Indonectes) for species in which radial costae are limited to the umbonal region, choosing Camptochlamys obscura as the type species. In view of Johnson's (1984) more extensive study of Jurassic Camptochlamys, in which the umbonal limitation of radial costae is considered to be a feature of a single variable species, Kanjilal's subgeneric name seems unnecessary. Johnson (1984, p. 142, 149) considered Camptochlamys obscura to be the ancestor of C. clathrata, the former having evolved probably from Camptonectes auritus through heterochronic change associated with phyletic size increases. He further suggested (p. 136, 142) that Camptonectes cinctus (J. Sowerby, 1822) may be a phyletic descendant of Camptochlamys obscura. Camptonectes cinctus is the type species of Boreionectes Zakharov, 1965, which has been shown to be a junior synonym of Mclearnia Crickmay, 1930 (Kelly et al., 1984, 1986). The hy- pothesis that Mclearnia (= Boreionectes) is descended from Camptochlamys is unlikely, however, in the light of information that has been published since Johnson's monograph was completed. The oldest species of Mclearnia thus far described, Mclearnia broenlundi (Ravn, 1911), occurs in the middle Oxfordian of east Greenland (Fürsich, 1982), and differs from later occurring Mclearnia in having a narrower umbonal angle, a less flattened right valve, and a smoother shell without well-developed commarginal flanges. None of the Mclearnia species are known to have radial costae. The earliest Camptochlamys appear in the Aalenian and Bajocian (Johnson, 1984), before the first Mclearnia, and the earliest members of the former already have a flattened right valve with well-developed radial costae. It thus appears that Camptochlamys and Mclearnia are each independently derived from Camptonectes.» WALLER, T. R. & L. MARINCOVICH JR. 1992. New species of Camptochlamys and Chlamys (Mollusca: Bivalvia: Pectinidae) from near the Cretaceous/Tertiary boundary at Ocean Point, North Slope, Alaska. Journal of Paleontology, 66 (2): 215-227, figs. 1-5. [p. 219, 220]
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