Loxochlamys Waller & Stanley, 2005
WALLER, T. R. & G. D. STANLEY JR. 2005. Middle Triassic Pteriomorphian Bivalvia (Mollusca) from the New Pass Range, west-central Nevada: Systematics, biostratigraphy, paleoecology, and paleobiogeography. Journal of Paleontology, [Memoir Paleontological Society 61], 79 (1): 1-59, figs. 1-14. [p. 40]
«Genus LOXOCHLAMYS new genus
Type species.— Loxochlamys corallina n. sp., Middle Triassic (late Ladinian), South Canyon, New Pass Range, Nevada (Fig. 12.1–12.6).
Included species.— Pecten chiwanae McLearn, 1941a (Fig. 12.7–12.9), and Pecten sasuchan McLearn, 1941a (Fig. 12.10), both from the Upper Triassic (Carnian) of the Peace River foothills, northeastern British Columbia, Canada.
Diagnosis.— Equivalved, slightly opisthocline Pectinidae with a persistent byssal notch of moderate depth; ornament of disk consisting of noncarinate radial plicae or radial undulations overlain by a distinctive shagreen or near-shagreen microornament that merges into antimarginal ridgelets near and on disk flanks and is replaced by coarse commarginal lamellae near ventral margin.
Description.— Shape equivalved, procrescentic, and slightly opisthocline. Disk ornament consisting of simple radial plicae that vary in amplitude among species from obscure radial undulations to high-amplitude plicae but in all cases lacking internal carinae, and fine commarginal growth lines that tongue ventrally in plical interspaces and dorsally across plicae, becoming replaced by coarse commarginal lamellae in late ontogeny; shagreen or near-shagreen microornament present through most of ontogeny, giving way to antimarginal ridgelets in anterior and posterior sectors of disk and extending onto disk flanks. Byssal notch of moderate to substantial depth, with a fine but persistent functional ctenolium. Auricles commonly non-costate, rarely with a few weak costae. Outer shell layer of mainly antimarginal fibrous calcite and an extensive inner aragonitic layer extending nearly to margins on both disk and auricles; presence of an outer prismatic layer in early ontogeny of right valve not observed.
Etymology.— The new genus name combines the Greek adjective, loxos, with the pectinid genus name Chlamys. The Greek adjective, meaning slanting or crosswise, calls attention to the peculiar shell microornament present in all members of the new genus.
Occurrence.— The new genus is known only from the Triassic of Nevada and British Columbia, with a stratigraphic range from Ladinian (Nevada) to Carnian (British Columbia).
Discussion.— The combination of characters described above sets Loxochlamys apart from all other known pectinid genera. Among Triassic Pectinoidea, a near-shagreen microornament is known only in Pecten rosaliae Salomon, 1895, from the Ladinian of Italy (Allasinaz, 1972, p. 307). The holotype of this species in
Münich (Bayerische Staatssammlung für Paläontologie und historische Geologie, Catalogue No. 1892.X.32), which I examined, is a right valve that bears a ctenolium associated with a deep byssal notch and a distinct byssal fasciole. Its outer shell layer is of undetermined microstructure but does not appear to be simple columnar prismatic. The presence of a ctenolium and lack of a persistent columnar prismatic shell layer contradict Allasinaz’s (1972, p. 307) placement (with a query) of this species in Filopecten Allasinaz, 1972, because Filopecten is an entoliid genus that lacks a ctenolium throughout ontogeny and has a persistent columnar prismastic outer shell layer on its right valve. Although clearly a member of the Pectinidae, Pecten rosaliae differs from members of Loxochlamys in lacking both radial and commarginal ornament and in having broad, low commarginal undulations. Other aspects of the shell microstructure of P. rosaliae are unknown. The antimarginal fibrous calcitic outer shell layer, configuration of commarginal growth lines that parallel a dentate disk margin, and presence of simple plicae that commonly are trigonal in cross section are features that Loxochlamys shares with Pseudopecten dentatus (J. de C. Sowerby, 1827), a species that is distributed mainly in the Lower and Middle Jurassic (lowest Hettangian to highest Bajocian) of the western Tethys (Johnson, 1984, p. 75). Pseudopecten dentatus, however, is prosocline, more equilateral and flaring, and lacks shagreen or near-shagreen microornament. Furthermore, the plicae of P. dentatus are internally carinate, its ctenolium becomes overgrown and ceases to be functional in late ontogeny, its antimarginal fibrous calcite is limited to only the outermost part of the outer shell layer, which otherwise is foliated calcite, and its ligament system migrates ventrally during ontogeny (new observations). A possible ancestor of P. dentatus postulated by Johnson (1984, p. 77), Pseudopecten coronatiformis (Krumbeck, 1924), from the Upper Triassic of Timor, has the sculptural features of Pseudopecten (Pseudopecten) Bayle, 1878 and not of Loxochlamys. The same applies to Chlamys coronata (Schafhäutl, 1851) from the uppermost Norian of Europe (Johnson and Simms, 1989, p. 192). On both stratigraphic and morphological grounds, the origin of Loxochlamys is probably from an ancestral species in Praechlamys, a genus that was already well diversified by Ladinian time in Europe (Allasinaz, 1972). If so, the lack of well-developed auricular costae in Loxochlamys is a carryover from Praechlamys, as is also the opisthocline shape, apparent lack of typical antimarginal microornament, and the antimarginal or radial fibrous microstructure of the outer calcitic shell layer.» THOMAS RICHARD WALLER & GEORGE D. STANLEY, 2005
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Loxochlamys corallina n. gen. and sp.; T. R. Waller & G. D. Stanley Jr., 2005, Middle Triassic Pteriomorphian Bivalvia (Mollusca) from the New Pass Range, west-central Nevada: Systematics, biostratigraphy, paleoecology, and paleobiogeography, figures 12.1-12.6.
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