Spathochlamys Waller, 1993
WALLER, T. R. 1993. The evolution of Chlamys (Mollusca: Bivalvia: Pectinidae) in the tropical western Atlantic and eastern Pacific. American Malacological Bulletin, 10(2): 195-249, figs. 1-14. [p. 229]
«Spathochlamys, new genus
Etymology.— Based on the Greek "spathe," meaning spade, in reference to concave-up scales, combined with the genus name Chlamys, which in turn is based on the Greek word "chlamys," meaning "mantle".
Diagnosis.— Mimachlamydini with rounded or broadly trigonal ribs separated by interspaces each containing a single very narrow riblet; crests of major ribs bearing narrow erect scales that are concave on their upper (dorsal) sides; edges of ribs on inner surfaces of valves strongly carinate; microsculpture in early rib interspaces obscure or dorninated by commarginal lirae which cross interspaces without strong curvature.
Type species.— Pecten benedicti Verrill and Bush in Verrill, 1897, extant, western Atlantic.
Other Species.— Pecten vestalis Reeve, 1853 [= Pecten (Chlamys) lowei Hertlein, 19351, extant, eastern Pacific, and P. vaginulus Dall, 1898, fossil, Miocene-Pliocene, Caribbean region and southeastern United States.
Geographic range.— Western Atlantic from at least Massachusetts to Brazil and throughout the Gulf of Mexico and Caribbean; eastern Pacific and Gulf of California from southern California to Ecuador and westward to the Galápagos Islands.
Stratigraphic range.— Upper Miocene to present.
Discussion.— The new genus differs in a number of qualitative features from Mimachlamys. Both the extant and fossil species of Spathochlamys have much more strongly developed carinate edges on the ribs on the inner shell surface than do species of Mimachlamys, where these carinae are sometimes minimally developed or restricted to early growth stages (as in M. varia, an extant species of the eastern Atlantic). Concave-up scales, which form atop at least some of the ribs in Spathochlamys, are unknown among extant species of Mimachlamys, where scales are concavedown (i.e. concave side facing toward the ventral margin). Some extinct European middle and upper Tertiary Mimachlamydini, e.g., M. angelonii (De Stefani and Pantanelli, 1878), have concave-up scales but lack medial riblets, and other aspects of their morphology suggest that they have a phyletic history that is independent of that of Spathochlamys.
There is a strong resemblance in ribbing pattern between Spathochlamys and Dimarzipecten Ward, 1992, a genus recently introduced to accommodate what its author believed to be a single species, Pecten crocus Cooke, 1919, that was originally described from the early Miocene (or late Oligocene) of Anguilla. Ward (1992) stressed that Dimarzipecten has a narrow stratigraphic range (restricted to lower Miocene and possibly upper Oligocene) and a broad geographic range extending from the Antilles to the Belgrade, Edisto, and Tampa Formations and correlative stratigraphic units of the Carolinas, Georgia, and Florida. However, I have identified a second apparently undescribed species of Dimarzipecten in the Chipola Formation of Florida (USNM(P) 474643 and 474644, from Farley Creek, NE1I4 sec. 21, TIN, R9W, Calhoun Co.). This extends the stratigraphic range of the genus into the upper lower Miocene (Burdigalian in age; see Akers, 1972: 10). Like Spathochlamys, Dimarzipecten has a Chlamys-like shape, ribs that become trigonal in cross-section at least in late ontogeny, narrow scales atop the ribs, and a fairly regular ribbing pattern with a single medial riblet in each interspace. Dimarzipecten, however, differs from Spathochlamys in lacking internal rib carinae and in having convex-up rather than concave-up scales on the rib crests. Furthermore, Dimarzipecten has a plesiomorphic microsculptural pattern of coarse antimarginal striae in the rib interspaces. The configuration of these striae is like that present in many modem Mimachlamys, including a herringbone-like configuration in rib interspaces in the center of the disk in early ontogeny. In Spathochlamys, antimarginal striae are absent or obsolete in early ontogeny except on and near the disk flanks, and commarginal lirae are present in the rib interspaces. A third genus, possibly new and referred to herein as Genus A pending further research, is characterized by Pecten sansebastianus Maury, 1920, of which P (Chlamys) portoricoensis Hubbard, 1920, is a junior synonym. This species occurs in the San Sebastian Shale and lower Lares Limestone of Puerto Rico (Maury, 1920; Hubbard, 1920), formations that are considered middle to late Oligocene in age (Maurrasse, 1990, fold-out correlation chart, Column 20). Genus A has a high, narrow shape like that of Dimarzipecten, and as in members of that genus, a small medial riblet appears in most rib interspaces in late ontogeny. Like Spathochlamys, Genus A has minute concave-up scales on its rib crests and the ribs have strongly carinate edges on the inner surfaces of the valves. Genus A differs from both Dimarzipecten and Spathochlamys in having a distinctive Aequipecten-like microsculpture in rib interspaces in early ontogeny. This pattern consists of wavy cornmarginal lirae that are strongly looped in a dorsal direction on the rib flanks and in a ventral direction in the rib interspaces; the looped lirae on the rib flanks may become cuspate, as in some Aequipecten and in most Cryptopecten. Like Dimarzipecten and Spathochlamys, Genus A lacks a foliated-calcite transgression on the inner surface of its umbonal region, but this is a plesiomorphic state that cannot serve as an indicator of close relationship. Tentatively, Genus A is considered a member of the tribe Aequipectinini based on its rnicrosculptural pattern. If Spathochlamys originated from a species of Dimarzipecten, then it seems likely that this origin was within the middle Miocene. The latest Dimarzipecten thus far uncovered is the Chipola (Burdigalian) species referred to above, and the earliest undoubted Spathochlamys are the specimens of S. vaginula, late Miocene and early Pliocene, referred to below. Although fossil specimens of Spathochlamys are rare, those that have been brought together for this study suggest that S. vaginula of the Miocene and early Pliocene gave rise to both of the extant species but at different times. S. vestalis of the eastern Pacific probably originated in the late Miocene; S. benedicti of the western Atlantic probably originated in the late Pliocene.» THOMAS RICHARD WALLER, 1993
|
Spathochlamys.benedicti; T. R. Waller, 1993, The evolution of Chlamys (Mollusca: Bivalvia: Pectinidae) in the tropical western Atlantic and eastern Pacific, figures 10a-10k.
|