Argopecten vicenarius vicenarius (Conrad, 1843)
CONRAD, T. A. 1843. Descriptions of a new genus, and of twenty-nine new Miocene, and one Eocene fossil shells of the United States. Proceedings of the Academy of Natural Sciences of Philadelphia, 1st series, 1(30-31): 305-311. [p. 306]
1843 Pecten vicenarius Conrad, 1843
Argopecten vicenarius (Conrad, 1843); E. J. Petuch, 2004, Cenozoic Seas, pl. 75, fig. G.
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«Suborbicular, inequivalved, the superior valve ventricose, the inferior plano-convex; ribs about 20, somewhat flattened on the back; ribs of superior valve narrower and more distant than those of inferior valve; surface of both with crowded regular concentric wrinkles; ears equal, moderate in size; sinus of inferior valve not profound.
Locality. Wilmington, N. C. Mr. Hodge.» TIMOTHY ABBOTT CONRAD, 1843
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«DISCUSSION.— Waller (1969: 49) noted that Argopecten vicenarius vicenarius differs from Argopecten gibbus (Linnaeus, 1758), its Pleistocene and Holocene descendant, in having steeper disk flanks, which are generally noncostate, a more strongly left-convex shell with wider disk gapes, larger auricular denticles, and a ctenolium that is commonly retained into maturity. Waller presumed that the ecological requirements of Argopecten vicenarius vicenarius were the same as living Argopecten gibbus because of morphological similarity and because of the existence in other areas of apparently well-differentiated bay scallops that were probably contemporaneous with, Argopecten vicenarius vicenarius. Waller (1969: 49) stated that these requirements include open, relatively warm marine waters of shallow to moderate depths (5-200 fathoms). This is independently substantiated by the paleoecological studies of DuBar and Howard (1963) dealing with the type Waccamaw outcrops on the Waccamaw River, Horry County, South Carolina, where A. vicenarius, although not abundant, is the most common scallop present. DuBar and Howard (1963: 27) concluded that "essentially all of the type Waccamaw Formation was deposited in an open, unrestricted ocean on the inner shelf (2- 12 fathoms) or the shallow part of the intermediate shelf (12-35 fathoms)."
FIGURED SPECIMEN.— USNM 203900 from USGS 25364. OCCURRENCES.— This species is found at sections 2 (D), 4 (D), and 5 (E). TYPE LOCALITY.-Wilmington, North Carolina (probably Waccamaw Formation), Mr. Hodge, collector. Waller (1969) designated a right valve, syntype of Conrad, ANSP 31416, as the lectotype.» WARD, L. W. & B. W. BLACKWELDER. 1987. Late Pliocene and Early Pleistocene Mollusca from the James City and Chowan Rives Formations at Lee Creek Mine. In: Geology and Paleontology of the Lee Creek Mine, North Carolina, II, C. E. Ray (ed.), Smithsonian Contributions to Paleobiology, 61: 113-283, pls. 1-47. [pp. 142, 143
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Argopecten vicenarius vicenarius (Conrad); L. W. Ward & B. W. Blackwelder, 1987, Late Pliocene and Early Pleistocene Mollusca from the James City and Chowan Rives Formations at Lee Creek Mine, plate 5, figures 7-9.
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«Comparison. Argopecten vicenarius vicenarius differs from A. gibbus in having steeper disk flanks, which are generally noncostate, a more strongly left-convex shell with wider disk gapes, larger auricular denticles, and a ctenolium that is commonly retained into maturity. The features that set the new subspecies apart from A. gibbus are the same features that produce a striking similarity with living thin-shelled forms of A. purpuratus. From this Pacific species, A. vicenarius vicenarius differs in having narrower disk gapes, a shallower byssal notch, less well-developed auricular denticles, generally noncostate disk flanks, and less flattened plicae that lack fringing concentric lamellae and single costae in interspaces.
Stratigraphic range, distribution, and ecology. Argopecten vicenarius vicenarius is known with certainty only from the Pliocene Waccamaw Formation of North and South Carolina. Fragments from the Miocene of New Jersey that Heilprin (1887b, p. 400, 402) and Whitfield (1894, p. 31, 32) questionably identified with Pecten vicenarius have not been reexamined. The ecological requirements of the species are presumed to have been similar to those of living A. gibbus because of morphological similarity and because of the existence, in other areas, of apparently well-differentiated bay scallops that were probably contemporaneous (see discussion of ecology under A. anteamplicostatus). These requirements include open, relatively warm marine waters of shallow to moderate depths (5-200 fathoms). This is independently substantiated by the paleoecological studies of Du Bar & Howard (1963) dealing with the type Waccamaw outcrops on the Waccamaw River, Horry County, South Carolina, where A. vicenarius, although not abundant, is the most common scallop present. Du Bar & Howard (p.27) concluded, "Essentially all of the type Waccamaw Formation was deposited in an open, unrestricted ocean on the inner shelf (2-12 fathoms) or the shallow part of the intermediate shelf (12-35 fathoms). The water temperature is judged to have been in the range of that off the present coast of central and southern Florida." Remarks on classiffcation. Conrad (1843, p. 306) introduced Pecten vicenarius with a brief description and without illustration or comparison. Although Heilprin (1881, p. 420) considered the species to be valid, noting that it is closely related to Argopecten comparilis (Tuomey & Holmes), Dall (1898, p. 249) placed Conrad's species in synonymy wlth A. eboreus (Conrad, 1833), apparently believing that Conrad's Wilmington shells were immatnre individuals. Subsequent anthors have followed Dall and have incorrectly identified these common Waccamaw Argopectens with A. eboreus, A. comparilis, or A. gibbus.» WALLER, T. R. 1969. The evolution of Argopecten gibbus stock (Mollusca: Bivalvia), with emphasis on the Tertiary and Quaternary species of Eastern North America. The Paleontological Society. Memoir 3 [Journal of Paleontology, 43 (5, supplement)]: 125 pp. [p. 49, 50]
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Argopecten vicenarius vicenarius (Conrad); T. R. Waller, 1969, The evolution of Argopecten gibbus stock, plate 6, figures 5-14.
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