Argopecten charltonius (Mansfield, 1936)
MANSFIELD, W. C. 1936. Stratigraphic significance of Miocene, Pliocene and Pleistocene Pectinidae in the Southeastern United States. Journal of Paleontology, 10 (3): 168-192, pls. 22-23. [p. 190, pl. 23, figs. 10-11]
1936 Pecten (Chlamys) charltonius Mansfield, 1936
W. C. Mansfield, 1936, plate 23.
|
«Shell rather small, solid; valves inflated, the left slightly more so than the right, inequilateral, the posterior region being more produced. Right anterior ear with a moderately deep notch, sculptured with about 4 rather strong radials. Right posterior ear sculptured with 3 or 4 weak radials on figured cotype but on some other cotypes with 7 or 8 radials. Ribs low, rather wide, nearly smooth, flat, 17 on each valve; interspaces shallow, round bottomed, a little narrower than ribs. Submargins without radials. Concentric lamellae more strongly developed between the ribs than on them.
Dimensions of syntypes (U. S. Nat. Mus. 373080): Right valve, length, 40 mm.; height, 37 mm.; convexity, 11 mm. Left valve, length, 40 mm.; height, 37 mm.; convexity, 12 mm. Pecten charltonius appears to be closely related to Pecten evergladensis charlottensis Mansfield, from the Pliocene Caloosahatchee marl, but differs from the latter in having a smaller shell, and fewer, wider, and less squarely cut ribs. Type locality: Station 6427, south abutment of Atlantic Coast Line R.R. bridge, St. Marys River, Nassau County, Florida; G. C. Matson, collector. Occurrence : Charlton formation, probably of Pliocene age.» WENDELL CLAY MANSFIELD, 1936
|
«Comparison. Argopecten charltonius only superficially resembles A. vicenarius charlottensis and A. anteamplicostatus of the Caloosahatchee Marl. The Charlton species differs from these in having very low and curved disk flanks, a right anterior auricle with a more commonly truncate anterior margin and a more prominent byssal fasciole, and more rounded plicae. In the nature of its disk flanks and right anterior anricle, A. charltonius is much closer to A. choctawhatcheensis, which it resembles also in having the early growth of the right valve proceeding at a low spiral angle that then increases later in ontogeny. A. charltonius differs, however, in having more prominent, broader auricles with straighter posterior free margins.
Stratigraphic range. Known only from the Charlton Formation, here considered middle Miocerre in age, possibly equivalent to the Arca Faunizone (see section on stratigraphy). Geographic distribution and ecology. A. charltonius is thus far known only from the area of exposure of the Charlton Formation along the Saint Marys River in northeastern Florida and southeastern Georgia. A number of peculiarities concerning the occurrence of the largest known sample (locality 71) of the species are relevant to an interpretation of its living habitat. First, the lithology in which the sample has been found is highly unusual for Argopecten. It is an exceedingly fine, tan, waxy clay with some sand-size grains of quartz and some tiny lenses of pure quartz sand. Second, the pectens are associated with abundant, wellpreserved ostracodes, apparently all of which are Cyprideis ovata (Mincher) and are representative of a restricted environment of low salinity (Philip Sandberg, pers. comm., 1968). The ostracodes are scattered within the fine clay but also occur greatly concentrated in small lenses of "ostracod coquina." Third, the pectens themselves are all highly abraded and appear to fall into two size classes (modal classes of 15 to 20 mm and 30 to 35 mm in height). Furthermore, almost all of the individuals in the larger size class have an almost identical irregular pattern of growth rings, indicating that they lived simultaneously and that their growth was highly irregular, with numerous pauses or retardations in rates of shell extension. Because no living scallops are known to survive in brackish water and no Argopectens are known to live on extremely soft mud or in turbid waters, these facts appear to indicate that the specimens on hand lived inshore, in an estuary or sound, in waters of normal or nearly normal salinity. Periodic changes in the environment —perhaps changes in salinity or in turbidity— interrupted or retarded growth in the population. One of these changes probably killed the population, which then may have been eroded or corroded and subsequently buried by brackish-water muds.» Nothing is known of geographic variation of the species or whether it was also able to live under open-marine conditions.» 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. 56, 57]
|
Argopecten charltonius (Mansffeld ); T. R. Waller, 1969, The Evolution of Argopecten gibbus Stock, plate 4, figures 3-8.
|