Argopecten choctawhatcheensis (Mansfield, 1932)
MANSFIELD, W. C. 1932. Miocene Pelecypods of the Choctawhatchee Formation of Florida. Florida Geological Survey Bulletin, 8: 7-164, pls. 1-34. [p. 63, pl. 9, figs. 4-8]
1932 Chlamys (Plagiocternium) choctawhatcheensis Mansfield, 1932
1932 Chlamys (Plagiocternium) choctawhatcheensis redbayensis Mansfield, 1932
1932 Chlamys (Plagiocternium) choctawhatcheensis redbayensis Mansfield, 1932
W. C. Mansfield, 1932, plate 9.
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«Shell rather small, suborbicular, left valve more convex than right, inequilateral — the anterior region more produced than the posterior. Right valve sculptured with 21 to 22 broadly rounded to nearly flat ribs, separated by interspaces that are about half the width of ribs. Left valve strongly inflated over the middle of the disk, sculptured with about 21 ribs similar to those on right valve. Submargins without radials. Right ear longer than left, deeply sinuate, and ornamented with four rather strong. crenulated radials. Left ear ornamented with about two more radials than right.
Cotypes (Cat. No. 371142, U. S. N. M.) measure: Right valve: Length, 37 mm.; height, 36 mm.; diameter, 8 mill. Left valve: Length, 41 mm.; height, 42 mm.; diameter, 14 mill. This species differs from Chlamys comparilis (Toumey and Holmes) in having a more convex left valve and lower, less distinct, and more closely set ribs. The margin of the ribs on Chlamys comparilis projects over the interspaces, whereas in C. choctawhatcheensis the interspaces are narrow, shallow, and rounded. The new species is closely related to Chlamys levicostatus Toula from the Gatun formation of Panama and perhaps should be regarded as a subspecies of C. levicoslatus. C. levicostatus has a smaller shell with more closely spaced ribs and more inflated left valve than the new species. Occurrence: Upper middle Miocene: Arca zone — vicinity of Red Bay, Walton County, Fla. (Cotypes from Mr. Anderson's farm, three-fourths of a mile east of Red Bay); station 12046, Vaughan Creek, upper locality, Walton County: Station 12047, Vaughan Creck, lower localily (spccimens small, identification uncertain) . The specimens from staton 12046 may be a variety of C. choctawhatcheensis, but they are more closely related to this species than to Chlamys (Plagioctenium) nicholsi Gardner from its type locality». WENDELL CLAY MANSFIELD, 1932
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«Comparison. Argopecten choctawhatcheensis differs from A. comparilis in being more left-convex, particularly in the upper portion of its size range, and in having lower, generally more rounded plicae, a less rounded anterior margin on its right anterior auricle, and disk flanks that are lower and more rounded in planes perpendicular to them. Certain localities in the Alaqua Creek Valley (localities 10 through 12), which carry the earliest fauna of the Arca Faunizone according to Mansfield (1932a, p. 14), also contain A. choctawhatcheensis that very closely resembles A. nicholsi, from the upper portion of the Shoal River Formation. In fact the dividing line between the species is quite arbitrary, indicating that the time differences between these beds must be slight (see comparison in following description of A. nicholsi),
Stratigraphic range. Upper middle Miocene ("Red Bay Formation" and Arca Faunizone). Geographic distribution and ecology. Thus far A. choctawhatcheensis has been found only in Western Florida in the area of exposure of the Arca Faunizone. Little has been written about the detailed paleoecology of this unit, except that it represents cooler waters than does the Shoal River Formation but warmer waters than do the succeeding Ecphora and Cancellaria Faunizones. Puri (1953, p. 49), without referring to any given locality, concluded that the Arca "facies" represents an outer-neritic environment at water depths between 30 and 100 m. In the absence of any more specific information, it may be inferred that A. choctawhatcheensis lived in a similar environment. Remarks on classiffcation. Mansfield (1932a, p.64) based a subspecies, A. choctawhatcheensis redbayensis, upon a single, poorly preserved left valve that differs from typical members of the species only in having fewer plicae (18 or 19 compared to 20 to 22) and in having a disk outline that appears to have a lower height/length ratio. However, other specimens in the collections of the U.S. Geological Survey from the same locality as the type of redbayensis (locality 21) appear to be typical choctawhatcheensis. Apparently, therefore, no good evidence, either morphological or geographical, exists for the recognition of the subspecies». 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. 55, 56]
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Argopecten choctawhatcheensis (Mansfield); T. R. Waller, 1969, The Evolution of Argopecten gibbus Stock, plate 3, figures 9-18; plate 4, figures 1,2.
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