Pecten mclellani Gibson, 1987
GIBSON, T. G. 1987. Miocene and Pliocene Pectinidae (Bivalvia) from the Lee Creek Mine and adjacent areas. Smithsonian Contributions to Paleobiology, 61: 31-112, pls. 1-31. [p. 43, pls. 1, 2; pl. 13, fig 6]
1897 Pecten maclellani Gibson, 1987
T. G. Gibson, 1987, plates 1, 2, 13.
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«DESCRIPTION.— Shell Outline: Shell medium to large in size, with specimens having heights of 80 to 100 mm common in the population sample, and reaching a maximum height of 130 mm; slightly longer than high, with a maximum length of slightly greater than 120 mm; right valve low to moderately convex with a maximum convexity of 17 mm; left valve flat to slightly convex with a maximum convexity of 9 mm; outline of disk equilateral, with almost identical anterior and posterior half-diameters.
Auricles and Outer Ligament: Right anterior auricle with a slightly convex surface; dorsal margin slightly dorsal to groove of outer ligament and folded; byssal notch very shallow with broad apex; byssal fasciole poorly developed; ctenolium observable on right valves. Left anterior auricle with surface nearly planar to slightly convex; dorsal margin slightly folded and nearly coincident with trace of outer ligament; free margin slightly rounded with shallow byssal sinus having a rounded apex. Posterior auricles similar in size to anterior; dorsal margin slightly dorsal to groove of outer ligament and folded; free margin slightly convex, especially near dorsal margin. Posterior and anterior outer ligaments generally about equal in length. Exterior Shell Surface: Both valves with about 12 plicae; in early growth stages plicae defined from interspaces by low, moderately rounded sides until shell heights of approximately 50 mm are reached, thereafter plicae becoming less strongly defined from interspaces with very broad, low sides, until becoming obscure at margins of large valves; no secondary radial ornamentation on plicae or interspaces; concentric lamellae well developed on left valves, less so on right valves being primarily preserved on the disk flanks. Anterior and posterior auricles have approximately 8 radial costae, moderately developed on the initial 10 mm of the auricle, becoming obsolete beyond this point; concentric lamellae well developed on auricles. Disk flanks of right valves have 4 moderately developed radial costae on initial 15 mm along with moderately developed concentric lamellae; left valves have smooth disk flanks except for welldeveloped concentric lamellae. Interior Features: Resilial insertions slightly less than 1½ times as high as long, orientation ranging from perpendicular to hinge line to slightly sloping either anteriorly or posteriorly; single, strongly developed auricular denticle both anteriorly and posteriorly; additional one or two costae found within the interior plicae. DISCUSSION.—The only closely related species in the Miocene strata ofthe Atlantic Coastal Plain is Pecten humphreysii Conrad, which is found in slightly lower beds ofthe Pungo River Formation in the Lee Creek Mine and in the Calvert Formation in northernmost Virginia, Maryland, and New Jersey. Pecten mclellani is distinguished from this species by the ornamentation, position ofthe byssal notch relative to the end of anterior outer ligament, and convexity of the right valve. The number of plicae in P. mclellani is greater, ranging from 10 to 13 in number, while P. humphreysii has from 8 to 10 (7 or 8 broad and prominent ones and two lateral ones quite reduced in size, which arc more prominent on the interior ofthe valve). Pecten humphreysii has much wider plicae on the right valve, with relatively narrower interspaces and, correspondingly on the left valve, has relatively narrow plicae with wide interspaces. The plicae on both valves are quite pronounced with vertical sides in some specimens. P. mclellani has considerably narrower plicae on the right valve with the interspaces more nearly approaching equal width with the plicae; on the left valve correspondingly are wider plicae than in P. humphreysii with narrower interspaces. The plicae are very slightly raised in comparison with P. humphreysii and do not have the vertical sides common in the latter. Figures 7 and 8 show the significantly greater width of the plicae on the right valves of P. humphreysii. (The measurements are taken internally as the sharpness of the plicae is increased on the interior, giving more accurate measurements.) Although the byssal notch is shallow in both species, the position of the notch in relation to the anterior end of the outer ligament is different between the two species (Figure 9). In P. humphreysii the posterior end ofthe byssal notch is 1 to 3 mm posterior to the anteriormost point of the outer ligament. In P. mclellani the posterior end of the byssal notch is from 0.5 mm posterior to almost 1 mm anterior to the anteriormost end of the outer ligament. The right valves of P. humphreysii have considerably greater convexity compared to those of P. mclellani as shown in Figure 10. As P. humphreysii is found in beds of the Pungo River Formation, below those in which P. mclellani occurs, and as the two species have similar morphology, it is considered likely that P humphreysii is ancestral to P. mclellani. One important phylogenetic change between the two species is a decrease in the strength and width of the plicae in the descendant species. Mature specimens of P. humphreysii do show a decrease in the strength of the plicae in the later stages of the valve to a height and sharpness similar to that found in the early stages of P. mclellani. There is a general similarity in the shell shape, ornamentation, both internal and external, and byssal characters between P. mclellani and the later occurring Pecten holmesii Dall from the Waccamaw Formation in South Carolina. No related species of an intermediate age is known, and the similarity may not reflect any relationship. An increase in convexity ofthe right valve of P. mclellani, however, along with a further reduction in the largely obscure plicae would give a form similar to that of P. holmesii. Pecten ochlockoneensis Mansfield from the Choctawhatchee Formation in Florida and P. smithi Olsson from the Yorktown Formation in Virginia differ from P. mclellani in having many more plicae on both valves and in being much smaller in size. In addition, P. ochlockoneensis has a more convex right valve. These two species are most similar to P. mclellani, but are doubtfully congeneric. NAME.— The species is named in honor of Mr. Jack McLellan of Texasgulf Inc., who collected and donated many specimens of pectens and other invertebrates from the Lee Creek Mine. TYPES.— Holotype: right valve, USNM 218830. Figured paratypes: right valve, USNM 218828; right valve, USNM 218829; Icfl valve, USNM 218831; left valve, USNM 218865; unfigured paratypes, USNM 218933, 362976, 362977, 362978, and 362979. STRATIGRAPHIC AND GEOGRAPHIC RANGE.— The only known occurrence of this species is in the upper part of the Pungo River Formation of early middle Miocene age in the Lee Creek Mine in North Carolina. Well-preserved specimens are found in the upper 6-12 feet (1.8-3.7 m) (units 4-7 of Gibson, 1967) of the Pungo River Formation, some being found just an inch (2.5 cm) below the contact with the overlying Yorktown. This mine contains the only known outcrop of this formation in North Carolina, the formation being limited to the subsurface except for this artificial exposure. Strata of this age containing calcareous fossils do not naturally crop out south of north central Virginia. TABLE 1.— Measurements (in mm) of a representative sample of
Pecten mclellani, new species.
MEASURED MATERIAL.— Total specimens measured include: 3 right valves and 1 left valve from USGS 25743 and 3 right valves and 1 left valve from USGS 25757. A population sample from the Pungo River Formation at Lee Creek Mine, USGS 25743 and 25757, consisfing of 8 specimens, 6 right valves and 2 left valves, was measured. Measurements of a representative sample of 3 specimens are given in Table 1 (USNM 218830, holotype, and USNM 218828, both from USGS 25743; USNM 362976 from USGS 25757).»
THOMAS G. GIBSON, 1987
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«Gibson (1987) figured a number of specimens of P. humphreysii humphreysii, P. humphreysii woolmani, and P. mclellani, and it appears to this author that they may all be variants of the same species.»
WARD, L. W. 1998. Mollusks from the Lower Miocene Pollack farm site, Kent County, Delawere: a preliminary analysis. In: R. N. Benson, ed., 1998, Geology and paleontology of the lower Miocene Pollack Farm Fossil Site, Delaware: Delaware Geological Survey Special Publication No. 21, p. 59–131. [79]
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