"Pecten" perplanus poulsoni Morton, 1834
MORTON, S. G. 1834. Synopsis of the organic remains of the Cretaceous group of the United States. 88 p. and Appendix, pls. 1-19. W. P. Gibbons, Printer. Key & Biddle, Minor Street. Philadelphia. [p. 59, pl. 19, fig. 2]
S. G. Morton, 1834, plate 19.
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«Specific character. Suborbicular; superior valve flat; ribs fourteen, not profoundly elevated, with crowded wrinkled striae; inferior valve ventricose, with prominent rounded ribs, the intervals striated; ears subequal.
Common in the newer cretaceous deposits near Claiborne, Alabama. I have much pleasure in naming this shell after my friend Charles A. Poulson, Esq., of this city.»
SAMUEL GEORGE MORTON, 1834
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«Glawe (1969), by plotting the frequency distribution of several morphotypes within stratigraphically successional population samples, showed that the "Pecten" perplanus stock during the early Oligocene split into two lineages, both derived from "Pecten" perplanus perplanus Morton, 1833. The first lineage evolved from this species to "P." perplanus poulsoni Morton, 1834, to "P." perplanus byramensis Gardner, 1945. The other evolved from "P." perplanus perplanus to "P." howei mariannensis Glawe, 1969, to "P." howei howei (Mansfield 1940). The "Pecten" perplanus stock ended, so far as known, with the last occurrence of "P." howei howei in the Chickasawhay Limestone and Paynes Hammock Formation of the American Gulf Coastal Plain in the late Rupelian or early Chattian (Glawe 1969: 57; Seisser 1984, fig. 3; Berggren et al. 1995: fig. 3).»
WALLER, T. R. 2006. New Phylogenies of the Pectinidae (Mollusca: Bivalvia): reconciling Morphological and Molecular Approaches. In S.E. Shumway & G.J. Parsons (Ed.) 2006: Scallops: Biology, Ecology and Aquaculture, 1-44. [p. 22]
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Pecten poulsoni Morton; C. W. Cooke & S. Mossom, 1929, Geology of Florida, plate 7, figures 1a, 1b.
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«DIAGNOSIS: Pecten perplanus having ribs of the right valve of medium height, rounded to square in transverse profile and unilirate in the adult portion of the disc of the dominant form.
DESCRIPTION: Shell orbicular, thin to moderately thick, small; maximum height 34.3 mm (mean height 20.17 mm), maximum length 37.2 mm (mean length 20.88 mm); unequally biconvex, the right valve moderately to very deeply convex (mean I:H = 0.31), the left slightly to moderately convex (mean I:H = 0.14); hinge line slightly concave in right valve, nearly straight in left valve; ribs of medium height, rounded to square shouldered and smooth, unilirate, or trilirate in the adult portion of the disc; ribs slightly wider than interspaces; right valve with 16 to 21 ribs (mean 18.5); left valve with 15 to 22 ribs (mean 18.4); concentric lamellae widely spaced with ventrally directed imbrications over the ribs. Ears rather large, subequal, prominently striated with 3 to 6 radial threads on each; byssal notch moderately deep; ctenolium visible in small individuals. Hinge with provinculum, two pairs of cardinal crura (occasional specimens have a rudimentary third posterior crus); one pair of auricular crura terminating distaily in a rounded, oblong ridge; resilial pit rather small, fringed with laterai ridges in the right valve. Adductor muscle scars large; a single scar in the left valve; scar in right valve observable but structure (i.e., whether divided or not) not discernible; interior of valves with faint internal ribs which are fluted along the ventral margin. VARIATION: Populations of P. perplanus poulsoni include morphotypes A, B, and C, the proportion of B being 50 percent or more (see fig. 15). Variation in other characters of P. perplanus poulsoni is shown in table 5. Specimens from the detrital facies differ from those from the nondetrital facies in having more convex right valves with slightly fewer ribs, Though the difference in number of ribs is statistically significant, the numerical difference is so small as to be of doubtful paleontological significance. OCCURRENCE: Mint Spring Marl, Marianna Limestone, and lower Glendon Limestone of Mississippi; and upper Marianna and lower Glendon Limestone of southwestern Alabama. LOCALITIES: Samples 00, 01, 02, at Vicksburg, Miss. (composite section 01); sample 00 at Forest Hill, Miss. (composite section 02); samples 00, 01 at Brandon, Miss. (section 03); samples 00, 01, 02 near Sylvarena, Miss. (composite section 04); samples 00, 01, 02 near Heidelberg, Miss. (section 05); samples 00, 01, 02, 03, 04 near Waynesboro, Miss. (composite section 06); samples 01, 02, 03, 04 near Millry, Ala. (composite section 07); samples 03, 04, 05, 06, 07 near St. Stephens, Ala. (section 08); sample 03 at Little Stave Creek near Jackson, Ala. (section 09); and sarnples 05, 06 near Suggsvilie, Ala. (section 10). For additional localities, see catalogue of references. AFFINITIES: P. perplanus poulsoni is intermediate between P. perplanus perplanus and P. perplanus byramensis. All three subspecies include populations composed of morphotypes A, B, and C. P. p. poulsoni differs frorn the other two in having 50 percent or more morphotype B. Comparisons of size, convexity, and rib number in the subspecies of P. perplanus are shown in figures 18, 19, and 20. Comparative data for P. perplanus and P. hawei are given in table 8. REMARKS. The syntypes of Pecten poulsoni Morton are labeled Acad. Nat. Sci. Philadelphia, no. 274 from st, Stephens, Alabama (fide Tucker-Rowland, 1936). They consist of five right valves and nine left valves mounted on a card. The third specimen from the top in the right hand column is here designated the lectotype.» GLAWE, L. N. 1969. Pecten perplanus stock (Oligocene) of the Southeastern United States. Geological Survey of Alabama Bulletin 91: 1-179, pls. 1-5. [p. 48, 49, 51]
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Pecten perplanus poulsoni Morton, 1834; L. N. Glawe, 1969, Pecten perplanus stock (Oligocene) of the Southeastern United States, plate 1, figures 3, 5; plate 3, figures 1, 3, 4, 6.
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«Pecten (Pecten) poulsoni Morton has long been used as an index fossil for the Vicksburg Group of the Gulf Coastal Plain by field stratigraphers and paleontologists. The Pecten poulsoni problem arises because more than one form of Pecten poulsoni is present in the Vicksburg Group. To solve this problem many stratigraphers and paleontologists: (1) group together all Vicksburgian Pectinidae and refer to them collectively as Pecten poulsoni, (2) recognize several forms and refer to them as Pecten cf. P. poulsoni, or (3) refer to the lower Vicksburg (Mint Spring, Marianna and Glendon) form as Pecten poulsoni Morton and the upper Vicksburg (Byram) form as Pecten byramensis Gardner (Text-fig. 1).
In general, the Pecten poulsoni problem is due not to ignorance or to disinterest on the part of workers, but rather to inaccessibility and confusion in the literature and of the holotype locality. Pecten poulsoni Morton was originally described from "the newer cretaceous deposits near Claiborne, Alabama" (Morton, 1834). This locality is so vague, both geographically and stratigraphically, that collections made in this general area subsequent to Morton's have yielded several species of Pecten ranging in age from Claibornian (Eocene) to Vicksburgian. Morton's generalized drawing and description of Pecten poulsoni are such that one cannot differentiate one form of Vicksburg Pecten from another, although it is obvious that Morton was describing the common Vicksburg Pecten rather than the Claibornian or Jacksonian species. Furthermore, the present whereabouts of the syntypes of Pecten poulsoni are not known (personal communication, Curator, Philadelphia Academy of Science). In 1834, Morton described Pecten perplanus and, in 1933, Cooke demonstrated it to be the left valve or Pecten poulsoni and therefore a junior synonym of Pecten poulsoni Morton. Rowland-Tucker (1936) examined the syntypes of Pecten poulsoni and expanded the original description to include all Vicksburgian Pectinidae with the exception of Pecten perplanus Morton, which she referred to as Chlamys perplanus. She contrasted Pecten poulsoni and Chlamys perplanus, but failed to discuss or photograph independently Chlamys perplanus. She unfortunately overlooked the work of Cooke (1933). Gardner (1945) split Pecten poulsoni in two species, "Pecten (Pecten) sp. cf. P. (P.) poulsoni Morton and Pecten (Pecten) byramensis Gardner." She established P. byramensis on the folowing criteria: (1) greater convexity of the right valve, (2) higher rib count, (3) development of secondary liration on the adult primaries, and (4) constant difference in its stratigraphic occurrence in the Vicksburgian Group. It can be demonstrated by quantitative and qualitative methods that she named only the extremes of variation in the species.» DELANEY, P. J. V. 1963. Stratigraphic Significance of Vicksburgian Pectinidae. Journal of Paleontology, 37 (6): 1265-1271, pl. 178. [p. 1265, 1266]
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Pecten (Pecten) poulsoni Morton; P. J. V. Delaney, 1963, Stratigraphic Significance of Vicksburgian Pectinidae, plate 178, figures 1-14.
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