Placopecten decemnarius (Conrad, 1834)
CONRAD, T. A. 1834. Observations on the Tertiary and more recent formations of a portion of the Southern States. Appendix. Descriptions of new Tertiary Fossils from the Southern States. Journal of the Academy of Natural Sciences of Philadelphia, 7 (1): 116-157. [p. 151]
1834 Pecten decemnarius Conrad, 1834
1840 Pecten virginianus Conrad, 1840
1843 Pecten tenuis H. C. Lea, 1843
1845 Pecten dispalatus Conrad, 1845
1840 Pecten virginianus Conrad, 1840
1843 Pecten tenuis H. C. Lea, 1843
1845 Pecten dispalatus Conrad, 1845
Chlamys decemnaria (Conrad); J. Gardner, 1943, Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina, plate 5, figures 1, 2, 6, 7.
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«P. decemnarius. Shell ovate, slightly convex, with about ten broad flattened ribs, disappearing on the umbo, some of them sulcated; radiating striae numerous, distinct, subscabrous; ears unequal.
Locality. Same as the preceding. [P. rogersii, James River, Va.] I possess but a single superior valve of each of these Pectens.» TIMOTHY ABBOT CONRAD, 1834
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«The position of Placopecten in the phylogeny of the Palliolinae is based on the demonstration of morphological transition between members of this genus and the fossil species "Chlamys" decemnaria (Conrad, 1834) by Gibson (1987: 68). At one end of its spectrum of variation "C." decemnaria is chlamydoid in form, with a complex ribbing pattern beginning ontogenetically with fine costellae that merge and cluster into coarser secondary plicae. This ribbing pattern, coarse antimarginal microsculpture between the ribs and across the disk flanks, and limited areas of "pseudoshagreen" microsculpture on the disk resemble features that are present in "Palliolum" tigerinum and higher lineages in the phylogeny (Fig. 1.2). At the other end of the spectrum of variation, "C." decemnaria approaches the shape and ribbing style of typical Placopecten, including the fossil species P. clintonius (Say, 1824), P. princepoides (Emmons, 1858), and P. sp. aff. P. magellanicus (Gibson 1987; see also Appendix Note 3). Given this transition, it is reasonable to place "Chlamys" decemnaria in the genus Placopecten (in the combination Placopecten decemnarius) and to expand the concept of Placopecten to include chlamydoid forms that show variation into Placopecten-type morphology. The reason that Placopecten in this expanded concept is placed as a sister group of the clade that originates at Node P4 (Fig. 1.2) rather than being a member of that clade is because or differences in auricular shape and auricular sculpture. The posterior margins of the posterior auricles of Placopecten decemnarius and other species of the genus are plesiomorphically straight or convex, not concave as at Node P4, and they lack the other auricular character states mentioned above for Node P4.»
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, figs. 1.1-1.4. [p. 19, 20]
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T. R. Waller, 2006, New Phylogenies of the Pectinidae, figure 1.2.
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«DISCUSSION.— Samples of more than 300 specimens of Pecten decemnarius and Pecten virginianus from the Lee Ceek Mine show the conrplete intergradation of these pleviously separated species. These two forms are only moderately common in most localities and, although people have noted the co-occurrence and similarities between them, heretofore they have not been considered as belonging to a single variable species. For example, Mansfield (1936: 178) noted that "Pecten decemnarius Conrad and Pecten virginianus Conrad are closely related and usually occur together." In the same paper, Mansfield considered both species questionably to have evolved from Placopecten clintonius. A population sample was composed of specimens occurring together on the spoil piles in a similar matrix, coming from the indurated silty sand in the lower part of unit 5. Chlamys decemnaria and rotten specimens of Turritella are about the only fossils present.
The presence of articulated specimens in the Lee Creek material shows the variation in the strength of the ornamentation on the two valves of an individual specimen. Regardless of whether the general development of the ornamentation is strong or weak, the right valve has considerably weaker development than the left (Plate 15: figures 2, 3, 6, 7; Plate 16: figures 4, 5). Within the large sample, it is possible to denronstrate an intergrading sequence of ornamentation from fine radial costae to coarse plications with coarse concentric lamellae (Plates 19, 20). On the right valves (Plate 19) the ornamentation exhibits wide variation, ranging from fine costae (figure 1) to moderate costae on a slightly undulating surface marked by more conspicuous grooves (figure 2), to moderately coarse costae with the undulations and grooves developing into plicae (figures 3-5), and then to increasingly better developed plicae with coarser costae (figures 6-13). Coarse concentric lamellae occur commonly in the coarser stages of the plicae (figures 10, 13). A similar increase in the development of the plicae and other ornamentation is observable on the left valves (Plate 20). Because the costae and/or plicae are coarser on the left valve than on the right, specimens with a finely costate right valve usually have moderately strongly sculptured left valves. Therefore, finely sculptured left valves are not common, but do occur (figure 2). The costae increase in coarseness (figure 3), and then valves occur with raised areas or plicae composed of two or three costae formed by bifurcation of an earlier raised single one (figure 5). The plicae become stronger, and along with them the intervening costae (figures 6, 7), with the development commonly of coarse concentric lamellae as on the right valve (figure 8). The extreme stage consists of three to five strong plicae along with a number of weaker plicae in varying stages of development, reflecting the coarsening of the costae between the plicae, commonly those formed by bifurcation from the original strong one (figure 9). In this variation, i.e., Pecten dispalatus Conrad, both the stronger and weaker plicae have coarse concentric lamellae. Although the ornamentation is intergradational between the two nominal species, a check of other possibly significant morphological characters was made to see if they also showed overlap. Specimens were divided into three groups on the basis of the strength of the ornamentation. These groups included the following three forms: (1) those with fine to moderate costae, without plicae, i.e., typical Pecten virginianus; (2) those with moderately developed costae and the incipient to moderate appearance of plicae, i.e., Pecten decemnarius; and (3) those forms with well-developed plicae and moderate to coarse costae along with concentric lamellae, i.e., typical Pecten decemnarius. A comparison among the three morphologic groups was made on the basis of external shell morphology, including features of the disk, auricles, and byssal notch. None of the comparisons indicates any difference among the three groups. Whether the characters used have a very high correlation (Figure 21) or only a moderate correlation (Figure 22), the three groups show no significant differences. Characteristics of the byssal notch, which have proven irnportant in other species and genera, show that although the relatively deep byssal notch is a variable character within each group, it is similar among the three groups (Figure 22). The considerably greater length of the anterior auricle in comparison to the posterior auricle also is consistent among the three groups (Figure 21). Because of the indurated matrix on the specimens, only external characters were used. The absence of significant differences in any of the studied characters among the three groups reinforces the conclusion that the range in ornamentation is just a variable characteristic of a single species. This wide range of ornamentation is greater than in most species of pectens. As both the finer and more coarsely ornamented forms are generally found in most localities, it appears that the wide range of ornamentation is a common genetic characteristic throughout the time range of the species. It also appears that as far as can be determined no phylogenetic or geographic trends toward finer or coarser plicae are present in the North Carolina-Virginia area. A wide variety of species have been placed in Chlamys, including the more recent placements of "Pecten" decemnarius. Although the ornarnent pattern in "Pecten' decemnarius is within the range of the type-species of Chlamys and other closely related species, it differs in not having the elongated shape of the disk characteristic of Chlamys islandicus, the type-species. Until a thorough reclassification of the Pectinidae is available, "Pecten" decemnarius is placed in the genus Chlamys. "Pecten" virginianus generally has been placed in the then subgenus Placopecten (Dall, 1898: 727; Tucker-Rowland, 1934: 617; 1938: 55; Gardner, 1944: 38), although these workers noted that "Pecten" virginianus and Placopecten clintonius differ by the former having a much deeper byssal notch and a strongly developed ctenoliurn. These two species were placed in the then subgenus Placopecten because of a general similarity in the shape and convexity of the two valves and their fine radial ornamentation. Differences in the byssal notch, however, and the intergradation with coarser radial ornamentation including plicae in the "P." virginianus and "P." decemnarius suite, make this placement questionable. The transition between "P." virginianus (usually placed in the subgenus Placopecten) and "P." decemnarius (usually placed in the subgenus Chlamys) does indicate that there may be a close relationship between the two groups, here regarded as genera.» GIBSON, T. G. 1987. Miocene and Pliocene Pectinidae (Bivalvia) from the Lee Creek Mine and Adjacent Áreas. Smithsonian Contributions to Paleobiology, 61: 31-112, pls. 1-31. [p. 66, 68]
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Chlamys decemnaria (Conrad); T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine, plate 15, figures 2, 3, 5-7 (above); plate 16, figures 3-5 (below).
Chlamys decemnaria (Conrad); T. G. Gibson, 1987, Miocene and Pliocene Pectinidae from the Lee Creek Mine, plate 19, figures 1-13 (above); plate 20, figures 1-10 (below).
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«This species is notably irregular in its sculpture, the disk being sculptured either by numerous more or less distinctly fasciculated, small, radial threads, or the fasciculi may be replaced partially by stout, elevated, rounded ribs, with wide, radially threaded interspaces. The radial sculpture may be nearly smooth or covered with a conspicuous, dense, concentric lamellation. Three or four of the ribs may be more prominent than the others, and the smaller ones uneven in size and rugose, forming the variety dispalatus. When the fasciculi are riblike they are usually dichotomous. The umbonal region in typical decemnarius is usually feebly sculptured, but in the variety dispalatus the ribbing approaches the beaks more nearly. The type of the 'latter has been carefully compared, and the ears and surface agree exactly with those of the decemnarius form. Large valves of the latter attain a height and width of 68 millimeters; the type of dispalatus measures 24 millimeters. The cardinal crura are parallel with the hinge line and moderately developed. The byssal notch is wide and conspicuous, the posterior ears small.
In sculpture this form almost exactly parallels the recent northwest American P. hericeus in its mutations.—Dall, 1898. The plates represent end members of complete series of this exceedingly variable form.
Distribution: Virginia: Miocene, Yorktown formation. Indian Field Point, Yorktown, and Bellefield, York County; Sycamore and a quarter to half a mile below Sycamore, Southampton County.
North Carolina: Miocene, Yorktown formation. 1½ miles above Murfreesboro and \ mile above Murfreesboro, Hertford County; Halifax, Halifax County; ½ mile above Bells Bridge and 1/8 mile below Bells Bridge, Edgecombe County; 2 miles west of Greenville, Pitt County.» GARDNER, J. 1943. Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina. Part 1. Pelecypoda. United States Geological Survey Professional Paper, 199-A: 1-178, pls. 1-23. [p. 31, 32]
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Chlamys decemnaria (Conrad) Dall; J. Gardner, 1943, Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina, plate 5, figures 1, 2. 6, 7.
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