Pernopecten Winchell, 1865
WINCHELL, A. 1865. Descriptions of new species of fossils from the Marshall Group and its supposed equivalents in other States; with notes on some fossils of the same age previously described. Proceedings of the Academy of Natural Sciences of Philadelphia, 17: 109-133. [p. 125]
«PERNOPECTEN, new genus.
Etymology.— Perna and Pecten, from a combination of some of the characters of the two genera.
Generic Characters.— Shell bivalve, sub-equivalve, monomyary. Valves more or less inequilateral and auriculate. Hinge line straight; hinge furnished with a central, triangular cartilage pit, and a transverse plate bearing on each side of the middle a series of smaller pits diminishing in size and depth from the centre outwards. The shell seems to be thin, and probably has a structure more like Pecten than Perna.
This genus, or subgenus is founded on Aviculopecten limaeformis, White and Whitfield, (Proc. Bos. Soc. Nat. Hist. vol. viii. p. 295.) My attention was first directed to the peculiarity of the hinge structure in two or three specimens sent me by Dr. White himself; and an examination of a number of specimens previously referred, to this species shows tliat they all possess it. The genus Aviculopecten, happily constituted by McCoy to receive a number of paleozoic species having affinities with Pecten in their external form, and with Avicula in their cardinal structure, is made by its author to differ from Pecten by the absence of a central ligamentary pit, and from Avicula by its nearly equilateral outline. The present genus differs from Avicula and Aviculopecten, and approaches Pecten and Monotis, in the presence of a mesial ligamentary pit; and it differs equally from Pecten, Aviculopecten and Avicula, and approaches Perna, by the presence of a series of isolated ligamentary pits in the cardinal area. It differs from Perna in its sub-central beaks, witli ligamentary pits on both sides. It agrees with Amusium in its sub-symmetrical ears, central cartilage pit, and the absence of radiating ridges, but differs in its straight hinge line and lateral cartilage pits. The position of the genus is apparently between Perna and Pecten, with a preponderance of aflinities for the latter, sufficient, perhaps, to throw it into the family of Pectinidae, White. Aviculopecten is grouped with the Aviculidae. It is probable that in addition to the two following species, others referred to Avicula, Pterinea, and more especially Aviculopecten, Amusium and Pecten, will be found to possess the assemblage of characters shown in Pernopecten Lima? obsoleta, Hall, (Rep. 10th Dist. N. Y., p. 265) = Pecten subobsoletus, d'Orb., is stated to have a "crenulated hinge line," while its external characters are quite conformable to Pernopecten. Not improbably Lima glaber, Hall, belongs in the same association. The same may be said of Pecten densistria, Sandb., from the Posidonomyenschiieer of Nassau: Avicula tumida and Avicula Iaevigata, de Koninck, from the carboniferous limestone of Belgium, &c. &c. This genus is known to have existed in the Chemung of Phillipsburg, New York,* whence it probably continued to the epoch of the Burlington limestone. An undescribed species occurs in the fine grained sandstone of Ohio. PERNOPECTEN LIMAEFORMIS, Winchell. (Aviculopecten limaeformis, White and Whitfield.) In this typical species, the number of ligamentary pits is about seven on each side of the mesial one. The hinge line is short, and the auriculations are small and Lima-like.»
* I find that Prof. Hall has also marked this form as a distinct genus.
ALEXANDER WINCHEL, 1865
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Pernopecten limaformis (White and Whitfield); T. W. Hutchinson & E. C. Stumm, 1965, Upper Devonian and Lower Mississippian Pectinoid Pelecypods from Michigan, Ohio, Indiana, Iowa, and Missouri, plate 6, figures 2-4, 6-10.
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Syncyclonema Sowerbyi (McCoy); R. B. Newton, 1907, Relics of coloration in fossil shells, plate 24, figure 11.
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«A ‘filosus structure’ is a term first explained by Cox (1924: 70) and later by Allasinaz (1972: 302) in descriptions of Triassic ‘entoliids’ (Entolioididae herein). The name for this term comes from the Triassic ‘entoliid’ Pecten filosus Hauer, 1857 non Reeve, 1852, the type species of Filopecten Allasinaz, 1972. The structure consists of fine antimarginal to radial fila or tiny ridgelets that develop on the inner surface of the outer, calcitic layer of the left valve within the narrow marginal secretory band of this layer (Allasinaz, 1972: plate 41, Fig. 1b; Hagdorn, 1995: Fig. 5a, b). Secretion of the fila ends when the inner crossed-lamellar aragonitic layer transgresses over them on the inner shell surface. These fila have never been observed on the inner growth surfaces of columnar prismatic calcite or on the inner surface of aragonitic layers. In Pernopecten, and in some Triassic Entolioididae, the structure is not present on the right valve, because no comparable nonprismatic calcitic layer is present on this valve. In other Triassic entolioidids (see below), a filosus structure may also be present on the right valve because a thin nonprismatic calcitic layer is present between the outer columnar prismatic layer and the inner crossed-lamellar aragonitic layer. The fila commonly exhibit divergent branching, in some areas of the shell producing nested sets of ‘V’s with narrow apical angles pointing dorsally. This fine structure is revealed on the outer surface of the valve only when this surface is abraded or corroded, and it is visible on the inner surface only when the inner aragonitic layers are differentially dissolved. A filosus structure occurs commonly but not universally in Pernopecten (Fig. 3) and does not appear to be useful as a taxonomic indicator at the genus or species level. However, the structure appears to be phylogenetically significant because it is identical to the filosus structure present in the Mesozoic family Entolioididae and in some Triassic Propeamussiidae (see below).
In addition to a filosus structure, some Pernopecten have preserved pigment patterns in the form of broad commarginal or radial bands, including broad dorsally pointing chevron patterns, as shown for Pernopecten sowerbyi (McCoy, 1844) by Newton (1907: plate 24, Fig. 11). Through diagenetic changes and compression, these broader patterns, originally developed in organic pigments within the shell fabric of the outer shell layer, may assume topographic relief or ‘pseudosculpture’ in the manner described by Hagdorn (1995). (Hagdorn thought that the filosus structure was also another type of ‘pseudosculpture’ produced by the diagenetic alteration of a pigment pattern. This matter is explored further below in the discussion of the new family Entolioididae.) Such pigment patterns are rare and do not appear to have any taxonomic or phylogenetic significance.»
WALLER, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society, 148: 313-342, figs. 1-12. [p. 317]
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