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Atomodesma (Bivalvia), and Permian Species of the United States

Erle G. Kauffman and Bruce Runnegar
Journal of Paleontology
Vol. 49, No. 1 (Jan., 1975), pp. 23-51
Stable URL: http://www.jstor.org/stable/1303315
Page Count: 29
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Atomodesma (Bivalvia), and Permian Species of the United States
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Abstract

Atomodesma is an uncommon, but geographically widespread, Temperate, epifaunal to semi-infaunal Permian bivalve which occupied a variety of inner sublittoral habitats in association with at least three distinct benthonic assemblages. These habitats are clearly defined by functional morphologic interpretations of shells of Atomodesma and related taxa, by analysis of sediment-bivalve relationships, and by associated organisms. Atomodesma thrived best in simple communities which it strongly dominated, on possibly mobile clastic substrates actively swept by currents, in the inner sublittoral shelf zone. Fine-grained sandstone and siltstone of shallow water origin support the highest abundance and diversity of Atomodesma. A detailed study of the shell morphology, and especially of interior characters such as muscle insertion areas, the ligamental area, the umbonal septum, and byssal slit in Atomodesma and related taxa (Kolymia, Intomodesma, Aphanaia = Maitaia) sheds considerable light on the phylogenetic relationships of this group. Lack of a true duplivincular ligament, the nature of the pedalbyssal musculature, and the equivalve to subequivalve shells of Atomodesma suggests closer relationships to the Early and Middle Paleozoic Ambonychiidae and Mesozoic Inoceramidae than to the Myalinidae, to which these genera and subgenera have previously been assigned. Most atomodesmids have a nearly smooth, channel-shaped ligamental groove similar to that developed by convergence in the youngest Inoceramidae (Tenuipteria). Rare specimens ("Permoceramus" of Waterhouse, 1970), possibly macromutants of single atomodesmid populations, developed a multivincular ligament like most Inoceramidae. These factors strongly suggest Atomodesma is descended from the Ambonychiidae, but belongs within the Inoceramidae, and is directly ancestral to Mesozoic inoceramids. Such an interpretation implies no direct relationship between inoceramids and superficially similar Isognomoniidae, and suggests that the multivincular ligament has arisen independently in ambonychian, pteriacian, and pectinacean stocks as a convergent morphologic feature. Genera and subgenera of Permian atomodesmid bivalves are redefined and species of Atomodesma are described from North America for the first time.

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