A New Permian Myalinid Genus, Elversella, of West Texas

Despite significant morphological diversity, late Paleozoic myalinid genera are conservative in their development of hinge structures, which facilitates interpretation of their taxonomic relationships and evolutionary history. The new species differs from other myalinids in the extraordinary, but not unique, ornamentation in the form of widely spaced, uniform rugae on the left valve. The right valve is smaller than the left one, a condition that has been termed discordancy (Newell and Merchant, 1939).

This may indicate that the missing marginal portions of the right valve were composed of conchiolin that is only rarely fossilized. Like limids and some other pleurothetic bivalves, the right valve is unornamented and less convex than the left. In both valves the umbonal angle retains a low value throughout growth, resulting in accelerated posterior growth in the adult shell SYSTEMATIC PALEONTOLOGY 1 Research Associate, Division of Paleontology (Invertebrates), American Museum of Natural History; Assistant Professor, Department of Geology, State University of New York at Cortland, P.O. Box 2000, Cortland, New York 13045. 2 Curator Emeritus, Division of Paleontology (Invertebrates), American Museum of Natural History.

Fig. 1. a, b: Shape differences between two myalinids: Elversella (a) and Orthomyalina (b), demonstrating contrasting growth forms along the axis of the umbonal ridge. In Elversella, growth in the mature shell curves backward (retrocrescent), in Orthomyalina it is dominantly downward (infracrescent); c: Liebea, illustrating duplivincular ligament characteristic of all myalinids, as with Elversella (b and c after Newell, 1942: figs. 2 and 5 respectively).

(fig. 1a), a growth strategy referred to as retrocrescence. Furthermore, the umbonal angle decreases during ontogeny, enhancing the retrocrescent aspect characteristic of this new species. As in all myalinids, the ligament is duplivincular (fig. 1c).

The significance of this new form is that it further exemplifies the remarkable taxonomic diversity of Permian Myalinidae (e.g., Newell, 1942; McRoberts and Newell, 1997). COLLECTION: The material before us includes fragments of many silicified valves from the U.S. National Museum of Natural History. They were collected by G.A. Cooper and his colleagues, in the Glass Mountains of West Texas, as byproducts of their search for brachiopods (Cooper and Grant, 1972).

Unfortunately, the best specimens are incomplete.