NONMARINE BIVALVES (UNIONOIDA: ANTHRACOSIIDAE) FROM THE LOWER PERMIAN (WOLFCAMPIAN) WELLES QUARRY, ARROYO DEL AGUA, NORTH-CENTRAL NEW MEXICO

The Welles Quarry (NMMNH locality 4825) is one of several pond/crevasse splay deposits in the Cutler Formation exposures near Loma Salazar in the Arroyo del Agua collecting area, Rio Arriba County, New Mexico. The quarry yields fish, amphibian, reptile, invertebrate, and plant fossils of Wolfcampian age. We describe a rich deposit of thin-shelled, freshwater bivalves from the quarry. The clams are preserved as external and (rarely) internal casts with a few shell fragments in laminar gray and dark red, micaceous muddy shale. Typical presentation is with the paired valves wide open (~180^o), the hinge intact, and exterior surfaces facing up. Fish scales, coprolites, and disarticulated bones are common in the clam-bearing facies.

The clams are equivalved, inequilateral, and elongate oval in shape. Ligaments are external and opistodetic, hinges are straight and edentate, and adductor muscle scars are absent or not preserved. Length ranges from ~ 1 to ~ 23 mm. Umbones are slightly inflated and located at ~ 0.25 of length from the anterior end. Ornamentation consists only of concentric growth ridges. Two variants, one with a rounded posterior end, and the other more blunt, may represent sexual dimorphs. Allometric height-to-length ratio (=0.45) and overall morphology are identical to the Guadalupian-Lopingian age anthracosiid, Palaeanodonta parallela, known from the Lower Beaufort Series of the Karroo basin in South Africa, and the Tatarian of Russia. However, due to the large temporal and geographic range differences between P. parallela and the Welles Quarry specimens, we provisionally assign the clams to Palaeanodonta cf. P. parallela. This is the first report of Palaeanodonta from the Permian of North America, and a substantial extension of its stratigraphic range from the Middle Permian to nearly the base of the Permian.