Nonmarine bivalves from the Lower Permian (Wolfcampian) of the Chama Basin, New Mexico
Spencer G. Lucas and Larry F. Rinehart
The Welles quarry is an Early Permian (Wolfcampian) vertebrate fossil locality developed in a pond deposit in the El Cobre Canyon Formation of the Cutler Group near Arroyo del Agua, Rio Arriba County, New Mexico. We describe an extensive assemblage of thin-shelled, freshwater bivalves from the quarry preserved as external and (rarely) internal casts. Typical preservation is with the paired valves wide open (~180o), the hinge intact, and exterior surfaces facing upward. 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 tolength ratio (≈ 0.45) and overall morphology are essentially identical to the Late Permian anthracosiid Palaeanodonta parallela (Amalitzky), known from South Africa and Russia. However, due to the large temporal and geographic range differences between P. parallela and the Welles quarry specimens, we provisionally assign them to P. cf. P. parallela. This is the first report of Palaeanodonta from the Permian of North America, a substantial extension of its stratigraphic range from the Middle Permian to nearly the base of the Permian and suggests that too little is known of late Paleozoic nonmarine bivalves for them to be of great biostratigraphic utility.
- Lucas, Spencer G.; Rinehart, Larry F., 2005, Nonmarine bivalves from the Lower Permian (Wolfcampian) of the Chama Basin, New Mexico, in: Geology of the Chama Basin, Lucas, Spencer G.; Zeigler, Kate E.; Lueth, Virgil W.; Owen, Donald E., New Mexico Geological Society, Guidebook, 56th Field Conference, pp. 283-287.