New Mexico Geological Society Annual Spring Meeting — Abstracts

We describe a rich deposit of thin-shelled, freshwater bivalves that constitute the first documented occurrence of these clams in the Pennsylvanian-Permian Sangre de Cristo Formation of the Rowe-Mora basin. The clams are from laminar, greenish-grey, micaceous shale beds of lacustrine origin at NMMNH locality 7889 near Glorieta Pass (T15N, R11E, Santa Fe Co.) in the upper part of the Sangre de Cristo Formation. These strata are of probable Early Permian (Wolfcampian) age. The clams are preserved as external and (rarely) internal casts that typically lie on the bedding planes with the articulated valves closed, although some show the paired valves wide open (~180^o ) and the hinge intact. Plant debris, including partial walchian fronds and cordaitalean material, and poorly-preserved conchostricans are common in the clambearing stratum.

The clams are equivalved, inequilateral, and elongate oval in shape. The hinges are straight and adductor muscle scars are absent or not preserved. Length ranges from ~ 8 to 26 mm. The beaks are anteriorly inclined, and the 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. Average height-to-length ratio (= 0.35), allometric growth constant, and overall morphology including the two variants are very similar to the Permian anthracosiid, Palaeanodonta, recently reported from the El Cobre Canyon Formation of the Cutler Group in the Chama basin. The Cutler Group record is Wolfcampian and in a different Paleozoic basin from the Rowe-Mora basin record, implying possible riverine connections between the two basins.