New Mexico Geological Society Annual Spring Meeting — Abstracts

North of Rancho de Chaparral in the Nacimiento Mountains of Sandoval County (sec. 2, T19N, R1E), Middle Pennsylvanian (Atokan) strata of the Sandia Formation rest on Precambrian basement. Poorly and incompletely exposed, the Sandia Formation is at least 10 m thick and consists of interbedded greenish gray/brown shale and coarse-grained to conglomeratic, quartzose sandstone. Overlying cherty limestones (“Gray Mesa Formation”) contain the fusulinacean Wedekindellina, indicative of a Desmoinesian age. In a 0.7-m-thick bed of conglomeratic sandstone that is ~ 4 m below the top of the Sandia Formation, we recovered an isolated bone that is the first tetrapod fossil from the Sandia Formation and New Mexico’s oldest fossil tetrapod. This bone is columnar, incomplete and ~ 40 mm long with a flat articular end that is 9.5 mm wide. The shaft is slightly bowed on its long axis, shallowly concave on one side, shallowly convex on the other side and widens toward the less complete articular end. It closely resembles the fibula or possibly a presacral rib of a primitive temnospondyl amphibian such as Greererpeton. However, we only tentatively identify the fossil as temnospondyl. Most other Pennsylvanian tetrapod records from New Mexico are of Late Pennsylvanian age (Bursum, El Cobre Canyon and Atrasado formations, most notably the Kinney Brick quarry), and the oldest previously reported record was a captorhinomorph bone from the Desmoinesian Flechado Formation in Taos County. The occurrence of tetrapod bone in the Sandia Formation thus pushes back New Mexico’s fossil record of tetrapods into the Atokan.