Vertebrate fossils in the upper Pennsylvanian (Missourian) Tinajas Member of the Atrasado Formation, Socorro County, New Mexico
— Susan K. Harris and Spencer G. Lucas

In the Quebradas area of Socorro County, New Mexico, the Middle-Late Pennsylvanian Atrasado Formation yields diverse nonmarine and marine fossil assemblages. South of Arroyo del Tajo, NMMNH (New Mexico Museum of Natural History) locality 12323 is a recently discovered and unique fossil locality in floodplain deposits of the Upper Pennsylvanian (Missourian) Tinajas Member of the Atrasado Formation. The vertebrate fossil assemblage of NMMNH locality 12323 includes vertebral centra that pertain to a rhizodont sarcopterygian, cf. Strepsodus, postcranial elements comparable to the eupelycosaurian genus Sphenacodon, and a large number of elements of uncertain identity. Vertebrate coprolites and plant stems also occur in the deposit, which is a 1.3 m thick interval of red (moderate reddish-brown) mudstone 4–5 m above the black shale lake deposits of the Tinajas. This plant- and bone-bearing bed is at the top of a 3.4 m thick interval of moderate reddish-brown mudstone and intercalated laminar micaceous sandstone. The red mudstones are mottled, contain kaolinitic lenses, and are interpreted as pedogenically modified floodplain deposits. Fossils of NMMNH locality 12323 represent the oldest known assemblage of both aquatic and terrestrial vertebrates from New Mexico and document the oldest record of the eupelycosaur cf. Sphenacodon.