New Mexico Geological Society Annual Spring Meeting — Abstracts

The first vertebrate fossils of Late Triassic age discovered in the western United States were collected by E. D. Cope in 1874 near Gallina in Rio Arriba County, New Mexico. The exact geographic and stratigraphic location of Cope's collection, which included the holotypes of the aetosaurian reptile Typothorax coccinarum Cope and the unionid bivalve Unio cristonensis Meek, have remained uncertain for more than a century. Using Cope;s field notes in the archives of the American Museum of Natural History and published sketches, we have relocated his locality which is near Cerro Blanco in the SW¼ NW¼ NE¼ sec. 4, T23N, RIE, in the upper part of the Petrified Forest Formation of the Chinle Group. Cope's description states that he collected from a conglomerate bed of "bluish tint" that resembled the "Potomac marble" (an Upper Triassic conglomerate of the Newark Supergroup used to build the columns in the Capitol building in Washington, D.C.). We collected fossils from an intraformational conglomerate in the upper part of the Petrified Forest Formation at the location stated above that include vertebrate coprolites, phytosaur-scute fragments and scute fragments of Typothorax coccinarum identical in preservation to the specimens of Cope's original collection. However, we found no additional unionid bivalves.

Typothorax is an index fossil of Norian time in the Chile Group of the western United States known from the Painted Desert Member of the Petrified Forest Formation and the Owl Rock Formation in Arizona, the Bull Canyon and Trujillo Formations in eastern New Mexico and the Cooper Member of the Dockum Formation in West Texas. The occurrence of the holotype of Typothorax in the upper part of the Petrified Forest Formation in the Chama basin of north-central New Mexico thus indicates a Norian age for these strata consistent with physical stratigraphic correlations and phytosaur-based biochronology.