New Mexico Geological Society Annual Spring Meeting — Abstracts

We document a rich Upper Triassic bonebed (NMMNH locality 3775--discovered by RK) in Borden County, Texas in either the uppennost Tecovas Fonnation (Adamanian-Iatest Carnian) or, more likely, the Trujillo Fonnation (early Revueltian-early Norian) of the Chinle Group. The principal fossiliferous horizon of NMMNH locality 3775 consists of a moderate brown, clast-supported conglomerate of rounded, flattened, intrafonnational mud pebbles up to 2 cm in diameter. This lithology strongly resembles the dominant lithologies of the Trujillo Fonnation, although the possibility exists that these strata instead represent an outcrop of a coarse-grained channel body within the stratigraphically lower Tecovas Formation. In either case, this is the stratigraphically highest occurrence of the larger morph of the aberrant archosauromorph reptile Trilophosaurus, although small fossils that may represent Trilophosaurus are known from the younger (mid-late Revueltian-middle Norian) Owl Rock Fonnation in Arizona. The associated fauna includes indeterminate coprolites, unionid bivalves, osteichthyans, indeterminate reptiles, a possible omithischian dinosaur, and the putative theropod dinosaur Spinosuchus caseanus Huene. Fossils of Trilophosaurus are the most abundant vertebrate remains at NMMNH locality 3775, and include teeth, skull elements, and diagnostic postcrania, including humeri and ilia. Osteichthyans are represented by fragmentary, indeterminate scales. A single, slightly recurved,
subtriangular tooth with dentic1es oblique to the tooth margin could represent an ornithischian dinosaur. We assign several vertebrae with tall neural spines to the poorly understood taxon Spinosuchus caseanus, and note that associated hollow limb elements support previous
hypotheses of the theropod affinities of this taxon.