TRILOPHOSAURUS JACOBSI IS NOT A PROCOLOPHONID: DATA FROM A NEW QUARRY FROM THE UPPER TRIASSIC CHINLE GROUP, WEST TEXAS

A new, extensive bonebed from the Upper Triassic Chinle Group in Borden County, Texas is either in the uppermost Tecovas or lowermost Trujillo formations, and consists primarily of intraformational conglomerates overlain by color-mottled bentonitic mudstones. We interpret these strata as a series of crevasse-splays, at least some of which transported and buried abundant reptile bones on the floodplain floor. The fauna consists primarily of the unusual reptile Trilophosaurus, but also includes an aberrant archosauromorph similar to Spinosuchus as well as isolated phytosaur and dinosaur teeth, diverse fish scales, and vertebrate coprolites. The only other associated faunal elements are shells of unionid bivalves that derive from an immediately overlying (< 1 m) bed. Bones are disarticulated and not even clearly associated. The site yields all major elements of Trilophosaurus, including skulls, lower jaws, vertebrae, pectoral, pelvic, forelimb, and hind limb elements. Duplication of sided elements (especially left tibiae) indicates an MNI (minimum number of individuals) of at least 16. A well-preserved, nearly complete juvenile skull and lower jaws is directly comparable to the holotypes of T. buettneri Case and T. jacobsi Murry. Contra to the assertion of some recent workers, T. jacobsi is not a procolophonid reptile (“Chinleogomphius”) but instead, as originally identified, a form closely related to T. buettneri. Features of the new Texas material that support assignment to T. jacobsi include tricuspid teeth that, unlike T. buettneri, have pronounced cingula anteriorly and posteriorly, as well as connecting the central cusp to both the lingual and labial cusps. This is the first associated skull and postcranial material of T. jacobsi. T. jacobsi also differs from T. buettneri in having paired crests near the midline along the postfrontal and parietal. T. jacobsi appears to be primarily Adamanian in age, and the first occurrence datum of T. jacobsi is stratigraphically higher than that of T. buettneri, but either or both may persist until Revueltian time, so their biostratigraphic utility is limited. T. jacobsi appears to be an anagenetic descendant of T. buettneri and was of comparable size, so the holotypes of T. jacobsi must represent a hatchling individual.