NEW MICROVERTEBRATE FAUNAL ASSEMBLAGE FROM THE UPPER TRIASSIC BLUE MESA MEMBER OF THE PETRIFIED FOREST FORMATION IN THE BLUE HILLS, EAST-CENTRAL ARIZONA

Located in the Blue Hills outside St. Johns Arizona, NMMNH locality 6818 is a previously unpublished Upper Triassic microvertebrate site. The locality lies in the upper of two bone bearing horizons of the Blue Mesa Member of the Petrified Forest Formation of the Chinle Group, approximately 12.2 meters from the base of the unit. The Blue Mesa Member in the Blue Hills yields a fauna of Adamanian (Carnian?) age. The fossiliferous horizon is a green and purple mottled mudstone-dominated interval with pedogenic alteration, including color mottling, reduction spots, and calcareous concretions. Larger elements are highly concreted, disarticulated, and scattered throughout the horizon. Microvertebrates are less concreted and most commonly found clustered in highly lenticular, minute (cm-thick) sand pockets distributed throughout the mudstone, and may represent one of Camp’s “meal pots” localities from the 1920s. The non-tetrapod portion of the assemblage includes a tooth of the xenacanth shark Triodus moorei, various osteicthyan scales, and six toothplates of the lungfish Asiatoceratodus. The toothplates are small, ranging in size from 9-13 mm long, but the internal angle and intercrest angle measurements are consistent with Asiatoceratodus (=Arganodus). Tetrapods in the assemblage include amphibians and, more commonly, amniotes. Amphibians are represented by tiny (juvenile?) metoposaurid jaw, interclavicle, clavicle, and skull fragments. Phytosaurs are common at the site, indicated by the presence of a complete femur, several osteoderms, and numerous type C, U, I, and B teeth. The teeth vary in size from mm- to cm- scale, indicating the presence of juveniles and adults in the assemblage. A fragmentary aetosaur osteoderm and possible rauisuchian teeth indicate the presence of other large archosaurs. Smaller taxa are represented by numerous teeth, including archosauriform tooth morphotypes B, C, J, K, and N. Coprolites are common at the site, and are diverse in size, texture, and shape. The density of the fossils is less than 2.75 g/cm_­³, which may hinder the use of heavy liquids, such as sodium polytungstate, for fossil separation. This locality shows how microvertebrate sites are much more diverse than typical tetrapod faunas in the area.