MICROVERTEBRATE ASSEMBLAGE OF THE UPPER TRIASSIC (ADAMANIAN) DOWNS’ QUARRY, ST. JOHNS, ARIZONA

The Downs’ Quarry is an Upper Triassic fossil site located near the Placerias Quarry outside of St. Johns, in eastern Arizona. The Placerias quarry has yielded the most diverse Upper Triassic vertebrate assemblage in the world, and is traditionally placed within the Blue Mesa Member of the Petrified Forest Formation, but multiple factors complicate stratigraphic placement. Within the Downs’ Quarry, there are two fossiliferous layers, and the lower bone horizon of the Downs’ Quarry is thought to correlate with the Placerias Quarry, which has recently been assigned a maximum age of 219.39 ± .16 Ma based on detrital zircons. The purpose of looking at the macro- and microvertebrate assemblages is to further understand the biodiversity of both quarries and the paleoenvironment in which they lived. To determine the assemblage present at the Downs’ Quarry, fossiliferous matrix was collected by J. Bolt of the Field Museum of Natural History (FMNH) in Chicago in the 1980s, and crews from the North Carolina Science Museum (NCSM) in Raleigh, in collaboration with students and faculty of Appalachian State University, from 2010-2014. The matrix was screen-washed, sieved, picked, and sorted into vials, a process that is ongoing. The macrovertebrate assemblage of the Downs’ quarry includes phytosaurs, the aetosaurs Calyptosuchus, Desmatosuchus, and Acaenasuchus, as well as other archosaurs. The known microvertebrate assemblage from the FMNH sample, thought to represent the upper horizon, includes Osteichthyes (indeterminate actinopterygians), temnospondyls, archosauriforms (Protecovasaurus, Uatchitodon, and other indeterminate archosauriforms), and a reveultosaur, as well as possible lepidosauromorphs (represented by acrodont and pleurodont jaw fragments). The most recent excavation of matrix from the lower horizon (NCSM sample) has revealed abundant teeth and tooth fragments, mostly belonging to phytosaurs, but the matrix also includes probable rauisuchians or theropods, and morphotyope O or Q archosauromorph teeth. Comparing assemblages between the two quarries, it appears that the Downs’ quarry fossils are less diverse, but preserve a more terrestrial assemblage than the more aquatic (paludal) setting of the Placerias Quarry.