New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Triassic Redonda Formation is east-central New Mexico consists of fluvial, lacustrine and lacustrine-margin strata deposited during latest Triassic time. The macroinvertebrate body fossil record of the formation is understudied, but known to include the redfieldiids Cionichthys and Synorichthysstewarti, the semionotids Semionotus and cf. Hemicalypterus, the lungfish Arganodus, and indeterminate coelacanth, the temnospondyl Apachesaurus gregorii, a large cynodont, the archosauroporph Vancleavea, the phytosaur Redondasaurus, the aetosaur Redondasuchus, a giant sphenosuchian, and possible theropod dinosaurs. The microvertebrate fauna is essentially unstudied, but is known to include broadly similar fish fauna. Screenwashing for microvertebrates at Shark Tooth Hill near San Jon yielded a microvertebrate fauna composed of redfieldiid and semionotid fish, indeterminate reptiles, several morphotypes of archosauriform teeth, small phytosaurs, and possible ornithischians. Chondrichthyans are conspicuously absent. Many of the archosauriform tooth morphotypes are known from much older (Adamanian) taxa, and thus are not age-diagnostic. The microvertebrates do, however, provide some insight into the small-bodied fauna of the Redonda Formation, which appears to have been dominated by small archosauriforms. The putative ornithischian teeth, while fragmentary, constitute the only record of ornithischian body fossils in the Redonda Formation. The diversity from this preliminary sample hints at a large microvertebrate fauna that remains largely undiscovered, and should spur additional interest in the microvertebrate record of the Redonda Formation. Indeed, the Redonda Formation is the most fossiliferous stratigraphic unit of latest Triassic age in western North America, and is clearly the key to understanding latest Triassic vertebrate evolution.