THE MICROVERTEBRATE FAUNA OF SHARK TOOTH HILL, REDONDA FORMATION (LATE TRIASSIC: APACHEAN), QUAY COUNTY, NEW MEXICO

The Upper Triassic Redonda Formation in east-central New Mexico consists of fluvial, lacustrine, and lacustrine-margin strata deposited during latest Triassic (Apachean: latest Norian-Rhaetian) time. The macrovertebrate body fossil record of the formation is understudied, but known to include the redfieldiids Cionichthys and Synorichthys stewarti, the semionotids Semionotus and cf. Hemicalypterus, the lungfish Arganodus, an indeterminate coelacanth, the temnospondyl Apachesaurus gregorii, a large cynodont, the archosauromorph Vancleavea, the phytosaur Redondasaurus, the aetosaur Redondasuchus and another, unnamed, aetosaur, a giant sphenosuchian, and possible theropod dinosaurs. The microvertebrate fauna is essentially unstudied, but is known to include a broadly similar fish fauna. Screenwashing for microvertebrates at Shark Tooth Hill near San Jon, New Mexico 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 not represented in the macrovertebrate record. 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 substantial 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. As a lacustrine-influenced unit of latest Triassic age with a known magntostratigraphic record, the Redonda Formation and its fauna should be used to test hypotheses of Triassic-Jurassic chronology and faunal turnover developed in the Newark Supergroup of eastern North America.