REVISION OF THE REDONDA FORMATION (UPPER TRIASSIC CHINLE GROUP) VERTEBRATE FAUNA AND ITS IMPACT ON THE APACHEAN LAND-VERTEBRATE FAUNACHRON

The vertebrate fauna of the Redonda Formation, Chinle Group in east-central New Mexico, has been studied for over 80 years, with the first paleontological surveys of Redonda strata occurring in the 1930s. However, the first faunal analysis of the unit only appeared in the 1970s. Since then, few analyses have focused solely on the Redonda fauna, instead focusing on the overall Chinle Group faunas in east-central New Mexico or only key taxa within the Redonda. Our recent re-analysis of the Redonda Formation vertebrate fauna has yielded significant results: (1) a more robust diagnosis for the phytosaur Redondasaurus, which is a biostratigraphically important taxon whose first appearance datum (FAD) defines the beginning of the Apachean land-vertebrate faunachron (LVF); (2) the recognition of sexual dimorphism within Redondasaurus; (3) a new aetosaur genus, documenting that two distinct aetosaur genera were present within the Redonda; (4) a new trirachodontid cynodont taxon, confirming extension of the range of this group into the Late Triassic; and (5) incorporation of the biostratigraphically significant vertebrate and invertebrate taxa into a more rigorous definition of the Apachean LVF. Overall, we found that the beginning of the Apachean is stratigraphically lower than previously concluded; it is not at the base of the Redonda Formation, but high in the underlying Bull Canyon Formation, based on the FAD of Redondasaurus. In addition, vertebrate biostratigraphy allows for correlation of the Redonda locally within the American Southwest and invertebrate biostratigraphy, specifically conchostracans, allow correlation of the Redonda Formation with late Norian strata of the eastern United States and to the Germanic basin. The Apachean LVF is thus confirmed as a robust biochronological construct with which to correlate late Norian tetrapod faunas.