TETRAPOD BIOSTRATIGRAPHY OF THE TRIASSIC-JURASSIC BOUNDARY, FOUR CORNERS REGION, USA

Nonmarine fluvial, lacustrine and eolian strata on the southern Colorado Plateau preserve one of the most extensive records of tetrapod body fossils and footprints across the Triassic-Jurassic boundary (TJB). A new and detailed lithostratigraphic framework enables recognition of five, temporally successive tetrapod fossil assemblages. The Owl Rock Formation (Chinle Group) yields a tetrapod fossil assemblage that includes the phytosaur Pseudopalatus and the aetosaur Typothorax coccinarum, index taxa of the Revueltian land-vertebrate faunachron (lvf), which is of Norian age. The Rock Point Formation (Chinle Group), especially at Ghost Ranch, New Mexico, yields a tetrapod fossil assemblage that includes the phytosaur Redondasaurus, index taxon of the Apachean lvf. Rock Point footprint assemblages include numerous Brachychirotherium, a crurotarsan track restricted to Triassic strata. Aetosaurus in the Rock Point Formation in the Eagle basin of Colorado suggests a Norian age, so we reject previous ideas that the Apachean is wholly Rhaetian in age. Most of the Dinosaur Canyon Member of the Moenave Formation and laterally equivalent Wingate Sandstone also are Triassic in age. These strata yield the phytosaur Redondasaurus and the footprint ichnotaxon Brachychirotherium, and they lack any Jurassic index taxa. They can thus be assigned an Apachean age, and may be equivalent to all or part of the Rhaetian. The uppermost Dinosaur Canyon Member, the entire Whitmore Point Member of the Moenave Formation and the uppermost Wingate Sandstone appear to be of Jurassic age. These strata yield Protosuchus, Megapnosaurus (“Syntarsus”) and the footprint ichnotaxa Eubrontes and Otozoum, taxa traditionally considered to be Jurassic. Eubrontes, however, now is known to have demonstrable Triassic records, but the lack of any Triassic index taxa in the upper Moenave-Wingate interval supports the idea that it is earliest Jurassic (Hettangian) in age. The Kayenta Formation yields a tetrapod fossil assemblage that is clearly of Jurassic age. A key taxon is the dinosaur Scelidosaurus, known from marine lower Sinemurian strata in England. Tetrapod biostratigraphy thus supports palynostratigraphy and magnetostratigraphy to place the TJB within the Moenave Formation and equivalent strata on the Colorado Plateau.