A NEW LATE TRIASSIC DINOSAUR FROM THE CHINLE GROUP OF ARIZONA

Upper Triassic Chinle Group strata in the Petrified Forest National Park (PEFO) in Arizona yield some of the oldest known dinosaurs. Most PEFO dinosaur fossils are from the Revueltian (early-middle Norian) Painted Desert Member of the Petrified Forest Formation. We report here an older PEFO theropod dinosaur, from the Adamanian (late Carnian) Blue Mesa Member of the Petrified Forest Formation. The fossil is from the “Dinosaur Ridge” locality in color-mottled, grayish-purple and yellowish-gray mudstone of the upper part of the Blue Mesa Member. The fossil bed is a pedogenically-modified floodplain overbank deposit that yields fossils of sphenosuchians, phytosaurs, aetosaurs and dinosaurs, and well represents the Chinle paleosol taphofacies at PEFO. The fossil consists of associated cranial material (notably parts of the antorbital region, skull roof and braincase), both lower jaws, many teeth, vertebrae from all along the axial column and assorted limb elements. These are from an animal about the size of a small Coelophysis bauri, with an estimated skull length of ~195 mm. One of the most distinctive features of this dinosaur is the maxillary fragment, which possesses an antorbital fossa unlike any other theropod taxon. The portion of the left maxilla preserved includes a dorsoventrally short jugal process, similar to coelophysoids, and a reduced antorbital fossa, which is generally expanded in coelophysoids, with an elliptical outline.. This mosaic of characters distinguishes this specimen from all other contemporaneous theropod taxa. The discovery of a new and relatively old (Adamanian) dinosaur taxon adds to the rapidly growing knowledge of early dinosaur diversity.