VERTEBRATE TRACK SITES IN THE CHINLE FORMATION (LATE TRIASSIC) OF THE CIRCLE CLIFFS AREA, SOUTHERN UTAH

Two track localities in the Owl Rock Member of the Chinle Formation in Grand Staircase-Escalante National Monument provide good evidence of the vertebrate groups present in the area during the Late Triassic. At one of the sites, preservation of mudcracks, natural casts and impressions of araucarian conifer foliage, and tracks/undertracks of the same print on a single slab, collectively provide insights to the paleoenvironment of the area near the end of the Triassic Period. Vertebrate ichnogenera identified so far include: Apatopus, Rhynchosauroides, Gwyneddichnium, Grallator, and Pseudotetrasauropus. These tracks indicate the presence of phytosaurs, lepidosauromorphs, small theropod dinosaurs, and small prosauropod dinosaurs. Except for phytosaurs, skeletal remains of these animals are unknown or very rare in southern Utah. Prosauropod material is extremely rare in the Chinle Formation in general, and most vertebrate fossils from this unit in the Monument are from the Petrified Forest Member. The track record from the Owl Rock Member is therefore important in that it preserves both the best data on the vertebrates of the Chinle in the Monument and the only data in the Monument so far for that member. The sample is still too small to draw even preliminary conclusions about relative abundances, but at one of the sites Pseudotetrasauropus is nearly as common as Grallator.