A COLLECTION OF TRACE FOSSILS FROM THE LOWER TRIASSIC MOENKOPI FORMATION (ARIZONA): AN EXAMPLE OF THE SCOYENIA ICHNOFACIES AND CHANGING SUBSTRATE CONDITIONS

Along with anatomy and kinematics, substrate conditions constitute a third major contributing factor to overall trace fossil morphology. Therefore, trace fossil suites (ichnofacies) provide insights into paleoenvironmental conditions not readily apparent from sedimentary structures alone. Here we report preliminary details on a collection of 25 trace fossil specimens from the Lower Triassic Moenkopi Formation housed at the Raymond M. Alf Museum of Paleontology (RAM). These specimens are important for interpreting terrestrial biodiversity and paleoenvironmental conditions during the ecological recovery period after the end-Permian mass extinction. The specimens were collected near Cameron, Arizona (RAM V94005), between 1950–1970. The specimens exhibit vertebrate traces (tracks, slip traces, and swim tracks), desiccation cracks, and invertebrate traces, with bioturbation indices ranging from 1–3. Vertebrate ichnotaxa include Chirotherium barthii, Chirotherium rex, Isochirotherium marshalli, and Rhynchosauroides, with C. barthii being most common. Invertebrate ichnotaxa include Arenicolites, Planolites, and Skolithos. This trace fossil assemblage, comprising vertical domiciles and locomotion traces with locally high abundances, represents the Scoyenia ichnofacies that is associated with terrestrial floodplain paleoenvironments, such as abandoned channels and desiccated overbank deposits. Crosscutting relationships and the disparate preservation of detail among traces are a product of changing substrate conditions. Some vertebrate tracks exhibit scale impressions, indicating that they were produced in a soft, but firm, fine-grained substrate that subsequently dried and produced mudcracks. After being cast by the overlying siltstone, the substrate was rehydrated and organisms continued to bioturbate the track surface, as evidenced by traces crosscutting the mudcracks and the tracks. Vertical burrows are preserved in both convex and concave hyporelief, indicating that they were produced within the substrate by organisms moving in both directions across the track surface. Future relocation of and collection at this locality will further elucidate details about paleoenvironmental conditions and paleoecology in this region during the Early Triassic.