AN ARTHROPOD-DOMINATED ICHNOASSEMBLAGE FROM SOUTHWESTERN UTAH: ICHNOTAXONOMY, ICHNOFACIES AND THE EARLY TRIASSIC TAPHONOMIC WINDOW

The end-Permian extinction was the largest such event in earth history and resulted in the ecologic restructuring of both terrestrial and marine environments. The dynamics and timing of marine ecologic recovery following this event has been the subject of numerous studies incorporating body and trace fossils. We report a relatively low diversity ichnoassemblage at a Utah Geological Survey fossil site (UGS locality 42Ws643T) in the upper Timpoweap Member (Smithian) of the Moenkopi Formation. The ichnoassemblage consists of predominantly horizontal locomotion and feeding traces typical of the Cruziana ichnofacies. Most of the traces are preserved in concave or convex epirelief and come from an approximately meter-thick package of coarsening upward, fine-grained, silty limestone bounded above and below by coarser-grained limestones. The trace fossil horizon is interpreted as a shallow, marine lagoon or shelf environment below fair weather wave base. Traces presumably produced by arthropods are dominant (Kouphichnium lithographicum, Protichnites, Petalichnus, and Cruziana) with rare occurrences of other invertebrate (Helminthoidichnites and Thallassinoides) and vertebrate (Undichna) traces. Occurrences of Petalichnus may actually be deep lateral impressions of Kouphichnium. Many of the trace fossil surfaces also preserve wrinkle structures, suggesting the presence of microbially stabilized substrates that may have played a role in increasing the preservation potential of the traces. The prevalence of microbial mats and the exceptional preservation of these traces are most likely due to greatly reduced infaunal bioturbation as a result of delayed ecologic recovery after the end-Permian mass extinction. This resulted in a loss of the sedimentary mixed layer providing a taphonomic window for detailed preservation similar to that observed in Early Cambrian and Lower Triassic deposits, including elsewhere in the Moenkopi. The ichnoassemblage preserved at UGS 42Ws643T also provides additional information concerning the diversity of shallow water environments during the Early Triassic and has implications for interpreting the global timing of ecologic recovery following the end-Permian mass extinction.