TETRAPOD SWIM TRACKS THROUGH TIME: INCREASED PRESERVATION POTENTIAL DURING THE EARLY TRIASSIC DUE TO DELAYED RECOVERY OF INFAUNA FOLLOWING THE END-PERMIAN MASS EXTINCTION

Fossil swim tracks attributed to various tetrapods have been reported from deposits throughout the world and ranging in age from the Carboniferous (Mississippian) to the Neogene (Pleistocene). A normalized analysis of these occurrences demonstrates that Early Triassic deposits contain an anomalously high number of occurrences compared to other epochs. Early Triassic swim tracks also tend to be better preserved than those from other epochs showing exceptionally detailed features such as scale striae and disc-shaped claw margins. Due to the subaqueous conditions under which these swim tracks are produced the potential track-recording substrate must be firm and semi-cohesive in order to maintain track detail before and after burial. Swim track localities from the Lower Triassic Moenkopi Formation in Utah show a suite of sedimentary and trace fossil characteristics that demonstrates the widespread development and persistence of firmground substrates in the interdistributary bays and channels of a large delta plain. Here the low diversity invertebrate trace fossil assemblage is comprised of locally high densities of relatively diminutive traces characteristic of stressed brackish water faunas. The normally depauperate infauna was even more repressed due to delayed biotic recovery following the end-Permian mass extinction resulting in extremely low degrees of bioturbation. This lack of biogenic mixing promoted semi-consolidation of dewatered mud substrates and the widespread production and persistence of firmgrounds capable of recording and maintaining swim tracks. Thus there existed a unique combination of factors during the Early Triassic that increased the preservation potential of detailed swim tracks. These factors were 1) depositional environments that promoted the production of firmground substrates, 2) delayed ecologic recovery resulting in the lack of well-mixed layers, and 3) the swimming behavior of various tetrapods. It was the prevalence of unbioturbated substrates resulting from the unique combination of ecological and environmental conditions during the Early Triassic that led to the abundant production and preservation of detailed swim tracks during this time period.