Complex ichnofabrics in the lower member of the Union Wash Formation, East-Central California: Failed recovery from the Permian-Triassic mass extinction

Recovery from the Permian-Triassic mass extinction was temporally and spatially complex as the result of the uneven distribution of environmental stress in the wake of the extinction. Regions with delayed recovery are thought to be influenced by the presence of stressful environmental conditions that persisted throughout the Early Triassic. In regions where oceanic conditions returned to normal, organisms recovered quickly. The Early Triassic Lower Member and Middle Member of the Union Wash Formation near Darwin, California were examined in order to better establish the relationship between biotic recovery and environmental conditions in a region where pervasive and persistent environmental stress has been previously documented. The Lower Member of the Union Wash Formation was examined to determine the extent of the biologic recovery by using trace fossil intensity, trace fossil diversity, and burrow diameter as recovery proxies. Despite the presence of complex ichnofabrics and abundant trace fossils, the small burrow diameters and monotypic trace fossil type (Thalassinoides suevicus) indicate that only limited recovery occurred, likely within the lowermost reaches of a "habitable zone," and was ultimately squelched. The results of this study suggest that the presence or absence of harsh environmental conditions was a strong control for determining the timing of biotic recovery. Current recovery models, while useful across the entirety of the Early Triassic, need to be adjusted to account for local and regional variance in ecosystems and environmental conditions.