Early Triassic Trace Fossils of the Western United States and their Implications for Prolonged Environmental Stress from the End-Permian Mass Extinction

The end-Permian mass extinction left an indelible mark on trace-fossil assemblages. This is evident during the lag phase of the biotic-recovery interval that occurred in the Early Triassic. Research on the Spathian Virgin Limestone Member of the Moenkopi Formation, western United States, has yielded a mixed carbonate–siliciclastic trace fossil assemblage. The presence of such traces as Thalassinoides, Laevicyclus, and Gyrochorte indicate that previously unreported metazoan behaviors had reappeared in equatorial regions by the close of the Early Triassic. Whereas diversity of trace-fossil assemblages increased from earliest to late Early Triassic time, persistent small size, low average ichnofabric index, low bedding-plane coverage, and reduced tiering point to prolonged stressful environmental conditions following the end-Permian mass extinction—conditions for which there is an abundance of global sedimentological evidence. This trace-fossil assemblage provides a record of soft-bodied organisms that might not otherwise be detected from the study of body fossils alone; it therefore acts as a constraint on the timing of the biotic recovery. This assemblage from the Virgin Limestone Member also serves as an indicator of environmental conditions that might not otherwise be gleaned, illustrating the utility of using trace fossils as environmental proxies during the lag phase of a biotic recovery. The presence of some of these traces (i.e., Thalassinoides) in lowermost Triassic strata of western Canada suggests that the recovery of trace-makers after the end Permian mass extinction was asynchronous, and that northerly latitudes may have experienced a less-protracted biotic recovery than in equatorial regions.