DIVERSE EARLY TRIASSIC TRACE FOSSIL ASSEMBLAGE SHOWS RECOVERY FROM END-PERMIAN MASS EXTINCTION VARIED IN TIME AND SPACE

The end of the Permian period is demarcated by the largest Phanerozoic mass extinction, and some evidence suggests that at least 5 million years elapsed before life on Earth returned to pre-extinction complexity in the Middle Triassic (e.g., Lehrmann et al., 2006). It is generally accepted that the entire Early Triassic is an interval characterized by low biodiversity and ecological complexity, but emerging research suggests that marine ecosystem recovery was not uniform around the globe following the extinction, as previously hypothesized. Paleolatitude (Wignall et al., 1996), length of time after the mass extinction (Twitchett and Barras, 2004), and shoreface architecture (Beatty et al., 2008) are proposed to have had roles in shaping the biotic recovery after the end-Permian mass extinction.

To test the hypothesis that biotic recovery following the mass extinction was spatially and temporally varied, we conducted field studies on Lower and Middle Triassic strata exposed in Ghuizhou Province, SW China. We report here on the trace fossil assemblage recorded in the Dienerian Second Member of the Daye Formation, which comprises the most diverse Early Triassic trace fossil assemblage known (~30 ichnogenera). This one-of-a-kind ichnocoenosis is recorded in thinly-bedded, cross-bedded silty micrite (Enos et al., 2006; Luo et al., 2007) deposited on a shallow, gently-sloping ramp. Though the assemblage is characterized by high diversity, trace fossils extend only within the lowest tier (0 to -6 cm); vertical bioturbation is low (ii 1 to ii 3); size of burrows is small (mm-scale); and bioturbation is primarily horizontal (BPBI 4-5).

This diverse ichnocoenosis deposited during the first half of the Early Triassic at a low paleolatitude in relatively shallow water suggests that recovery from the end-Permian extinction was complex and cannot be explained by one spatial or temporal attribute. Benthic communities during the Early Triassic were likely influenced by a combination of organisms’ environment, paleolatitude, geography, and proximity to the mass extinction. Understanding the timing and geography of ecosystem restoration will further our ability to more completely understand the inter-relatedness between environmental conditions and evolutionary processes.