COMPARING EARLY TRIASSIC PATTERNS OF BENTHIC ECOLOGICAL RECOVERY FOLLOWING THE END-PERMIAN MASS EXTINCTION: A TEST OF THE HABITABLE ZONE HYPOTHESIS IN DISPARATE SHOREFACE ENVIRONMENTS

The end-Permian mass extinction devastated life on our planet by eliminating approximately 95% of its species 252 million years ago. In the Early Triassic, the first five million years following the extinction, volcanic eruptions of the Siberian Traps caused pelagic extinctions and anachronistic carbonate deposition. The surviving fauna diversified during a time of varying environmental conditions such as anoxia, hypercapnia, and acidification. It is hypothesized that there may have been certain shoreface conditions that ameliorated the effects of anoxia for the benthic fauna within a region called the habitable zone. This area defined by Beatty et al. (2008) from analysis of trace fossil data includes the lower shoreface and offshore transition where fair weather and storm wave activity would have mixed in atmospheric oxygen, relieving harsh ocean conditions for the benthic fauna there.

In this study two localities from the Moenkopi Formation are compared to better define and test the habitable zone hypothesis while simultaneously exploring differences in paleoecology. The differences between the localities include time elapsed since the extinction event, the range of environments represented, and benthic faunal composition. The earlier Smithian stage is represented by the Sinbad Member of Utah; a laterally extensive shelf populated by microgastropods and bivalves. The Sinbad represents shallow depositional environments many of which fall within the habitable zone. While anoxia was not prevalent, acidification events may have been facilitated by widespread wave aeration and dissolution of carbon dioxide produced by continuing volcanic eruptions. In contrast the later Spathian stage in Nevada is represented by the Virgin Limestone Member and dominated by crinoid, echinoid, and bivalve communities. The Virgin represents a diverse array of environments from the lower shoreface to distal offshore, only some of which fall into the hypothesized habitable zone. The differences in the proportion of habitable environments and the lateral extent of these environments may influence the differences in biodiversity observed for each locality. To better understand these differences, the composition, diversity, and abundance of the different fauna from each locality and their paleoenvironment are compared.