TESTING THE HABITABLE ZONE HYPOTHESIS FOR THE EARLY TRIASSIC BENTHIC FAUNA OF WESTERN PANGAEA

The end-Permian mass extinction was the most severe taxonomic and ecological extinction of the Phanerozoic. The Early Triassic, following the extinction event, was often viewed as a prolonged recovery period. Instead, recent research shows fluctuations in the global carbon isotope record, volatile pelagic extinctions, and the deposition of anachronistic facies; evidence of continuous environmental perturbations. This study focuses on the Virgin Limestone at Lost Cabin Springs (LCS), Nevada, which represents Spathian deposition during the Lower Triassic and includes one of the aforementioned carbon isotope excursions. Paleoenvironmental analysis previously performed at LCS revealed eight parasequences that illustrate how marine environments progressed during the Spathian in SW Nevada.

A recently described “habitable zone” refugium from northwestern Pangaea (Beatty et al. 2008) suggests that during the recovery from the extinction certain regions of the shoreface with the right amount of wave activity might have been best for the diversification and preservation of benthic organisms due to the oxygenation of upwelling anoxic deep water through wave activity. Beatty et al. (2008) found that trace fossils from the offshore transition and lower shoreface of the Lower Triassic were larger and more diverse compared to other environments.

Trace fossils at LCS also support the habitable zone hypothesis. Diversity is lowest and burrowing is shallow in offshore environments while diversity is highest in the lower shoreface where the most wave aeration occurred. Preliminary results show that benthic body fossil species composition, dominance, and ecology might also vary with location across a carbonate ramp and with wave activity. In distal offshore environments benthic diversity is low (3-4 bivalve species, 1 crinoid species) whereas areas of the ramp between storm wave base and fair weather wave base have the highest diversity and more complex ecological structure (up to 10 species of bivalve, 1 crinoid and 1 echinoid species). The steady increase in Early Triassic benthic diversity may be attributed to the protection provided to the taxa in these environments by wave aeration. Additionally it may explain the disconnect in timing between the recovery of benthic and pelagic taxa.