UNDERSTANDING EARLY TRIASSIC BENTHIC INVERTEBRATE DIVERSITY AND ECOLOGY AND CORRESPONDING CARBON ISOTOPE EXCURSIONS

The end-Permian mass extinction resulted in the demise of ~90% of marine genera. Recent work on the Early Triassic using carbon isotopes, ammonoids, conodonts, and some benthic fauna shows that this supposed recovery period was almost as turbulent as the extinction itself. Carbon isotope records from China, India, and Italy portray a global signal with major perturbations at stage boundaries in the Early Triassic most likely resulting from rapid and widespread volcanic eruptions of a magnitude that could alter environmental conditions. The coinciding nature of the global cycles of extinction and radiation of ammonoids and conodonts with global carbon isotope excursions as well as anachronistic microbialite and carbonate fan deposition, suggests that all three are related. In order to investigate the ubiquity of the connection between the carbon signal and biological changes, benthic diversity and ecology need to be more thoroughly investigated. A literature review showed evidence for gradual increases in both taxonomic and guild diversity. Taxonomic diversity increased from 26 genera in the Griesbachian to 66 genera by the Spathian. The lack of evenness in the recovery suggests ecological stagnation: dominance of a few disaster and opportunistic genera occupying only a few life habits. To better understand this turbulent period, we investigated the Spathian-Anisian benthic communities of the Virgin Limestone. These two Early to Middle Triassic stages were chosen for study because of a major carbon isotope excursion that occurs at the boundary, which corresponds in magnitude to the excursion at the end-Permian. Additionally, sequence stratigraphic analysis of the boundary finds beds of benthic invertebrates in stratigraphic contact with anachronistic microbialite facies suggesting a close proximity of an anoxic environment with more habitable ocean conditions. Patterns in benthic and pelagic diversity, in relation to the Spathian-Anisian excursion, both point to environmental instability in the Early Triassic as the probable cause for the protraction of the biotic crisis.