MICROVERTEBRATES DISPLAY HIGH DIVERSITY AND DISPARITY IMMEDIATELY PRIOR TO THE END-TRIASSIC MASS EXTINCTION IN EASTERN PANTHALASSA (Invited Presentation)

Marine vertebrates are essential to their ecosystems but marine vertebrate macrofossils often exhibit patchy distributions and low sample size in shallow marine facies, limiting their utility as indicators of marine ecosystem health. Data from sand/silt-sized vertebrate microfossils can expand the known environments and stratigraphic ranges of vertebrates across key extinction intervals.

In Muller Canyon (Nevada, USA), an important sedimentary succession representing shifting shallow marine conditions in the terminal Triassic, few vertebrates have been previously reported, and studies on Panthalassan vertebrate diversity during the end-Triassic mass extinction are scarce. We extracted and analyzed vertebrate microfossils from an ichthyosaur-bearing horizon below the units which bear isotopic indicators of the end-Triassic mass extinction (ETE) to determine the ecological structure of marine vertebrates in the terminal Triassic. Our survey of ichthyoliths from the upper Mount Hyatt and Muller Canyon members of the Gabbs Formation revealed abundant actinopterygians, sarcopterygians, chondrichthyans, and reptile elements with varied sizes and dietary modes, including piscivores, generalists, and durophages. This morphologically disparate assemblage in the upper Rhaetian units indicates a thriving shallow marine vertebrate community immediately before the ETE. These results contrast with previous suggestions of a gradual onset of harmful environmental conditions well before the end of the Triassic

The vertebrate assemblage is preserved above the facies transition from the carbonate-dominated Mount Hyatt Member to the more clastic Muller Canyon Member. Although this facies shift may have been driven by shallowing water depth due to marine regression, increasing ocean acidification is likely to be better evidenced by decreasing carbonate percentages up-section. Thus, ichthyolith morphotype diversity and disparity, when combined with sedimentological and geochemical data, provide unique insights into paleoecological response to regional and global scale environmental change.