Environmental change and biotic turnover in the Ediacaran: Evidence for the first major extinction of animals.
Scott D. Evans
Florida State University
Chenyi Tu, Adriana Rizzo, Rachel L. Surprenant, Phillip C. Boan, Heather McCandless, Nathan Marshall, and Mary L. Droser
University of California, Riverside
Shuhai Xiao
Virginia Tech
The oldest fossils of macroscopic, complex community forming organisms, including putative animals, belong to the Ediacara Biota. Over their more than 35-million-year existence, three distinct faunal assemblages are recognized amongst this biota - the Avalon (~575-560 Ma), White Sea (560-550 Ma) and Nama (560-538 Ma). We compiled a global database of Ediacaran fossil occurrences to test hypothesized drivers of changing diversity, specifically targeting an apparent extinction between the White Sea and Nama assemblages. Our results demonstrate that 80% of White Sea genera disappear from the fossil record prior to the establishment of the Nama assemblage, a loss of taxa comparable to that experienced during the ‘Big 5’ mass extinctions of the Phanerozoic. Variations in sampling and preservation do not match changes in diversity, indicating that this represents a true extinction event. A broad range of ecologies impacted is also inconsistent with biotic replacement and matches the predicted effects of major environmental change. Further, taxa whose morphology suggests a maximization of surface area relative to volume, a potential indicator of greater tolerance for low oxygen conditions, preferentially survive into the Nama assemblage. This is consistent with geochemical proxies demonstrating highly variable redox conditions in the Neoproterozoic, suggesting that the loss of White Sea taxa was caused by reduced global oxygen availability. Thus, as with the ‘Big 5’ mass extinctions in Earth history, this early event in the evolution of animals appears to have been caused by climate variability.
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