Due to the continuing corona virus situation, this meeting will be held online through Zoom.
AND EARTH’S FIRST
MAJOR EXTINCTION OF ANIMAL LIFE
Dr. Scott Evans
Agouron Postdoctoral fellow, Virginia Tech
Sunday, December 11, 2022 2:00 P.M.
THIS TALK WILL BE ONLINE
Note: This is the second Sunday of December, not our usual third Sunday of the month, so as not to interfere with members’ holiday’s plans
The oldest fossils of macroscopic, complex community forming organisms, including putative animals, belong to the Ediacara Biota. These largely soft-bodied forms predate the Cambrian explosion and include representatives of the three major body plans (Porifera, Cnidaria and Bilateria) found in animals today.
Over their approximately 40-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) – named for characteristic fossil localities from each interval. Many studies have attempted to determine the drivers of the evolutionary events that led to such forms, but few have tackled the question of what happened to the Ediacara Biota – most of which did not survive into the Cambrian. In a recent study, 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. We find 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. Although there are differences in paleogeographic distribution, environments sampled and mode of fossilization between all three Ediacaran assemblages, these do not match changes in diversity. This indicates that the observed decline in generic richness is not due to taphonomic or sampling bias, but instead represents a true extinction event.
A broad range of ecologies are impacted during this time, inconsistent with biotic replacement and matching the predicted effects of major environmental change. 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 Neoproterozoic and Cambrian oceans, suggesting that the White Sea-Nama extinction 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.