LIFE AT THE LIMITS: NEW DATA FROM SOUTHWESTERN MONGOLIA SHED LIGHT ON THE ROLE OF REDOX IN THE EARLY DIVERSIFICATION OF ANIMALS IN THE CAMBRIAN

The radiation of animals in the early Cambrian has been attributed to numerous biological and environmental factors including gene regulation, predator-prey dynamics, redox changes, or nutrient availability. While it was likely a combination of these and many other factors that allowed animals to proliferate, the contribution of each component can be investigated to understand how these elements drive animal evolution. Specifically, we can investigate the timing and magnitude of environmental shifts and determine how well they correlate with biological changes. Here we present new paleontological and geochemical evidence from the mixed shale and carbonate succession of the Bayan Gol Formation of southwestern Mongolia. This stratigraphic succession represents deposition in a shallow tropical foreland basin during the earliest Cambrian, notably marked by the first occurrence of small shelly fossils. We document abundant tubular animal and macroalgae fossils in shales underlying the first small shelly fossils, representing a lower diversity fauna that preceded the explosion of biomineralizing organisms. Iron speciation data suggest that oxygen levels may have been low, but potentially not anoxic through this interval, allowing animals to survive. However, these conditions were ephemeral and anoxic conditions returned after the first radiation of shelly taxa. These data contribute to mounting evidence that life in the early Cambrian lived at the edge of habitability and that even minor fluctuations in environmental conditions could potentially drive great shifts in marine ecosystems.