Meeting Abstract
The Ediacaran Period records the earliest evolution of complex macroscopic life, characterized by benthic eukaryotes, sponges, cnidarians, and simplistic burrows made by mobile bilaterians. These fossils appear concomitant with geochemical evidence for an increase in atmospheric O2 and oceans that were perhaps significantly warmer than modern. Interestingly, these ‘Ediacara biota’ occur globally as early as 571 Ma in deep-water, aphotic slope facies, but remain absent in shelf environments until 560-555 Ma. The Ediacaran record therefore displays a puzzling 15 Myr period when large complex eukaryotes arose and flourished in bathyal settings, but did not inhabit shallow-water environments. To explore this apparent evolutionary pattern, the O2 and capacity limited thermal tolerance of marine intertidal invertebrates with bodyplans functionally analogous to Ediacaran organisms was measured to better understand the synergistic effect of temperature in low O2 partial pressure (PO2) oceans. Across all measured polychaete and cnidarian taxa, the onset of anaerobiosis, measured as the critical PO2, was found to vary systematically with temperature above and below taxon-specific optima. These data suggest that the aerobic capacity of early macroscopic organisms would have required habitats with relatively stable temperatures. In other words, life may have been excluded from shallow waters in low-O2 redox landscapes due to seasonal and perhaps even diurnal temperature fluctuations. Ultimately, the evolution of earliest animals in isothermal deep-marine refugia may represent a physiological consequence of Ediacaran oceans characterized by low and dynamic PO2.
