Meeting Abstract
Animals originated and evolved during one of the most unique times in Earth history—the Neoproterozoic Era. This talk aims to discuss 1) when landmark events in early animal evolution occurred, and 2) the environmental milieu during these evolutionary milestones, and how such factors may have affected ecosystems and bodyplans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ~800 million years ago (Ma) (Tonian), the bilaterian body plan by ~650 Ma (Cryogenian), and divergences between sister phyla by ~560-540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. Considering environmental conditions, a large dataset of >10,000 Neoproterozoic-Paleozoic shale samples compiled by the Sedimentary Geochemistry and Paleoenvironments Project is interrogated here to better understand the oceanic landscape early animals inhabited. The results demonstrate that animals evolved in a relatively low-oxygen ocean, although perhaps not considerably less oxygenated than many times in the Paleozoic. Anoxic water columns were generally ferruginous (iron-rich) rather than euxinic (sulfide-rich, as in the modern ocean), and sulfide stress was likely limited. Analyses of sedimentary total organic carbon suggest that the Neoproterozoic ocean had lower primary productivity compared to the preceding Mesoproterozoic or following Paleozoic. Combined with an inability to inhabit productive regions in this low-O2 ocean, earliest animal communities would have likely been more food limited than generally appreciated, leading to important impacts on ecosystem structure and organismal behavior.
