IN THE WAKE OF A SNOWBALL EARTH AND IN THE DAWN OF ANIMAL RADIATION: TERMINAL PROTEROZOIC EVOLUTION OF MULTICELLULAR EUKARYOTES

Crown group eukaryotes may have begun to diverge and independently evolved multicellularity by the beginning of the Neoproterozoic, but their taxonomic diversity and morphological complexity remain low until the terminal Proterozoic. Paleontological data from the terminal Proterozoic Doushantuo (ca. 600 Ma) and Dengying (ca. 560 Ma) formations indicate that the diversity of multicellular eukaryotes increased significantly after the Nantuo (or Marinoan) snowball Earth event and before the Cambrian radiation.

Multiple taphonomic windows in Doushantuo cherts, phosphorites, and carbonaceous shales allow us to assemble a more complete picture of the Doushantuo biosphere in both microscopic and macroscopic regimes. Both microscopic (mm-sized) and macroscopic multicellular algae are diverse in the Doushantuo, – microscopic algae are silicified and phosphatized in cellular detail, and macroscopic algae from Doushantuo shales are preserved in style and quality comparable to those in the Burgess Shale. These algae display a level of morphological complexity similar to crown group algae. Doushantuo animals are poor in diversity. Microscopic metazoans, as represented by phosphatized animal embryos and tabulated micro-tubes, occur in Doushantuo phosphorites. Macroscopic non-bilaterian animals may also occur in Doushantuo shales, but their interpretations are equivocal. No macroscopic bilaterians have so far been unambiguously identified in Doushantuo shales, despite their preservational potential. This indicates that the evolution of macroscopic bilaterian body plans and associated pattern formation mechanisms is largely a post-Doushantuo event.

The Dengying Formation may be separated from the Doushantuo Formation by a possible post-Marinoan glaciation. Macroscopic carbonaceous algae do occur in the Dengying Formation, but what distinguishes it from the Doushantuo is a multitude of animal fossils, including phosphatized spheroids, lightly mineralized tubular fossils (cloudiniids), large bilaterian trace fossils, and Ediacaran fossils. Paleontological data from the Doushantuo – Dengying formations and other geological units therefore support that there may be causal relationships between Neoproterozoic climate changes and metazoan evolution.