DIVERSITY OF ANIMAL LIFE IN THE DOUSHANTUO FORMATION OF SOUTHWEST CHINA

Molecular biology studies show that many of the genes used to build animal body plans are in common between Cnidaria and Bilateria. Molecular clock studies, calibrated with the fossil record, unequivocally tell us that the divergence between the bilaterians and their sister group, the cnidarians, must have arisen well before what the macrofossil record evidence shows. One way to reconcile the apparent discrepancy between molecular biology and paleontology on the origin of Bilateria is that the protostome-deuterostome ancestor must have been a microscopic animal form already present in the Proterozoic. The Doushantuo Formation, southwestern China, represents a marine environment deposit younger than the terminal Snowball episode (~590 mya) and quite older than the Precambian-Cambrian boundary (543 mya). This deposit provides one of the largest sources of astonishingly preserved microfossils with details at the cellular level, and is thus an ideal site to search for different forms of early animal life. The presence of microfossils has been studied in thousands of thin sections under bright-field and polarized light, and SEM on acid residues. The biological affinity of the identified forms has been interpreted using the morphometric consistency and deformability of their soft structures as well as the overall morphology of their surface and internal structures. This approach has led to the identification of putative sponges, adult and embryonic cnidarians, as well as protostome and deuterostome bilaterian embryo-like microfossils. These results show an incredible diversification of life forms in the Proterozoic, consistent with the predictions of molecular evolution studies.