A hot-off-the-presses article in Science describes the discovery of bilaterian fossils in the Doushantuo Formation, from roughly 570 million years ago.
Ten phosphatized specimens of a small (<180 µm) animal displaying clear bilaterian features have been recovered from the Doushantuo Formation, China, 40 to 55 million years before the Cambrian. Seen in sections, this animal (Vernanimalcula guizhouena gen. et sp. nov.) had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure. The structural complexity is that of an adult rather than larval form. These fossils provide the first evidence confirming the phylogenetic inference that Bilateria arose well before the Cambrian.
This is exciting news, not because it revolutionizes our understanding of evolutionary history, but precisely because it is nothing surprising at all—we expect, from molecular/phylogenetic evidence, that complex animal life arose long before the Cambrian 'explosion', and what these fossils represent is a satisfying confirmation of that expectation (and they neatly fit predictions about bilaterian evolution that Erwin and Davidson made in 2002). It is actually expected, though, that bilaterian coelomates are even older than the 570 million years of the Doushantuo Formation; the last common ancestor of protostomes (arthropods and others) and deuterostomes (vertebrates and others) is estimated to have lived somewhere between 600 and 1200 million years ago.
Continue reading "Pre-Cambrian coelomate!" (on Pharyngula)