Discovery of the oldest bilaterian from the Ediacaran of South Australia.

Abstract

Analysis of modern animals and Ediacaran trace fossils predicts that the oldest bilaterians were simple and small. Such organisms would be difficult to recognize in the fossil record, but should have been part of the Ediacara Biota, the earliest preserved macroscopic, complex animal communities. Here, we describe <i>Ikaria wariootia</i> gen. et sp. nov. from the Ediacara Member, South Australia, a small, simple organism with anterior/posterior differentiation. We find that the size and morphology of <i>Ikaria</i> match predictions for the progenitor of the trace fossil <i>Helminthoidichnites</i>-indicative of mobility and sediment displacement. In the Ediacara Member, <i>Helminthoidichnites</i> occurs stratigraphically below classic Ediacara body fossils. Together, these suggest that <i>Ikaria</i> represents one of the oldest total group bilaterians identified from South Australia, with little deviation from the characters predicted for their last common ancestor. Further, these trace fossils persist into the Phanerozoic, providing a critical link between Ediacaran and Cambrian animals.