Ichnological evidence for meiofaunal bilaterians from the Ediacaran and earliest Cambrian of Brazil

08 Sep 2017

The evolutionary events during the Ediacaran–Cambrian transition (~541 Ma) are unparalleled in Earth history. The fossil record suggests that most extant animal phyla appeared in a geologically brief interval, with the oldest unequivocal bilaterian body fossils found in early Cambrian. Molecular clocks and biomarkers provide independent estimates for the timing of animal origins, and both suggest a cryptic Neoproterozoic history for Metazoa that extends considerably beyond the Cambrian fossil record. We report an assemblage of ichnofossils from Ediacaran–Cambrian siltstones in Brazil, alongside U-Pb radioisotopic dates that constrain the age of the oldest specimens to 555–542 Ma. X-ray microtomography reveals three-dimensionally preserved traces ranging from 50–600μm in diameter, indicative of small-bodied, meiofaunal tracemakers. Burrow morphologies suggest they were created by a nematoid-like organism that utilised undulating locomotion to move through the sediment. This assemblage demonstrates animal-sediment interactions in the latest Ediacaran Period, and provides the oldest known fossil evidence for meiofaunal bilaterians. Our discovery highlights meiofaunal ichnofossils as a hitherto unexplored window for tracking animal evolution in deep time, and reveals that both meiofaunal and macrofaunal bilaterians began to explore infaunal niches during the late Ediacaran.