SLIME TRAVELERS: EARLY EVIDENCE OF ANIMAL MOBILITY IN THE EDIACARA BIOTA OF SOUTH AUSTRALIA

The soft-bodied Ediacara Biota contains some of the earliest evidence for mobile animals in the fossil record. While there is unambiguous evidence for mobility demonstrated by the trace fossil Helminthoidichnites, interpretations of other potentially mobile taxa remain contentious. Recent analysis of over 300 square meters of bedding planes from the Ediacara Member of South Australia demonstrates clear evidence for mobility in both Dickinsonia and Yorgia. External molds of these taxa typically occur as negative impressions on the base of sandstone beds and are associated with positive relief structures interpreted to represent areas where the organism remained stationary for some period, resulting in a depression of the organic mat covering the Ediacaran seafloor that was subsequently filled with sand, colloquially referred to as “footprints”. Poorly preserved negative impressions and well-preserved positive structures (trace fossils) as well as the preferred alignment and similar size of associated trace and body fossils suggests that “footprints” do not represent decayed specimens. One bed, TC-MM3, contains a significant number of Dickinsonia trace fossils as well as independent indicators of current directionality to establish a random orientation of the inferred movement direction with respect to paleocurrent, demonstrating that "footprints" were not formed by the passive transport of these organisms.

Our data reveal that the size distribution of positive traces of Dickinsonia is skewed toward larger sizes than expected based on the size distribution of body fossils. This is likely due to regrowth of the organic mat which would have preferentially eliminated smaller “footprints” more rapidly. Trace fossils are often similar in size or even larger than associated body fossils, indicating that movement occurred on ecological timescales and possibly that Dickinsonia and Yorgia where able to expand to increase surface area for feeding. Distances between subsequent traces suggest that typical movement occurred on the cm to dm scale and that larger organisms could move further distances, with the largest specimens found more than one meter apart. Preferentially wrinkled and irregular specimens of Dickinsonia may indicate that movement was achieved via muscular peristalsis.