Ediacaran Oxygen Oases and the Emergence of Bilaterian Burrowing

The Ediacaran period (635-542 Ma) saw major biological diversification and innovation, including the rise of bilaterian motility as evidenced by the Helminthoidichnites trace fossil. At a glance, Helminthoidichnites appears to be leveed furrow that traces the horizontal movements of a small bilaterian animal across the seafloor, but it is frequently preserved in negative relief on the bottom thin, laterally discontinuous beds. This preservation style requires that the animal burrowed beneath the margins of these discontinuous sediments--and was thus among the first in Earth history to penetrate the sediment-water interface, albeit horizontally rather than vertically. Yet the motivation for this energy-intensive burrowing behavior is unclear. To illuminate this enigmatic behavior, we examine the spatial distribution of bed sole Helminthoidichnites in negative relief at Nilpena (Flinders Ranges, South Australia). We then interpret our results in the context of a simple numerical model for benthic O₂ diffusion in sandy sediments influenced by microbial mats, which were ubiquitous in the Ediacaran marine environment. We find that the margins of discontinuous sands likely hosted O₂ levels in significant excess of ambient seawater--potentially enticing the bilaterian responsible for producing the Helminthoidichnites trace to engage in burrowing behaviors prior to the advent of modern-style burrowing and large-scale mixing of sediments in the Phanerozoic. These results imply an important role for O₂ in the tempo and mode of biological innovation on Earth, including the emergence of sophisticated burrowing behavior.