Paleoecology of the enigmatic Tribrachidium: New data from the Ediacaran of South Australia

Abstract

Tribrachidium is a monospecific genus of the Ediacara biota found globally. In the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges of South Australia, the spatial distribution of Tribrachidium across the seafloor is best described as patchy. Although Tribrachidium is the dominant fossil on two of the twenty-six beds currently excavated and is present in large numbers on another, the genus most commonly occurs as no more than a handful of specimens on a bed. Tribrachidium size frequency distributions of each of the three beds with more than 5 specimens are all statistically distinguishable from one another. Additionally, the size range on any given bed is smaller than the overall size range observed for the genus. These patterns suggest that these organisms lived in populations composed of single generations. The beds with numerous Tribrachidium come from different facies and are characterized by the presence of dissimilar mixes of taxa and textured organic surfaces, indicating that Tribrachidium was a generalist, able to adapt to a variety environments. Uniquely, the base or internal structure of Tribrachidium is also found preserved in both positive and negative relief as a sequence of concentric ridges on beds where Tribrachidium is the dominant genus. The most parsimonious explanation for the presence of these concentric ridge fossils is that they are the fossilized form of a Tribrachidium preserved when the organism was buried upside-down, flipped over, or dead and partially decayed prior to burial.