FROM BEGINNING TO BELLY UP: SIZE DISTRIBUTIONS AND THE PRESERVATION OF THE BASE OF THE EDIACARA FOSSIL TRIBRACHIDIUM

The assemblages of macroscopic, soft-bodied organisms of the Ediacara Biota represent Earth’s earliest animal ecosystems. Tribrachidium is one of the iconic members of the White Sea assemblage and occurs relatively commonly in the Ediacaran Member of the Rawnsley Quartzite in South Australia. Its distribution across the Ediacaran seafloor as a whole is best described as patchy. Tribrachidium has been found on nine of the twenty-six beds that have been excavated at Nilpena, South Australia. On six of these beds, there are five or fewer Tribrachidium, but it is the dominant genus on two of the remaining beds. Additionally, although it is uncommon for genera of similar abundance to Tribrachidium to occur in diverse facies, the nine beds where Tribrachidium occurs come from a variety of the different facies observed in the Ediacaran of South Australia. The wave-base sand facies is the most common facies for Tribrachidium.

In South Australia, Tribrachidium ranges in size from about 3-30 mm, but the size range observed on individual beds is smaller. The size frequency distributions of the Tribrachidium on each of the three beds with more than five Tribrachidium are also each significantly different from each other and from the overall size frequency distribution of all Tribrachidium at Nilpena. Combined with the lack of extensive time averaging on individual beds and the sessile interpretation of Tribrachidium, the distinct size groupings on individual beds suggest that populations of these fossils are composed of cohorts.

Interestingly, two of the beds also reveal the base of Tribrachidium. The classic, tri-radial form of Tribrachidium is preserved in negative relief on the base of the excavated beds. On the two beds dominated by Tribrachidium, the base of the organism, which consists of concentric circles, is also found. The base of Tribrachidium has been preserved in both negative and positive relief: flipping the organism upside-down on the mat prior to burial would result in a negative external mold of the concentric circles on the base of the bed; a positive feature would form when the incoming sand filled the impression left in the mat by the base of a Tribrachidium that had been previously removed.