PALEOECOLOGY OF THE EDIACARAN FOSSIL TRIBRACHIDIUM: EDIACARA MEMBER, RAWNSLEY QUARTZITE, SOUTH AUSTRALIA

Tribrachidium is a classic tri-radial element of the Ediacara Biota from the Ediacara Member of the Rawnsley Quartzite of South Australia. The phylogeny of the genus remains enigmatic and the paleoecology has not been previously been examined. Excavation of over 20 beds in the area of Nilpena, west of the Flinders Ranges exposing over 300 square meters, provides an excellent opportunity to consider the paleoecology of this classic genus.

Tribrachidium occurs in negative relief on the base of 11 of the excavated beds. It is the dominant organism on two beds (1TTB, TB) and occurs commonly on one (MM2) that is otherwise dominated by Dickinsonia. It occurs in various facies ranging from those interpreted to represent shallow-marine settings above storm wave base (MM2, 1TTB) to below wave base within incised valley fill (TB). Tribrachidium is not typically associated with any particular genus, though it commonly occurs with Rugoconites. On 1TTB over 140 specimens occur in approximately 4 square meters with Dickinsonia, Rugoconites and Aspidella. Over 40 specimens of Tribrachidium occur on TB primarily with tube fossils. Tribrachidium is directly associated with textured organic surfaces on both 1TTB and TB. There is no evidence that Tribrachidium was mobile.

Within a bed there is no evidence for clustering, however, the variable abundance of Tribrachidium within and across facies reveals an overall patchy distribution of Tribrachidium populations on Ediacaran seafloors. While the diameter of Tribrachidium ranges from 4 to 30 mm across these three beds, it occurs with nearly non-overlapping size ranges on MM2, 1TTB and TB with mean diameters of 7, 12, and 20 mm respectively. We interpret Tribrachidium to have been a sessile, suspension-feeding organism, adhering to biomats, and recruited opportunistically.