RIDGE OVER TROUBLED WATER: INVESTIGATING THE LIFE HABIT OF ATTENBORITES JANAE

The Ediacara Biota existed during the Ediacaran Period and were the first community forming taxa. Currently, all members of the Ediacara Biota are interpreted as benthic and dependent on the microbial mat that dominated Ediacaran seafloors. Attenborites janae is a recently described fossil taxon of the Ediacara Biota, found in the Ediacara Member in South Australia, which previous researchers have suggested may have been pelagic based on preliminary data, setting it apart from other existing Ediacaran taxa. Attenborites is the fossil of a soft bodied organism preserved as a cast on the base of fine-grained sandstone beds from the Rawnsley quartzite found at the Nilpena Ediacara National Park (NENP). These sandstone beds capture snapshots of the Ediacaran seafloor as it was 550 million years ago, preserving in situ communities of the Ediacara biota. Attenborites is preserved on only 2 of the 40 excavated beds at NENP, with 99 out of 100 specimens preserved en masse on a singular bed, TB-ARB. Here we test the hypothesis that Attenborites was pelagic with nearly double (52 to 100) the number of previous specimens while also examining the morphology and taphonomy of the taxon in greater detail.

Attenborites is an irregular taxon possessing three morphotypes, circular, ovate, and triangular, which appear to be the result of differential orientation of the organisms when preserved upon the microbial mat. Additionally, the outer margins of Attenborites specimens are well-defined, supporting the conclusion that the taxon was not imbedded in the mat but distinctly separated from it during life. Attenborites are usually preserved with internal, longitudinal ridges, and the density and number of ridges on a singular specimen appear to be random with no correlation to the size of an individual. Additionally, these ridges commonly converge at one end of the fossil, forming an internal triangular pattern. The variation in the number and density of these ridges indicates that they likely formed during deflation of the organism, while the triangular convergence of the ridges on many specimens suggests that they preferentially form in a pattern reflecting morphological characteristics of Attenborites. These lines of evidence together support the position that Attenborites was most likely pelagic and living in the water column rather than on the seafloor.