SIZE ‘EM UP: BODY SIZE DISTRIBUTIONS OF DICKINSONIA COSTATA FROM NILPENA, FLINDERS RANGES, SOUTH AUSTRALIA

Dickinsonia costata, the iconic Ediacaran fossil of South Australia, occurs abundantly in the Ediacara Member of the Rawnsley Quartzite cropping out in the Flinders Ranges area. Excavation of 28 medium grained, rippled sandstone bedding surfaces at the National Heritage Site, Nilpena, provides the ideal setting for an ecological examination of in situ specimens. Thirteen beds contain ten or more Dickinsonia and the fossil dominates four of these bed surfaces. In addition, hundreds of specimens of D. costata housed in the South Australia Museum provide a robust picture of Ediacara Member Dickinsonia.

Body size measurements of over 900 specimens of Dickinsonia costata, taken as the diameter along the long axis, shows specimens ranging from 3.61 mm to 167.43 mm with a mean of 27.41 mm and an overall right-skewed distribution. This log-normal size frequency curve is indicative of a reproductive strategy known as high infant mortality. Body size distributions on individual beds with large numbers of Dickinsonia are also consistent with high infant mortality. For example, the body size distribution on a single bed containing 206 specimens, more than half of the D. costata at Nilpena, is strongly right skewed. Additionally, the absence of cohorts within this large population is suggestive of continuous reproduction. Size distributions on most other beds support this reproductive strategy although some beds appear to be skewed by low specimen numbers and the timing of depositional events. The presence of D. costata in numerous disparate lithologies within the Ediacara Member demonstrates that it lived in a variety of environments while abundance data suggests a preferred shallow water, fair-weather wave base habitat. Within this wave base habitat, examinations of D. costata on individual bed surfaces demonstrates a significant variability in density and size. Substrates dominated by fossils of Aspidella (and thus fronds) and Funisia limited the presence of D. costata. Based on evidence from organic mat coverings, the timing between depositional events limited the possible size ranges of D. costata, with small populations representing communities buried before they fully developed.