DOES SIZE REALLY MATTER?: BODY SIZE DISTRIBUTION AND GROWTH OF DICKINSONIA COSTATA FROM NILPENA, SOUTH AUSTRALIA

Dickinsonia costata, an iconic Ediacara fossil, occurs abundantly in thin-bedded ripple laminated sandstones of the Ediacara Member of the Rawnsley Quartzite, Flinders Ranges, South Australia. Excavation of 28 medium grained, rippled sandstone bedding surfaces at the National Heritage Site, Nilpena provides the ideal setting for a paleoecological examination of in situ specimens. Twelve of these beds contain ten or more Dickinsonia and the fossil dominates four of these beds.

Body size measurements of over 400 specimens of Dickinsonia costata, taken as the diameter along the long axis of the fossil, show specimens ranging from 5.98mm to 133.34mm with a mean of 35.58mm and a median of 25.54mm. This body size data demonstrates an overall right-skewed distribution, indicating the presence of numerous small specimens with relatively few reaching large size. Investigation of body size variation from bed to bed indicates that on three beds where they are relatively abundant individual D. costata populations display a right skewed distribution similar to the overall distribution of the organism. Interestingly, not all beds display this result. Four beds contain specimens all less than 35mm. Several of these beds have underdeveloped textured organic surfaces and thus, may represent single generations on a freshly exposed seafloor. The remaining five beds consist of normal body size distributions; however, all of these beds contain less than twenty total specimens. Importantly some of the surfaces containing these specimens also show evidence of high-energy storm events (strongly aligned ripped up fronds) and no other small fossils and thus, may be a result of size sorting during storm events. These data suggest that when examining multiple populations, or single populations with high abundances, D. costata exhibits a body size distribution consistent with that of a typical soft-bodied invertebrate. On beds where this is not the case factors beyond simple biology, such as environment, likely influenced these distributions as well, providing additional insight into the depositional and preservational history of the beds.