SNAPSHOTS AND LONG EXPOSURES: PICTURING LIFE IN THE EDIACARAN

Patterns of origination and evolution of early complex life on this planet are interpreted largely from the fossils of the Precambrian soft-bodied Ediacara Biota. Excavation and reconstruction of beds of the Ediacara Member of the Rawnsley Quartzite at the National Heritage Ediacara fossil site Nilpena, in the Flinders Ranges of South Australia has exposed nearly 300 square meters of fossiliferous bedding planes. As a result, the taphonomy and sedimentology of the succession are well-constrained, rendering it possible to disentangle ecological from environmental and taphonomic signals. The fossil record that is preserved on these beds is a function of a number of factors. These include: 1) sedimentary environment—there is a strong facies control on the distribution of taxa; 2) initial colonization—in certain circumstances colonization by one taxon appears to have restricted colonization by other taxa; 3) nature of textured organic surfaces (TOS); 4) time between episodes of disturbance—in particular, time between depositional events; and 5) biostratinomic processes, including both pre-mortem and post-mortem processes. These five factors are not completely independent. The development of TOS, for instance, depends on the duration of time between depositional events—which, in turn, is related to sedimentary environment. However, these five categories represent the first-order factors controlling not only Ediacara ecology, but also the nature of the Ediacaran fossil record.

The excavation and reconstruction of beds at Nilpena yield an exceptional and unique opportunity to examine not only the taxonomic composition of Ediacara communities but also their ecological character at various stages of development. Preserved ecological ‘snapshots’ of fossil assemblages range from immature communities of small-bodied individuals, associated with poorly developed organic mats to communities characterized by a high diversity of macrofaunal taxa, wide range of body sizes and the presence of dense textured organic surfaces.