VOYAGE TO THE BOTTOM OF THE EDIACARAN SEA: WHAT’S MISSING FROM THE PICTURE?

With a few notable exceptions (e.g. Clapham and Narbonne, Gehling and Narbonne, Grazhdankin), due to the lack of large bedding plane surfaces, studies of Ediacaran fossils are necessarily limited to individual specimens or small assemblages. Localities near Ediacara (Flinders Ranges, South Australia) allow for significant excavation of hundreds of square meters of bedding planes and thus provide an opportunity for study of spatial relationships between and among species as well as detailed comparative paleoecology and taphonomy. The fossiliferous beds were deposited as storm-sands that vary from stacked event sands deposited in channels near fair-weather wave base, to thin-bedded sands and silts deposited as waning storm surges below storm base.

Preliminary work at these localities suggests that there is a high level of heterogeneity in the bed-by-bed fossil record. At one locality, we examined 4 successive sandstone beds 2-10 cm in thickness. These beds are separated from each other by very thin (mm-cm) muddy siltstones and wafer-thin sandstones. Excavation reveals that the fossil assemblages of these beds are strikingly different. Bed 1 contains Dickinsonia. Bed 2 contains abundant Aspidella, under a basal surface entirely coated by microbial or algal material. The size-frequency plot for Aspidella is consistent with a single population. Bed 3 is dominated by Dickinsonia but includes other taxa such as Spriggina, Parvancorina, Tribrachidium and Rugoconites. Thus, there are two beds dominated by Dickinsonia. However, the size range of the Dickinsonia on these beds barely overlaps. The larger Dickinsonia show some evidence of physical reworking. Bed 4 has abundant distinctive problematic “mop-like” structures (possibly a form of actinian) that are commonly associated with disks, referred to as Aspidella and Eoporpita, as well as a very large frond attached to a holdfast.

All beds exhibit high taxonomic dominance and thus, low evenness. However, none (but for Bed 1 with large Dickinsonia) are monospecific.