THE ROOT OF THE PROBLEM: DISTINGUISHING ALGAE HOLDFASTS FROM PULL-OUT STRUCTURES IN THE EDIACARA MEMBER OF NILPENA EDIACARA NATIONAL PARK, SOUTH AUSTRALIA

The fossil record of macroscopic algae extends back to the Tonian, but the soft-bodied and fragile nature of these taxa results in less well-preserved and typically rarer fossils than their metazoan counterparts. The Ediacara Member of the Rawnsley Quartzite at Nilpena Ediacara National Park (NENP), South Australia, is one of only a few sites globally that preserves Ediacaran algae as casts and molds and the only site known that preserves an abundance of algae and metazoan fossils alike. At NENP, algae fossils are preserved as bundles of filaments on twenty-eight of the thirty-five bedding planes excavated from the shallow water Oscillation Rippled Sandstone facies. Although there are over 250 specimens of macroscopic algae preserved across this facies, less than 25 individuals are whole specimens with a holdfast, stipe, and thallus visible. All other specimens are incomplete, and a large portion of the algal record is composed of isolated holdfasts. These discrete holdfasts are assigned to algae based on their morphology. The preservation of isolated algal holdfasts mirrors the preservation of the holdfast form genus Aspidella and its assignment to frondose organisms like Arborea. Just as the preservation of Aspidella indicates a greater community presence of fronds than the preservation of fronds themselves, the preservation of abundant algal holdfasts indicates that algae were more abundant in Ediacaran communities than is currently recorded or considered. The definitive assignment of holdfast structures to algae is critical to understanding their abundance but is complicated by their simple, rhizoidal morphology. These holdfasts are essentially composed of converging linear filaments, causing algal holdfast morphology to converge with that of pullout structures left behind when Aspidella are pulled from the microbial mat by storms, referred to as Mop. We have examined over 100 probable algal holdfasts and 200 Aspidella pull-outs from multiple bedding planes to develop a model for the recognition of true algal holdfasts versus Mop based on their distinctive morphology and taphonomy. This model can be used to more accurately estimate the relative abundance of algae in the shallow water communities of the Ediacara Member.