Biostratinomy of the Ediacara Member (Rawnsley Quartzite, South Australia): implications for depositional environments, ecology and biology of Ediacara organisms

Abstract

The Precambrian Ediacara Biota—Earth's earliest fossil record of communities of macroscopic, multicellular organisms—provides critical insights into the emergence of complex life on our planet. Excavation and reconstruction of nearly 300 m 2 of fossiliferous bedding planes in the Ediacara Member of the Rawnsley Quartzite, at the National Heritage Ediacara fossil site Nilpena in South Australia, have permitted detailed study of the sedimentology, taphonomy and palaeoecology of Ediacara fossil assemblages. Characterization of Ediacara macrofossils and textured organic surfaces at the scale of facies, bedding planes and individual specimens has yielded unprecedented insight into the manner in which the palaeoenvironmental settings inhabited by Ediacara communities—particularly hydrodynamic conditions—influenced the aut- and synecology of Ediacara organisms, as well as the morphology and assemblage composition of Ediacara fossils. Here, we describe the manner in which environmental processes mediated the development of taphofacies hosting Ediacara fossil assemblages. Using two of the most common Ediacara Member fossils, Arborea and Dickinsonia , as examples, we delineate criteria that can be used to distinguish between ecological, environmental and biostratinomic signals and reconstruct how interactions between these processes have distinctively shaped the Ediacara fossil record.

Constructional and functional anatomy of Ediacaran rangeomorphs

“…Rapid sedimentary casting of bag-like compartments is also documented in a range of co-occurring non-rangeomorph Ediacaran taxa. Among the most spectacular examples are in situ, vertically oriented populations of Charniodiscus in Zimnie Gory sections of the White Sea (Grazhdankin, 2014;Ivantsov, 2016), reclined but similarly frondose Pambikalbae and Arborea from Nilpena in South Australia (Jenkins & Nedin, 2007;Laflamme et al 2018;Dunn et al 2019;Droser et al 2020) and globular tripartite Ventogyrus from the Onega River area of the White Sea (Ivantsov & Grazhdankin, 1997;Fedonkin & Ivantsov, 2007). All of these fossils have been variably infilled during event-bed sedimentation, yielding a taphonomic continuum from fully inflated 3D casts through to essentially 2D death-mask imprints.…”

Dickinsonia: mobile and adhered

“…The high activity of the environment during the formation of the Flinders-Belomorian localities and the low density of the soft-bodied organisms in comparison with mineral sediment should theoretically lead to the separation of bodies from the substrate, their transfer and their burial inside the more fine-grained part of the sedimentary cycle. This, apparently, happened to a part of the community (Ivantsov & Zakrevskaya, 2018;Droser et al 2020). However, the preservation of the majority of macrofossils here is exactly the opposite case, in the most coarse-grained part of the cycle, right on the bottom of the layer.…”

“…Fossil remains of Dickinsonia were found in the upper Ediacaran deposits of South Australia (Flinders Ranges) and Eastern Europe (Podolia, southeastern White Sea area, Middle Urals) (Sprigg, 1947;Keller & Fedonkin, 1976;Fedonkin, 1983;Krivosheev & Polenov, 2001). The habitat of these organisms is usually reconstructed as hydrodynamically active shallow-marine settings between fair weather and storm wave base, sporadically covered with siliciclastic suspension (Seilacher, 1989;Grazhdankin, 2004;Droser et al 2017Droser et al , 2020. There are observations that Ural Dickinsonia could inhabit shallower parts of the basin with deviating salinity (Bobkov et al 2019;Mitchell et al 2020).…”

“…The fossil remains of Dickinsonia and other numerous macroorganisms are preserved in situ under the covers of silty-sandy sediments in burial event horizons of the Flinders-Belomorian style (Grazhdankin, 2004;Narbonne, 2005;Droser et al 2006Droser et al , 2020Ivantsov & Zakrevskaya, 2018). It was shown that microbial mats represent the substrate inhabited by the Flinders-Belomorian community (Seilacher, 1989(Seilacher, , 1999Gehling, 1999;Steiner & Reitner, 2001;Bottjer & Clapham, 2006;Callow & Brasier, 2009;Gehling & Droser, 2009;Laflamme et al 2011;Tarhan et al 2017).…”

The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing

“…The incredible natural laboratory of the fossil-bearing Ediacara Member at Nilpena offers opportunities to address the specific impact of biostratinomy, and particularly current-mediated hydrodynamic conditions, on bedding plane assemblages of Ediacaran macrofossils. Droser et al [61] use the knowledge they have gained during excavation of multiple extensive bedding plane surfaces [62] to provide a palaeoecological baseline against which surfaces suspected of having been influenced by environmental conditions can be compared. Using the taxa Dickinsonia and Arborea as examples, they are able to tease apart the competing ecological, environmental and biostratinomic signals that have contributed to assemblage composition on specific surfaces.…”