Palaeoecology of Ediacaran communities from the Flinders Ranges of South Australia

Abstract

If we could capture a glimpse of the earliest macroscopic communities on Earth, what might they have looked like? The globally distributed fossils of the Ediacara biota represent the earliest-known examples of multicellular life, and are our best chance of understanding how early macroscopic life evolved on Earth. The Ediacaran fossils of the Flinders Ranges in South Australia (~ 555 million years old) record ancient marine-benthic communities as shallow impressions in large expanses of stratified, fossilized seafloors. The unique preservation style of Ediacaran fossils, where largely external impressions replicate the locations of individuals on the ancient seafloor as they were in life (pre-burial and subsequent fossilization), allows for the analysis of inter- and intra-taxon spatial distributions and interpretation of organism behaviour. Furthermore, crude yet detailed impressions of the individuals allows for the limited analysis of morphological characters, and occasionally tentative placement within specific phyla. Due to limitations in preservation, the phylogenetic affinities of Ediacaran fossils are still debated. Assignments have ranged from extinct relatives of extant marine animals, to terrestrial fungi and lichens, to an extinct kingdom of life altogether. However, many palaeontologists today recognize Ediacaran fossils as a diverse collective of enigmatic marine organisms, some of which might represent the earliest examples of molluscs, cnidarians, echinoderms, sponges and arthropods. The Flinders Ranges of South Australia preserves some of the world’s most diverse Ediacaran communities, so Ediacaran seafloors from there have been the subject of many studies of Ediacaran palaeoecology. In my thesis I investigate the palaeoecology of select Ediacaran seafloors excavated from two main fossil sites from the western flanks of the Flinders Ranges: Ediacara Conservation Park and the National Heritage Listed fossil site in Nilpena. Due to the high species diversity present on many Ediacaran seafloors, I explore the communities from a holistic perspective, comparing apparent ecological trends with living communities, as well as from a species-specific level. The community ecology of a new fine-grained Ediacaran fossil bed recently discovered in Ediacara Conservation Park (NECP Bed-1) is explored. This fossil bed preserves a highly diverse community including dozens of specimens of the small enigmatic shieldshaped fossil Parvancorina, and two new undescribed genera. The diverse Ediacaran community, highly textured organic surface (TOS) and trace fossils are evident of successive events occurring on NECP Bed-1, and are indicative of a mature community at late-stage succession. Foremost, I focus on the small and relatively common shield-shaped fossil Parvancorina, which has been controversially interpreted as an early arthropod. Through nearest-neighbour cluster analyses of the Parvancorina population on NECP Bed-1 (n = 202), I demonstrate that two size-classes are present, distinguishing ‘juveniles’ from ‘adults’. Furthermore, orientation analysis of the population showed a strong bimodal orientation in alignment with benthic currents, suggesting that orientation played an important role in its autecology. Globally, there are two described species of Parvancorina inferred from traditional bivariate analyses of specimen length and width, that demonstrate gross shape disparity: 1) P. minchami, specimens of which are laterally wider, whilst 2) P. saggita specimens are comparatively narrower. To more comprehensively assess the shape variability in the genus, I apply geometric morphometric shape analyses to 213 specimens from Ediacara Conservation Park, Nilpena and the White Sea of Russia collectively, revealing a continuous gradient in shape change from wide specimens through to narrow specimens. In light of the variability observed in its shape, I argue that the two currently described taxa are possibly extreme morphotypes of a species that demonstrates a high degree of morphological plasticity. In this thesis I also describe a new Ediacaran fossil with bilateral symmetry from Ediacara Conservation Park, an organism I have named Velocephalina greenwoodensis. This fossil shows a body structure previously undescribed among the Ediacaran genera, although it does share some similarities with the mollusc-grade Ediacaran fossil Kimberella. As such, I interpret Velocephalina to be a possible stem-group mollusc, and also suggest that bilaterian organisms were likely more prolific during the Ediacaran period than previously thought. Finally, I examine the palaeoecology of major fossil beds excavated from Nilpena using species-diversity models applied to living communities, to see if the same ecological assembly rules pertained to the earliest complex communities on Earth. The species-area richness (SAR) model, 𝑆 = 𝑐𝐴𝑧, where species richness (S) increases as a power function (z) of habitat area (A), is a fundamental ecological law that applies to all living communities. I apply the fundamental ecological law of SAR to a sample of 18 Ediacaran seafloor surfaces from Nilpena to see if the same ecological assembly rules pertained to some of the earliest communities on Earth. Remarkably, despite a lack of predation –one of the main drivers of Phanerozoic evolution– in the sampled Ediacaran communities, and vast changes in species composition, this study demonstrates that this fundamental ecological assembly rule persisted for over half a billion years.