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Palaeoecology of Ediacaran communities from the Flinders Ranges of South Australia
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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 shield- shaped 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 th𝑧e earliest complex communities on Earth. The species-area richness (SAR) model, 𝑆 = 𝑐𝐴 , 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.
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Recent excavations of Ediacaran assemblages have revealed striking bed-to-bed variation in diversity–abundance structure, offering potential insight into the ecology and taphonomy of these poorly understood early Metazoan ecosystems. Here we compare faunal variability in Ediacaran assemblages to that of younger benthic assemblages, both fossil and modern. We decompose the diversity of local assemblages into within-collection (α) and among-collection (β) components and show that β diversity in Ediacaran assemblages is unusually high relative to younger assemblages. Average between-bed ecological dissimilarities in the Phanerozoic fossil record are comparable to within-habitat dissimilarities typically observed over meter to kilometer scales in modern benthic marine habitats, but dissimilarities in Ediacaran assemblages are comparable to those typically observed over 10–100 km scales in modern habitats. We suggest that the unusually variable diversity–abundance structure of Ediacaran assemblages is due both to their preservation as near snapshots of benthic communities and to original ecological differences, in particular the paucity of motile taxa and the near lack of predation and infaunalization.
Middle-to-late Ediacaran (575-541 Ma) marine sedimentary rocks record the first appearance of macroscopic, multicellular body fossils, yet little is known about the environments and food sources that sustained this enigmatic fauna. Here, we perform a lipid biomarker and stable isotope (δ 15 N total and δ 13 C TOC) investigation of exceptionally immature late Ediacaran strata (<560 Ma) from multiple locations across Baltica. Our results show that the biomarker assemblages encompass an exceptionally wide range of hopane/sterane ratios (1.6-119), which is a broad measure of bacterial/eukaryotic source organism inputs. These include some unusually high hopane/sterane ratios (22-119), particularly during the peak in diversity and abundance of the Ediacara biota. A high contribution of bacteria to the overall low productivity may have bolstered a microbial loop, locally sustaining dissolved organic matter as an important organic nutrient. These oligotrophic, shallow-marine conditions extended over hundreds of kilometers across Baltica and persisted for more than 10 million years.
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 including 1) sedimentary environment; 2) initial colonization; 3) nature of textured organic surfaces; 4) duration of time between episodes of disturbance; and 5) biostratinomic processes. 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.
Establishing how Ediacaran organisms moved and fed is critical to deciphering their ecological and evolutionary significance, but has long been confounded by their non-analogue body plans. Here, we use computational fluid dynamics to quantitatively analyse water flow around the Ediacaran taxon Parvancorina, thereby testing between competing models for feeding mode and mobility. The results show that flow was not distributed evenly across the organism, but was directed towards localized areas; this allows us to reject osmotrophy, and instead supports either suspension feeding or detritivory. Moreover, the patterns of recirculating flow differ substantially with orientation to the current, suggesting that if Parvancorina was a suspension feeder, it would have been most efficient if it was able to re-orient itself with respect to current direction, and thus ensure flow was directed towards feeding structures. Our simulations also demonstrate that the amount of drag varied with orientation, indicating that Parvancorina would have greatly benefited from adjusting its position to minimize drag. Inference of facultative mobility in Parvancorina suggests that Ediacaran benthic ecosystems might have possessed a higher proportion of mobile taxa than currently appreciated from trace fossil studies. Furthermore, this inference of movement suggests the presence of musculature or appendages that are not preserved in fossils, but which would noneltheless support a bilaterian affinity for Parvancorina. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
Ediacara fossil surfaces from the Flinders Ranges (South Australia) commonly record excellent preservation quality and can provide a palaeoecological window into some of the oldest communities on Earth (ca. 555 Ma). An excavated semi-contiguous sandstone bed of 6.5 m² from a fossil locality at Crisp Gorge in the central Flinders Ranges records an abundance of taxa and structures characteristic of White Sea assemblage communities. Stratigraphic analysis places the fossil surface within the Oscillation Rippled Sandstone Facies of the Ediacara Member at Crisp Gorge. The community appears to be predominantly juvenile forms, with Dickinsonia costata, Parvancorina minchami and Tribrachidium heraldicum present only within interpreted juvenile size ranges. The textured organic surface contains structures including low relief ridges and round bosses, but overall records a smooth bed surface and is interpreted as representing a surface with only an immature microbial mat developed before burial. No effaced or decayed organisms were identified. Community analysis describes an intermediate Shannon diversity (1.27) and an uneven community dominated by the population of D. costata, which comprises more than 50% of the individuals. The examined parameters, when combined with the presence of small, juvenile taxa and an immature organic mat, suggests that the community inhabiting this surface prior to its catastrophic burial may have been comparable to a modern early-stage, primary successional community. The Crisp Gorge bed emphasizes that Ediacara fossil surfaces from South Australia span a range of developmental stages and offer a window into Ediacaran sea-floor communities at various stages of maturity.
Diverse interpretations of Ediacaran organisms arise not only from their enigmatic body plans, but also from confusion surrounding the sedimentary environments they inhabited and the processes responsible for their preservation. Excavation of Ediacaran bedding surfaces of the Rawnsley Quartzite in South Australia has provided the opportunity to study the community structure of the Ediacara biota, as well as the autecology of individual organisms. Analysis of two bedding surfaces preserving large numbers of Parvancorina illustrates that individuals display a preferred, unidirectional orientation aligned with current, as indicated by the identified current proxies: tool marks, overfolded edges of Dickinsonia, felled fronds and drag structures generated by uprooted frond holdfasts. Taphonomic and morphological evidence suggests that the preferred orientations of Parvancorina individuals are not the result of passive current alignment, but represent a rheotactic response at some stage during their life cycle. These results illustrate a previously unrecognized life mode for an Ediacaran organism and arguably the oldest known example of rheotaxis in the fossil record. The morphology and previously suggested phylogenetic affinities of Parvancorina are also re-evaluated. Apart from possessing a bilaterally symmetrical body, there are no unequivocal morphological characters to support placement of Parvancorina within the Euarthropoda or even the Bilateria.
The Ediacara Member of the Rawnsley Quartzite of South Australia hosts some of the most ecologically and taxonomically diverse fossil assemblages of the eponymous Ediacara Biota-Earth's earliest fossil record of communities comprised of macroscopic, complex, multicellular organisms. At the National Heritage Site, Nilpena, fifteen years of systematic excavation and reassembly of bedding planes has resulted in reconstruction of over 400 square meters of Ediacaran seafloor, permitting detailed and sequential sedimentary, paleoecological and taphonomic assessment of Ediacara fossilized communities and the shallow marine settings in which these ecosystems lived. Sedimentological investigation reveals that the Ediacara Member consists of successions of sandstone event beds and a paucity of other lithologies. Moreover, these Ediacara sandstones are characterized by a suite of sedimentary features and style of stratigraphic packaging uncharacteristic of Phanerozoic sandstone successions considered to have been deposited in analogous shallow marine, storm-dominated environments, including: (1) extremely thin (sub-mm- to mm-scale) bed thickness; (2) lateral discontinuity; (3) textural uniformity, including lack of disparity in grain size, between adjacent beds; (4) lack of amalgamation; (5) lack of erosional bed junctions; (6) doubly rippled bedforms defined by rippled bed tops and bases which crisply cast the tops of underlying rippled beds; (7) ubiquity of textured organic surfaces (TOS); (8) positive correlation between body fossil size and abundance and bed thickness; and (9) texturally immature assemblages of sandstone rip-up clasts along bed tops. We interpret these features to reflect the presence of widespread matgrounds, which facilitated seafloor colonization by and ecological development of Ediacara macroorganisms in high-energy environments. Further, we argue that pervasive matgrounds directly mediated the formation and preservation of non-uniformitarian sedimentary features and stratigraphic packaging in the Ediacara Member and were responsible for the anactualistically complete nature of the Ediacara stratigraphic record.
The snowball Earth hypothesis predicts that low-latitude glacia-tion lasted millions of years while CO 2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleo-magnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion–isotope dilution–thermal ioniza-tion mass spectrometry) from eight tuffs from multiple distant strati-graphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the mul-timillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to <9.5 m.y. after the Gaskiers glaciation. Thus, despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes, it appears that Earth narrowly escaped a third Neoproterozoic snowball glaciation just prior to the late Ediacaran expansion of metazoan ecosystems.