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Author(s): Surprenant, Rachel Lorraine | Advisor(s): Droser, Mary | Abstract: The Ediacara Biota represent a turning point in the evolution of life on Earth, signifying the transition from single celled organisms to complex, community forming macrobiota. The exceptional fossil record of the soft-bodied Ediacara Biota provides critical insight into the nature of this transition and into ecosystem dynamics leading up to the so-called “Cambrian Explosion”. However, the preservation of non-biomineralizing organisms in a diversity of lithologies goes hand-in-hand with considerable taphonomic complexity that often shrouds true ecological and biological signatures. We address the nature of this taphonomic complexity within the fossiliferous sandstones of the Ediacara Member in South Australia. Utilizing the most data-rich and well-preserved outcropping of the Ediacara Member, the Nilpena Station National Heritage Ediacara Fossil Site, we conduct a focused, taxon-level taphonomic characterization of the tubular organism Funisia dorothea. Funisia is the most abundant body fossil in the Ediacara Member, making its taphonomic characterization essential to the accurate interpretation of regional paleobiology and paleoecology. We identify two primary modes of Funisia population structure, cluster-type and surface-type packages, each of which exhibit distinct suites of taphonomic variation in individual Funisia. Within the two package types, four preservational modes of Funisia are identified: convex external molds, concave external molds, convex internal molds, and concave internal molds. Among macrofossils at Nilpena, this tiered preservational complexity is unique to Funisia; its systematic classification elucidates population-level biostratinomy at Nilpena as well as aspects of Funisia’s paleobiology and autecology.

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 ha...

Earth's earliest known metazoan ecosystems are represented by a handful of globally distributed fossil assemblages, collectively referred to as the Ediacara Biota. Although a number of these deposits have been extensively studied, a large proportion of Ediacaran diversity remains uncharacterized. As a result, our understanding of community structure during this important stage of early metazoan evolution is largely incomplete. Moreover, it is only by examining these deposits from a taphonomic perspective that we can hope to decipher these enigmatic forms and fully reconstruct the Ediacaran community. Using this approach, we describe the anomalous preservation of a distinct, prolific, and previously undescribed Ediacaran biogenic sedimentary structure, informally known as “mop,” from the Ediacara Member of the Rawnsley Quartzite in South Australia. Morphological resemblance, spatial association, size distribution, and examination of intermediary forms indicate a shared origin with the holdfast for...

Bioturbation long has been ‘blamed’ for eliminating late Proterozoic-style sedimentary structures and fabrics. While the presence of diverse and complex burrows in lowermost Cambrian strata is indisputable, analysis of Precambrian–Cambrian successions in southeast Newfoundland demonstrate that this burrowing style did not produce typical Phanerozoic-style ichnofabrics. Three hundred meters of the siltstone/sandstone facies of member 2 of the Chapel Island Formation were examined in the area of the Precambrian–Cambrian boundary stratotype. Gyrolithes, Planolites, and Skolithos occur as sand infills ubiquitously throughout siltstone beds, most commonly without direct contact with an overlying sandstone bed, as if “floating” in the siltstone. In contrast, Treptichnus pedum occurs as sand infills adhering onto the base of thin sandstone beds that have different grain size and texture than the burrow infills. Both of these burrow types represent a style of preservation in which the burrows are unattac...

A variety of sedimentary structures and patterns in Proterozoic siliciclastic sedimentary rocks cannot be explained by known inorganic processes. In particular, certain bed-surface textures, and domed and disrupted sand lamination, are demonstrably the mechanical products of microbially bound sediment and microbial mats. In all but the most wave and current active marine environments of the terminal Proterozoic, the absence of effective grazing and burrowing allowed mat-communities of cyanobacteria to colonize sedimentary surfaces. The resultant microbial mats inhibited sediment erosion, formed partings when buried between sand beds, and restricted vertical migration of pore fluid and gas in both exposed and subaqueous environments. Distinctive "petee" laminations, known from modern mat-bound, tidal-flat sediments, are recorded for the first time in the rock record from the terminal Proterozoic Rawnsley Quartzite of South Australia. The preservation of external molds of soft-bodied Ediacaran organisms is interpreted as a function of the early diagenesis of a sole veneer. A form of "death mask" resulted from bacterial precipitation of iron minerals in the sand that smothered decaying microbial mats and megascopic benthic organisms. The appearance of three-dimensional trace fossils in Early Cambrian strata signaled a behavioral revolution; the evolution of efficient grazing reduced the development of benthic mat communities in all but the most extreme environments, while bioturbation disrupted buried mats and closed a taphonomic window of preservation for soft-bodied organisms.