The earliest fossil communities of macroscopic organisms are preserved in the Ediacara Biota. While many are difficult to classify into well-known phylogenetic groups, the exceptional preservation of these mostly soft-bodied forms, provides a plethora of material for comprehensive investigations of their biology and ecology. Exceptional deposits of the Ediacara Member from the Flinders Ranges and surrounding areas of South Australia yield abundant Ediacara Biota fossils ideal for such studies. The research presented below targets modular Ediacaran taxa, in particular the iconic and abundant fossil Dickinsonia. Patterns of growth and development in Dickinsonia are explained most parsimoniously via posterior addition of modular units and variable growth rates of units through development regulated to maintain an ovoid shape. Fossiliferous Ediacaran bedding plane surfaces, including the recently uncovered Alice’s Restaurant bed, highlight the heterogeneous distribution of taxa on the Ediacaran seafloor. Trace fossil evidence demonstrates that Dickinsonia and Yorgia were mobile, muscular organisms that fed on the organic mat through their ventral surface. Comparisons with other bilaterally symmetrical modular taxa suggests that many Ediacaran forms may have been mobile but did not engage in activities conducive to trace fossil preservation. Dickinsonia was bilaterally symmetrical, marked by distinct anterior/posterior and likely dorsal/ventral differentiation in addition to modules that met precisely at the midline. Taphonomic variants of Dickinsonia reveal that it was structurally resilient for a soft-bodied organism, highly extensible compared with most modern biopolymers, capable of elastic and plastic deformation, and composed of relatively thick, differentiated tissue. While many of these characters are consistent with bilaterians today this fossil probably represents an extinct clade within the Eumetazoa. This work establishes that the physiology and morphology of Dickinsonia were specifically suited to Ediacaran environments, and that departures from those unique conditions may have led to the ultimate disappearance of this taxa.

Dickinsonia, an iconic member of the Ediacara biota, is abundant in the Ediacaran deposits found at the Nilpena field site, South Australia. Despite exquisite preservation at this site, many specimens of Dickinsonia appear to be incomplete. Orientation measurements from specimens on fossil beds suggest that “missing pieces” are aligned irrespective of the axial orientation of Dickinsonia. The nonrandom orientation of incomplete specimens matches that of aligned structures found on two beds. The directionality of this feature suggests the molding of incomplete specimens under the influence of current activity prior to or during burial. This feature originates where part of a Dickinsonia was lifted off of the substrate during a storm event and sand was deposited beneath this lifted portion. This suggests that Dickinsonia was easily separated from the sea floor and was not attached to the substrate. This is consistent with the data suggesting that Dickinsonia was mobile. Examination of Dickinsonia costata demonstrates ranges in length from 3.6 mm to 167.4 mm and occurs in varying abundance within and across facies. Density data show that this organism preferred a relatively shallow water, fair-weather wave base habitat. The substrate dominated by fossils of Aspidella and Funisia limited the presence of D. costata. Ubiquitous Funisia in the absence of Aspidella preserves larger populations of D. costata, while juvenile forms occur preferentially in an inferred algal-dominated seafloor. The timing of depositional events controlled the maximum size ranges of D. costata, with small populations representing communities buried before they fully developed. The biology of D. costata is reflected in the overall right-skewed, log-normal size frequency curve, demonstrating a pattern of high infant mortality. A single bed contains more than half of the D. costata at Nilpena and the absence of cohorts within this large population is suggestive of continuous recruitment.

Tetrapod swim tracks attributed to reptiles occur abundantly throughout Lower to Middle Triassic deposits across the western United States. In central and southern Utah these swim track localities are stratigraphically restricted to the upper portion of the Torrey Member of the Moenkopi Formation. Here they show detailed features such as longitudinal striae and disc-shaped claw impressions that must have required specific substrate conditions in order to be produced and preserved. A suite of sedimentologic and ichnologic observations at several swim track localities demonstrates the widespread development and persistence of subaqueous firmground substrates buried by crevasse splay deposits in the interdistributary areas of a large lower delta plain. The heterolithic stratigraphy is typical of interdistributary bays and channels and generally consists of decimeter-scale fining-upward cycles of mud- and siltstone beds below the track-bearing horizons and thicker, massive or cross-bedded sandstones above. Siltstone and sandstone beds show low angle accretion sets, vary laterally in thickness, and commonly pinch out so that they cannot be correlated regionally. Common sedimentary structures indicate predominately quiet subaqueous conditions with periodic higher energy events. These include trough cross stratification, climbing ripples, rip-up clasts, and soft sediment deformation. Track surfaces often exhibit localized dewatering structures and load casts in addition to flute casts and current crescents indicating unidirectional current flow. Mudcracks are rare. The low diversity invertebrate ichnoassemblage is comprised of locally high densities of relatively diminutive traces and reflects stressed brackish water faunas. These characteristics are almost certainly augmented by delayed biotic recovery following the end-Permian mass extinction and resulted in extremely low degrees of bioturbation. This lack of biogenic mixing promoted semi-consolidation of dewatered mud substrates resulting in the widespread production and persistence of firmgrounds capable of recording and maintaining detailed swim tracks. The subsequent burial of these firmground surfaces under coarser-grained crevasse splay deposits resulted in sharp heterolithic contacts further facilitating the preservation of detailed track features.

The radiation of burrowing metazoans in the early Phanerozoic is commonly assumed to have dramatically altered the properties of marine sediment. The advent of biogenically-mediated sediment mixing, by profoundly impacting nutrient burial fluxes, organic carbon remineralization, seafloor oxygenation and sediment ecology, likely instigated Phanerozoic biogeochemical cycling. However, the timing of the development of mixed seafloor sediment has, historically, not been well constrained. Mixing has commonly been assumed, in the absence of data, to occur at the Precambrian-Cambrian boundary with the appearance of the index fossil and three-dimensional burrow Treptichnus pedum (the &ldquoCambrian substrate revolution&rdquo or &ldquoagronomic revolution&rdquo). Likewise, instances of exceptional preservation--preservation of near-surface structures of high taphonomic fidelity, including bioglyphic detail--have previously been anecdotally reported from lower Paleozoic trace fossil assemblages worldwide. However, it has hitherto been unknown to what extent this exceptional preservation characterizes the lower Paleozoic record as a whole.

Herein I present new ichnological, stratigraphic and taphonomic data suggesting that, although significant developments in infaunalization-- paleobiologically complex animal-substrate interactions, particularly burrow construction--occurred during the early Paleozoic, mixing remained suppressed throughout this interval. I demonstrate that shelfal sediment in the earliest Cambrian was essentially unmixed and that, even as late as the middle Ordovician--over 75 million years after the Precambrian-Cambrian transition and well after the appearance and diversification of supposedly deposit-feeding trilobites, seafloor shelfal sediment remained largely unmixed on a global scale. Moreover, lower Paleozoic heterolithic shelfal successions are also characterized by widespread exceptional preservation of shallow-tier ichnological assemblages and bioglyphic structures, implying formation and preservation in a cohesive, i.e. unmixed substrate. Bed-sole assemblages are preserved through syndepositional casting rather than intrastratally. Well-mixed intervals are typically of limited spatial extent; macro-scale depositional fabrics are largely undisrupted. These findings challenge the current assumption that mixing occurred with the first appearance of three-dimensional burrows and that early Paleozoic infauna were efficient sediment mixers. The protracted development of the mixed layer holds important implications for exceptional preservation in both the trace and body fossil records, for the structure of lower Paleozoic benthic ecosystems, and for the advent and development of modern-style biogeochemical cycling.

Here we reconstruct a new tubular, serially divided organism with a bilateral morphology from the Ediacaran of South Australia. The organism, Plexus ricei n. gen. n. sp., was a broadly curving tube that resided on the Ediacaran seafloor. Plexus ricei individuals range in size from 5 to 80 cm long and 5 to 20 mm wide, and are comprised of two main components: a rigid median tubular structure and a fragile outer tubular wall. Plexus ricei is preserved as an external mold on bed soles, and as a counterpart cast on bed tops in sandstones interpreted to represent deposition between storm and fairweather wave-base. The phylogenetic affinities of P. ricei are uncertain; P. ricei symmetry implies a bilaterian origin, but a lack of defined anterior and posterior ends precludes definitive assignment.

The Ediacaran Period, spanning 635 to 538 million years ago, marks a pivotal phase in life’s evolutionary history, showcasing early multicellular animal life. This thesis examines the remarkable fossil bed, 1T-F, from the Ediacara Member of the Rawnsley Quartzite at Nilpena Ediacara National Park, South Australia, offering insights into a shallow marine ecosystem during the Ediacaran period. 1T-F, preserving over 400 fossils across 16 metazoan-grade genera and numerous multicellular algae, exhibits remarkable ecological complexity and diversity. A detailed examination reveals a stable, low-energy environment that fostered diverse life forms. Comparative studies with other fossil beds emphasize 1T-F’s unique ecological dynamics, providing valuable insights into the evolutionary transitions preceding the Cambrian Explosion and enhancing our understanding of early multicellular ecosystems.

A new species of Middle Ordovician receptaculitid is described from the Arrow Canyon Range in southeastern Nevada. Ischadites n. sp. exhibits a unique gross morphology, large body size, small, numerous outer plates, and an exceptionally large aperture to body size ratio.

Hydrocarbon seeps and their associated chemosymbiosis-based communities have been documented throughout modern oceans, as well as in Neoproterozoic through Pleistocene aged rocks in the stratigraphic record. A particularly good record of ancient hydrocarbon seeps occurs in Mesozoic strata (Tithonian-Maastrichtian) along the western North American continental margin, cropping out throughout the Great Valley Group forearc strata and the Franciscan Complex. This extensive record allows for the comparison of the paleoecological signatures of stratigraphically and spatially related hydrocarbon seeps, which are geotectonically, petrographically, and isotopically consistent with each other.

Ten hydrocarbon seeps were located throughout this region and their physical, morphological, and paleoecological signatures were documented. Analyses indicate that their chemical signatures are petrographically and paragenetically similar and their geochemical values are consistently anomalous, however, their physical and biological signatures are surprisingly variable. The localities are strikingly different from each other in regards to faunal composition and morphological characteristics. The deposits include variably sized lenses, pods, mounds, and continuous outcrops. Eighteen taxa were identified, the most common being vestimentiferan worm tubes. Gastropods are the most abundant clade, while bivalves are the most diverse. No taxon is found at all ten localities and each locality is dominated by a different taxon that is commonly only a minor constituent of another localities faunal assemblage. This variability indicates that, as with modern seeps, heterogeneity is driven largely by the ephemeral nature of the seep environment, including ebullition volume, rate, and area, rather than by the duration of fluid advection.

These ancient seep deposits cropping out in the GVG forearc strata and the Franciscan Complex preserve the physical, chemical, and biological signatures of a group of tectonically related ancient hydrocarbon seeps and are a record of 80 m.y. of intermittent fluid seepage, occurring over geologically short intervals. along the western North American continental margin. As the abundance of recognized hydrocarbon seeps in this area continues to increase, they can be viewed collectively as a quasi-continuous, dynamic fluid-sediment-biotic system, with the potential to reveal larger evolutionary, biogeographic, geotectonic, and geochemical patterns of hydrocarbon seep processes and associated faunas through geologic time.

The sharing of ecospace with organisms of the same or different species is one of the most fundamental properties of life. The excavation and conservation of nearly 40 bedding plans covered with in situ Ediacaran fossils at Nilpena Ediacara National Park in South Australia, allows for the application of spatial ecological techniques typically used on modern ecosystems to test hypotheses about spatial properties of Earth’s oldest animal communities. Using a combination of photogrammetry and spatial point pattern analysis, the spatial relationships of the sessile taxa Aspidella, Obamus, and Tribrachidium in the context of body size and abundance is presented here. Two Aspidella dominated beds, a Tribrachidium dominated bed and one Obamus populated bed from two of the four fossiliferous facies at Nilpena were examined for this study. Results reveal that Aspidella and Obamus are distributed into segregated clusters, while Tribrachidium are more spatially homogenous. The Aspidella clusters are heavily influenced by the taxon’s dominance on the beds and by environmental factors (surrounding taxa, taphonomy, etc.). Additionally, sized-based analysis reveals the potential for intraspecific competition amongst the Aspidella, while also fortifying a continuous reproduction hypothesis. Obamus show “hyper aggregation,” which is potentially a result of dispersal limitation. Finally, Tribrachidium spatial results support a seasonal reproductive method and passive suspension feeding strategy.

Tribrachidium is a monospecific genus of the Ediacara biota that is found globally. In the Flinders Ranges of South Australia, the spatial distribution of Tribrachidium across the seafloor is best described as patchy. Although Tribrachidium is the dominant fossil on two of the twenty-six beds currently excavated and is present in large numbers on another, the genus most commonly occurs as no more than a handful of specimens on a bed. Tribrachidium size frequency distributions of each of the three beds with more than 5 specimens are all statistically distinguishable from one another. Additionally, the size range on any given bed is smaller than the overall size range observed for the genus. These patterns suggest that these organisms lived in populations composed of cohorts. The beds with numerous Tribrachidium come from different facies and are characterized by the presence of dissimilar mixes of taxa and textured organic surfaces, indicating that Tribrachidium was a generalist, able to adapt to a variety environments. Uniquely, the base or internal structure of Tribrachidium is also found preserved in both positive and negative relief as a sequence of concentric ridges on beds where Tribrachidium is the dominant genus. The most parsimonious explanation for the presence of these concentric ridge fossils is that they are the fossilized form of a Tribrachidium preserved when the organism was buried upside-down, flipped over, or partially decayed prior to burial.