No description available.

Actualistic, sedimentological, and experimental approaches to the study of vertebrate trace taphonomy in the Kenya Rift Valley have permitted the characterization and even quantification of factors that influence trace morphology and preservation potential in semi-arid lake margins. Several important taphonomic factors were identified from the modern lake-marginal sediments of saline, alkaline Lake Bogoria and freshwater Lake Baringo in the Baringo-Bogoria basin (1º N and 36º E). The investigation of these factors, considering vertebrate ecology and sedimentology of the deposits, together with the study of early and later diagenetic processes that cement substrates, provided a framework for the paleoecological interpretation of three Pleistocene fossil footprint localities, also in the Baringo-Bogoria basin. The most important taphonomic factors appear to be related to the semi-arid climate (e.g., high evaporation:precipitation ratio), frequent lake level changes, the closed nature of the lake basins, bedrock geology (mainly volcaniclastic) within the catchment, and the chemical composition of lake and pore waters. Notable factors that cause the alteration, destruction, and stabilization of traces include efflorescent salt crystallization, which may temporarily cement substrates or destroy traces during crystal growth in the capillary fringe; substrate wetting and drying, which can induce soil-crusting and the shrinking and swelling of smectitic clays; and the presence of benthic microbial mats and biofilms, which may temporarily stabilize substrates or contribute to their early cementation by mediating carbonate precipitation. Experiments to quantitatively and statistically test the effects of salt efflorescence, the rate and temperature of substrate drying (e.g., sun-baking), and swelling and non-swelling clays supported field observations. Preservational processes interpreted from Pleistocene footprint-bearing sediments include the early cementation of substrates by carbonates (e.g., calcite), and during prolonged, stable dry phases, the precipitation of zeolitic cements and Mn- and Fe-oxide minerals. ****PLEASE NOTE: This thesis is formatted to be printed double-sided.

Actualistic, sedimentological, and experimental approaches to the study of vertebrate trace taphonomy in the Kenya Rift Valley have permitted the characterization and even quantification of factors that influence trace morphology and preservation potential in semi-arid lake margins. Several important taphonomic factors were identified from the modern lake-marginal sediments of saline, alkaline Lake Bogoria and freshwater Lake Baringo in the Baringo-Bogoria basin (1º N and 36º E). The investigation of these factors, considering vertebrate ecology and sedimentology of the deposits, together with the study of early and later diagenetic processes that cement substrates, provided a framework for the paleoecological interpretation of three Pleistocene fossil footprint localities, also in the Baringo-Bogoria basin. The most important taphonomic factors appear to be related to the semi-arid climate (e.g., high evaporation:precipitation ratio), frequent lake level changes, the closed nature of the lake basins, bedrock geology (mainly volcaniclastic) within the catchment, and the chemical composition of lake and pore waters. Notable factors that cause the alteration, destruction, and stabilization of traces include efflorescent salt crystallization, which may temporarily cement substrates or destroy traces during crystal growth in the capillary fringe; substrate wetting and drying, which can induce soil-crusting and the shrinking and swelling of smectitic clays; and the presence of benthic microbial mats and biofilms, which may temporarily stabilize substrates or contribute to their early cementation by mediating carbonate precipitation. Experiments to quantitatively and statistically test the effects of salt efflorescence, the rate and temperature of substrate drying (e.g., sun-baking), and swelling and non-swelling clays supported field observations. Preservational processes interpreted from Pleistocene footprint-bearing sediments include the early cementation of substrates by carbonates (e.g., calcite), and during prolonged, stable dry phases, the precipitation of zeolitic cements and Mn- and Fe-oxide minerals. ****PLEASE NOTE: This thesis is formatted to be printed double-sided.

No description available.

The western United States Deep Spring and Wood Canyon formations contain a variety of late Ediacaran fossils, representing the enigmatic Ediacara biota and a metazoan worm-like fauna. The latter, dubbed ?Wormworld?, is comprised of a number of tube-dwelling organisms whose tubes were preserved through pervasive pyritization and carbonaceous compressions, as well as abundant horizontal burrows and grazing traces. One particular site, within Ancient Bristlecone Pine Forest presents a challenge that is not faced at the other localities, as it contains abundant tube-shaped fossils that are lacking in morphological characteristics due to poor preservation. Through investigations into what remains of the fossils and the potential preservational pathways that could produce such fossils, it is possible to use the findings to identify additional tube worm assemblages that may have been overlooked due to the assumed restriction on exceptional preservation based on sedimentology. Alternatively, trace fossils in the area appear to be more readily preserved and abundant, allowing for investigations into trends in frequency and faunal occurrences leading into the Cambrian Explosion. These traces and the trace maker activities are of a particular interest, as they represent the first communities of established bioturbators, which helped oxygenate the seafloor and mix the previously stratified microbial mat-rich substrate. The trace fossils on the surface of the beds represented ichnotaxa that are well-known in Cambrian deposits. However, petrographic thin sectioning revealed an unexpected fossil, Lamonte trevallis, previously only reported from South China. The presence of Lamonte trevallis is evidence for more advanced, complex Cambrian-like feeding behaviors occurring prior to the Precambrian?Cambrian boundary. The diversity of the Ediacaran fossils from the western United States places it among the established Ediacaran exceptional preservation localities and justifies the designation of the Deep Spring/Wood Canyon assemblage as an Ediacaran Lagerst?tte.

The trace fossil, or ichnofossil record provides a different perspective on animal evolution compared to that of body fossil, as it is typically preserved in different facies and lacks the bias towards mineralised body parts. As trace fossils record behaviours, they often cannot be assigned to particular producers, except in rare cases where they are found in association with them. However, they play an important role in the study of evolution and biostratigraphy and are particularly useful in stratigraphic studies of otherwise unfossiliferous rocks.Rusophycus is a rare exception as it is a trace fossil that is typically assigned to a trilobite tracemaker. However, this does not exclude other arthropods from being considered as potential producers of Rusophycus. Indeed, the trace appears before the first trilobite body fossils and is found after their disappearance. The earlier occurrences of Rusophycus are also the earliest undoubted signs of arthropods and are from the Terreneuvian. Pre-trilobitic forms seem to be different from their younger counterpart in morphological aspect, but this has not been formally quantified.This study investigates these early Rusophycus to obtain a clearer picture of what the pre-trilobitic Rusophycus record is like and to search for temporal trends in their morphology. Pre-trilobitic occurrences are also compared with younger ones to quantify their differences and try to interpret their significance. Pre-trilobitic Rusophycus appear to have lower length:width ratios than the younger ones. Moreover, they are also, on average wider than the younger Rusophycus. This is significant as it could result from a change in producer and probably in appendages used for digging. It could as well reflect a reduction in the number of appendages used. However, these low ratios could also be the result of taphonomic variation and the preservation of only the deeper parts of the traces. A reflection on the potential producers of the pre-trilobitic Rusophycus is also provided based on what is known of the morphology of early arthropods. While no definitive conclusion can be made, the probability that at least some of the earlier Rusophycus were not made by trilobites appears to be clear.

No description available.

No description available.

No description available.

During the Late Pliocene and early Pleistocene, when the paleo-Orinoco delta system transited over the Amacuro Shelf and reached the paleo-shelf-break along the southeastern shoreline of Trinidad. At this time onwards, the shelf-edge delta clinoforms developed further eastward. These deltaic clastic wedges serve as the unique analog in the geological record for an accommodation-driven inner-shelf and shelf-edge delta, developed at an oblique foreland tectonic setting situated at a tropical-equatorial paleogeographic setting. These deposits were influenced by strong Atlantic longshore current, tropical storms, and phytodetrital pulses, and with an exceptionally high sediment accumulation rates. These four aspects make the clastic wedges unique candidates for sedimentological, ichnological, and stratigraphic investigation. The primary objectives of this thesis are to: (a) collect, analyze, and integrate outcrop data on lithofacies, trace fossils, and discontinuity surfaces into a comprehensive depositional and ichnological model for the first growth-fault-guided shelf-marginal pulse of the paleo-Orinoco delta, as recorded in the Mayaro Formation outcrops in southeast Trinidad; and (b) deduce the dominant sedimentary processes during the across shelf transit and their impacts on the benthic infauna as preserved in the Morne L’Enfer Formation outcrops of southwest Trinidad, which are possibly slightly older than the Mayaro Formation. The basal interval of the Morne L’Enfer Formation has specifically been investigated for this purpose, where the deltaic clastic wedges are preserved directly above shelf deposits. The entire Mayaro Formation megasequence is categorized into deposits belonging to twelve different subenvironments based on lithofacies associations and ichnological characteristics. Ichnological evidence indicates that the shelf-edge deltas are one of the most extreme marine environments for benthic metazoans to colonize. However, the combinations and ranking of stress factors affecting the colonizing fauna are diverse and distinct in every individual subenvironment indicating the relative dominances of river-influence, waves, and/or sediment-gravity-flows vis-à-vis slope instability. Due to variations in stress factors, the megasequence also displays dual ichnologic and sedimentologic properties of both the shelf-edge delta lobe(s) and the outer shelf delta lobe(s). A minor transient tidal influence can only be observed in the architectural elements, e.g. elongated interbar embayment and interlobe prodeltaic depocentres, which control topography and enhance tidal effect. Discovery of an unusual monospecific Glossifungites Ichnofacies along an incision surface in the midst of the Mayaro Formation succession enabled a substantial overhaul of the earlier understanding of the formation in terms of its depositional model and stacking pattern. The surface has been re-identified as a canyon/gully cut at the shelf-edge, which possibly acted as a conduit for (a) the mass movements and for (b) the coarse clastic (mostly silt to medium-grained sand) sediment transfer to deep marine settings. The monospecific nature of the Glossifungites Ichnofacies suite indicates that the incision surface was under substantial ecological stresses for the colonizing infauna. The stresses might have arisen from slope instability of the steep canyon/gully walls, mass movements above the incision surface, elevated water turbulence, and lowered salinity from river influx. Five different facies tracts have been identified within the canyon/gully-fill, which crosscuts the shelf-edge delta-front. The facies tracts are dominated by different types of sediment-gravity flow deposits, which are systematically stacked and are almost devoid of trace fossils due to rapid sedimentation rates and slope instability. They are also strikingly different from the surrounding deltaic facies. A high-frequency sequence stratigraphic model involving the influence of growth-fault tectonics on the relative sea-level curve has been invoked to explain the incision of the canyon/gully and its sequential filling processes. On the other hand, the transition from the open shelf to inner-shelf deltaic condition as displayed by the basal members of the Morne L’Enfer Formation is strongly dominated by evidences of river influence with the transient background action of fair-weather waves and storm waves. A peculiar pattern of disappearance of trace fossils produced by irregular sea-urchins highlight that the river influence was quite strong not only at the sediment-water interface but also in the water-column, which affected invertebrate larvae. The initial progradation of the clastic wedge on the shelf was dominated by hyperpycnal flows and waves in contrast to tidal domination in the younger members of the formation.