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The Lower Cretaceous (Valanginian) Mulichinco Formation of the Neuquén Basin in western Argentina is host to important hydrocarbon reservoirs, encompassing a wide variety of sedimentary facies, including thick eolian deposits in its lower interval in proximal positions. Ten ichnofabrics are characterized based on the analysis of cores from the El Mangrullo oil field. These ichnofabrics have been grouped in four associations, representing eolian dunes, eolian sand sheets, interdunes, and fluvial sheet floods. In terms of the ichnofacies model, both the Scoyenia and Octopodichnus-Entradichnus ichnofacies are identified. Ichnologic information has been integrated with sedimentologic and sequence-stratigraphic information. The studied succession comprises a 3rd-order depositional sequence that is subdivided into three 4th-order sequences stacked in a backstepping pattern as a result of a rise in the water table. The base of the 3rd-order sequence is represented by the intra-Valanginian unconformity that in this area separates marine deposits below from continental deposits above. Overall the 3rd-order sequence reflects the vertical transition from a dominance of eolian dune deposits to eolian sand sheet and fluvial sheet flood deposits, the latter intercalating with marginal-marine intervals. Application of available conceptual frameworks to understand trace-fossil distribution in desert environments suggests that the eolian Mulichinco ichnofauna indicates a temporal evolution from hyper-arid to arid and semi-arid conditions. The results of the present analysis argue against the common assumption of deserts as barren of life and provide further support to the notion of an archetypal eolian ichnofacies. This study expands the realm of application of ichnology in subsurface to eolian environments by providing a characterization of an ichnofauna from a desert setting based on core analysis.
The taphonomy of a well preserved ophiuroid-stylophoran assemblage from the Bokkeveld Group, Lower Devonian of South Africa is described using micro-CT scanning techniques. This assemblage provides a taphonomic window into the structure of Early Devonian, echinoderm-dominated communities within the Malvinokaffric Realm of SW Gondwana. Micro-CT scanning and 3D modeling of a portion of an obrution bed revealed an assemblage of over 700 ophiuroids observed of the newly described Gamiroaster tempestatisReid 2019 associated with two species of stylophorans of two species, the mitrates Paranacystis cf. petriiCaster 1954 and Placocystella africana (Reed 1925). The 3D model permitted detailed sedimentologic and taphonomic analysis of the bed including determination of the degree of fossil articulation, orientation, and faunal counts. The results indicate that deposition took place in a storm-influenced, shallow marine environment smothering both attached and vagrant taxa of the benthic community during a high-energy storm event. Micro-CT scanning is a powerful taphonomic tool for the non-destructive analysis of delicate echinoderm assemblages that would otherwise be difficult to study.
Well-preserved arthropod trackways are found in the Lower Triassic Daye Formation in the Huaxi area of the city of Guiyang, South China. Two types of trackways have been identified. The first type is composed of symmetrical to asymmetrical trackways containing several sets of repeated imprints. The main imprints in each set are typically tetradactyl at the front of the imprint and bifid towards the rear. Associated intermediate imprints are simple, short to long, curved scratches with a bifid termination on one end. The second type is composed of irregularly arranged imprints of variable morphologies. The morphological characteristics of both types of trackways suggest that these trace fossils should be referred to as Kouphichnium, which is most probably produced by limulids, marine arthropods that inhabit a wide range of marginal marine environments during their life cycle. The Kouphichnium specimens documented here are the earliest limulid fossil trackways found after the Permian–Triassic mass extinction (PTME) in South China. Both forms of limulid traces are closely associated with other ichnotaxa, including Helminthoidichnites, Helminthopsis, Planolites, and Phycosiphon, which indicates the emergence of a three trophic level system with limulids located at the top of the food web during the Dienerian (251.7–251.2 Ma). Paleoenvironmental analysis reveals that the trace-makers lived below fair-weather wave base, on a gently sloping bottom, at depths of about 50–60 m, while pyrite framboid data suggest that deposition occurred under dysoxic conditions. The complexity of the Dienerian food web and feeding behaviors is a reflection of the degree of marine benthic ecosystem recovery and a result of the environmental amelioration after the PTME.