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Trace-fossil distribution within the framework of three-dimensional fluvial architecture has been commonly overlooked. The Miocene Vinchina Formation in western Argentina preserves extensive outcrops of fluvial deposits, including architectural elements of both anastomosing and braided systems identified along the Quebrada de La Troya. Multistorey sandy channels, amalgamated sandy channels, heterolithic multistorey channels, channels with gravel bars, abandoned channels, muddy floodplains, crevasse splays, and crevasse channels have been identified. Of these, only the deposits of three elements were bioturbated, namely crevasse splays, anastomosing abandoned channels, and braided abandoned channels. Vertical simple burrows (Skolithos isp.), large-sized J burrows (Capayanichnus vinchinensis), and simple horizontal burrows (Palaeophycus tubularis) are the most common trace fossils in the Vinchina Formation. Other elements include the horizontal meniscate trace Taenidium barretti and the vertebrate footprints Tacheria troyana, Macrauchenichnus troyana, and Ardeipeda isp. The trace-fossil assemblages identified in the Vinchina Formation collectively illustrate the Scoyenia Ichnofacies. In addition, five ichnofabrics are characterized. The position of the water table, substrate consistency, flow energy, and time between depositional events under arid to semi-arid climate conditions were the main parameters controlling bioturbation. Based on detailed observation of the cross-cutting relationship among ichnotaxa, the ichnofabric distribution and the preservation features of the trace fossils studied, a colonization sequence for each of the subenvironments of the Vinchina Formation is proposed in this study. In addition to integration with conventional facies analysis, articulating ichnologic data and fluvial architecture provides further insights into the application of trace fossils to unravel the sedimentary dynamics of alluvial systems.
Environmental seasonality produces variations in the distribution and structure of biodiversity throughout the year. Understanding how ecological fidelity is affected by these variations allows taphonomists to assess not only the degree of fidelity, but also to identify which seasonal assemblage the fossil record reflects. The present study aims to evaluate the fidelity of gastropod mollusks from a tropical soft substrate environment (Ubatuba Bay, Brazil) collected over a 12-month interval. Samples were collected at four sites within a bathymetric interval of 5–15 m. After the taxonomic assessment, statistical analysis of the live-dead assemblages was conducted. For biodiversity, the total number of shells per sampling site, relative abundance, and species richness, as well as the Shannon index were used. Fidelity was based on the relative percentages of species between the living and dead assemblages and similarity between them was measured by the Sørensen index. Our results show that during stable rainfall/fluvial conditions, the number of dead individuals was low, and fidelity was low while during intense turbidity and subsequent death of gastropods, the abundance of dead individuals increased, and fidelity became high. Only at collection site P4, during intense rainfall/fluvial conditions, the diversity and similarity indices showed significant values. The clockwise circulation of water masses within the bay and the upwelling currents in summer, coupled with the physical-geographical barrier at P4, explain the accumulation of shells at the site, as well as its seasonal behavior. Thus, seasonality exhibited a direct impact on the live-dead fidelity of soft-substrate Caenogastropoda from Ubatuba Bay, Brazil.
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