CONICET | Buscador de Institutos y Recursos Humanos

Although it is commonly assumed that the trace-fossil record of eolian dunes and associated environments is invariablypoor, a systematic review of the available information indicates that this is not necessarily the case. Amodel involving five main phases of colonization of desert environments through the Phanerozoic is proposedin this paper. The first phase (Cambrian?Silurian) involved animal incursions into coastal dune fields directlyfrom the sea, although it is unlikely that these animals would have remained for long periods of time in coastaldeserts. The second phase (Devonian) reflects the activities of dune pioneers that left their fluvial habitat toenter temporary or permanently into inland deserts. The third phase (Carboniferous?Permian) involved the colonizationof deserts by tetrapods. The fourth phase (Triassic?Cretaceous) involved a major exploitation of the infaunalecospace as reflected by the appearance of more varied behavioral patterns in sub-superficial structures.The fifth phase (Paleogene?Recent) reflects the appearance of the ecological dynamic that characterizes moderndesert communities. The invertebrate ichnofacies for eolian dunes is re-named herein as the ?Octopodichnus?Entradichnus Ichnofacies? honoring the seminal work of previous workers who simultaneously tackled theissue of eolian dune ichnofacies. The Chelichnus ichnofacies is retained for vertebrate trace-fossil assemblagesin eolian settings. Both ichnofacies occur inmobile and temporary stabilized sandy substrates, subject to frequenterosion and deposition, and to strong seasonality. Desert settings consist of complex mosaics of habitats or physicalunits associated with organism activity. Trace-fossil distribution can be understood as reflecting thepartitioning of desert settings in a mosaic of landscape units, characterized by water content and its temporalfluctuations, nutrient availability, nature of the substrate, and the dominant organisms present. In turn, desertsystems are dynamic entities that change as a response to regional climate. Landscape units, such as eoliansand seas, salt flat and playa lake systems, ephemeral rivers and alluvial fans, interact in response to regionalscaleclimate variations in hyper-arid, arid, and semiarid climatic settings. Ancient deserts completely developedunder hyper-arid climatic conditions rarely preserve trace fossils due to the absence ofmoisture near the surface.However, the alternation of wet periods may represent windows for life development and thus, preservation ofbiogenic structures. Arid deserts display complex patterns of dunes combined with dry, wet, and floodedinterdunes. Dry desert elements (e.g. dunes, interdunes, sand sheets) typically record the Entradichnus?Octopodichnus and Chelichnus Ichnofacies. Slight rises in regional precipitation produce elevation of the watertable and increase of fluvial discharges that provide water and sediment to the system. These processes may resultin the local concentration of trace fossils in wet interdunes and ephemeral fluvial systems, illustrating theScoyenia and Chelichnus Ichnofacies. In semiarid systems playa lakes expand by the addition of freshwater,evolving into freshwater lakes, and fluvial systems may become more common; lake margins and fluvialoverbanks typically contain trace-fossil assemblages that may be ascribed to the Scoyenia Ichnofacies.