Field of Science

Two New Triassic Papers in Paleo3

The latest Triassic publication fest continues.

Marsicano, C.A., Mancuso, A.C., Palma, R.M., and Krapovickas, V. 2010. Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): taphonomy and significance. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2010.03.009.

Fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting. Therefore, in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track-bearing beds during imprinting. The analyzed section corresponds to thick Middle Triassic lacustrine/deltaic deposits of the Ischichuca/Los Rastros Formation (Ischigualasto-Villa Unión Basin) that crops out at the Quebrada de Ischichuca in northwestern Argentina. Part of the track-bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall. Cross-cutting relationships observed among ripple-marks, the footprints, and invertebrate traces of a softground suite of the Scoyenia ichnofacies suggest an omission surface. Measured trackway orientations in the sandstones are perpendicular to the paleo-shoreline, with the animals coming and going along the exposed top of the bars, probably for drinking. Laterally, the distal delta front deposits interfinger with track-bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment. Trackway orientations in the wackestones are, in contrast, consistent with the animals moving nearly parallel to the lake border, probably along a preferred route. Evidences of a relative high groundwater table at the time of imprinting in the track-bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces. Nevertheless, temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones. Moreover, footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure. Combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the Ischichuca/Los Rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution. Footprints can also provide valuable information about locomotion dynamics and trackmaker behavior. Thus, the sideways deformation observed in the studied footprints, attributed to basal archosaurs and putative basal dinosaurs, can be related to an outward rotation of the foot during the step cycle, a condition that might allied to the development of the parasagittal posture in Archosauria. Besides, the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts, the most important group of herviborous animals in the early Mesozoic terrestrial ecosystems throughout Gondwana.

Preto, N., Kustatscher, E., and Wignall, P.B. 2010. Triassic climates -- state of the art and perspectives. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2010.03.015.

ABSTRACT - The climate of the Triassic period was characterized by a non-zonal pattern, dictated by a strong global monsoon system with effects that are most evident in the Tethys realm. This strong monsoonal regime is related to the aggregation of the Pangaean supercontinent, which by Triassic time was already completed. Climate oscillations existed within this framework. The harsh hothouse climatic conditions that characterized the Late Permian, and perhaps precipitated the end-Permian mass extinction, were probably maintained during the Early Triassic and may account for the impoverished, but distinctive, faunal and floral Lower Triassic associations. Although metazoan reef builders were probably the most
affected group, carbonate production remained high at least in the western Tethys realm. The Middle Triassic was characterised locally by humid episodes, although their geographical distribution has yet to be clarified. The Carnian Pluvial Event, marks an episode of increased rainfall documented worldwide, was the most distinctive climate change within the Triassic. Different hypotheses have been proposed for its causes: changes of atmospheric or ocean circulation driven by plate tectonics; a peak of the global monsoon due to maximum continent aggregation; or triggering by the eruption of a large igneous province. Subsequently, the late Carnian and Norian seem to have been climatically stable, although minor climatic changes have recently been described even from this time period. Finally, the end Triassic extinction event is also associated with climate change, specifically warming and increased rainfall, but this evidence comes mostly from the northern parts of the Central Atlantic Magmatic Province, and the global pattern of climate change at the Triassic / Jurassic boundary has still to be resolved. Many facets of Triassic climate are intriguing and deserve further research. However, paleoclimate studies on the Triassic have so far been carried out only locally with different proxies. Those proxies will require inter-calibration, in order to depict correctly the temporal and geographical patterns of Triassic climate.


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