Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): taphonomy and significance

MARSICANO, C.A.; MANCUSO, A.; PALMA, R.M.; KRAPOVICKAS, V.

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Año: 2010 vol. 291 p. 388 - 399

0031-0182

Fossil tetrapod footprints not only provide valuable information abouttrackmaker paleobiology but also to give insight into details of the depositionalconditions of the substrate at the time of imprinting. Therefore, in the present studythe mode of formation and taphonomy of footprints in different substrates was used toinvestigate the gait and walking dynamics of the trackmakers as well as a source ofadditional information on the environmental conditions of the track-bearing bedsduring imprinting.The analyzed section corresponds to thick Middle Triassic lacustrine/deltaic depositsof the Ischichuca/Los Rastros Formation (Ischigualasto-Villa Unión Basin) that cropsout at the Quebrada de Ischichuca in northwestern Argentina. Part of the trackbearingsurfaces correspond to the top of sandy distributary channel mouth bars in adistal delta front setting that were exposed along the lake margin during a lake levelfall. Cross-cutting relationships observed among ripple-marks, the footprints, andinvertebrate traces of a softground suite of the Scoyenia ichnofacies suggest anomission surface. Measured trackway orientations in the sandstones areperpendicular to the paleo-shoreline, with the animals coming and going along theexposed top of the bars, probably for drinking. Laterally, the distal delta front depositsinterfinger with track-bearing wackestone beds of palustrine origin deposited in arestricted local embayment lateral to the delta influenced environment. Trackwayorientations in the wackestones are, in contrast, consistent with the animals movingnearly parallel to the lake border, probably along a preferred route. Evidences of arelative high groundwater table at the time of imprinting in the track-bearing surfacesare revealed by the well developed rims of extruded sediment and collapsed digits inthe studied tracks and the nearly absence of associated desiccation cracks on thesame surfaces. Nevertheless, temporary emergence cannot be ruled out whenpaleosoil formation was probably promoted as can be observed in the microstructureof both sandstones and wackestones. Moreover, footprint preservation in thewackestones might have been enhanced by partial hardening of the trampled surfaceduring subaerial exposure. Combining ichnofossil content and taphonomy with faciesanalysis we identified in the lower part of the Ischichuca/Los Rastros succession arelatively rapid withdrawal of the water basinward that was probably due to a forcedregression during early rifting of basin evolution.Footprints can also provide valuable information about locomotion dynamics andtrackmaker behavior. Thus, the sideways deformation observed in the studiedfootprints, attributed to basal archosaurs and putative basal dinosaurs, can be relatedto an outward rotation of the foot during the step cycle, a condition that might allied tothe development of the parasagittal posture in Archosauria. Besides, the denselytrampled surface described herein constitutes the first documented evidence ofputative social behavior among therapsid dicynodonts, the most important group ofherviborous animals in the early Mesozoic terrestrial ecosystems throughoutGondwana