Tetrapod swimming/wading traces in Lower Cretaceous marginal-marine environments from Patagonia:distribution and paloebathimetric implications

Abstract

During the past few years, the first integrated ichnological/sedimentological research of Lower Cretaceous, marginal-marine units from the Neuquén basin (Northern Patagonia, Argentina) has been carried out. Most of the ichnofossils included in those studies were invertebrate trace fossils. Only some were assigned to vertebrates (non-avian theropod dinosaurs, in particular). The aim of the present work is to communicate the presence of tetrapod swimming traces in two Lower Cretaceous units of this basin, to describe them and to inform of their use as paleobathymetric tools in a case study. The Neuquén Basin contains over 7000 m thick marine and continental deposits of Late Triassic to Paleogene age. The material analyzed in this work comes from two units: the Mulichinco Formation and the Agua de la Mula Member or Upper Member of the Agrio Formation (Mendoza Group). The first unit is a mainly clastic, marine to continental succession of early Valanginian age. Nearby the study area, the Mulichinco Formation comprises clastic and carbonate lithologies that have been interpreted as deposited in non-deltaic, open shallow marine environments. The second unit is a Late Hauterivian?Early Barremian mixed carbonate?siliciclastic marine and marginal-marine succession. In five localities, bi or tridactyl hypichnial marks composed of elongate, parallel, straight to slightly curved ridges were found. Some of the terminations are reflexed. The sets of ridges are 48 mm up to 18 cm in length, and 17 mm up to 9 cm in width. We have previously reported isolated material. Even though the specimens continue to be scarce, it is possible to acknowledge that they are more abundant than previously thought. Some are provisionally assigned to Characichnos Whyte and Romano, and are ascribed to the activity of swimming or wading tetrapods of relatively small size. During the last years it has been confirmed that shallow and/or marginal-marine environments where common in the studied units. In levels where the direct sedimentological evidence was inconclusive, the presence of these trace fossils allowed to infer shallower depositional settings.