Dinosaur tracks, xiphosuran trackway and tellinoid feeding trace fossils: A window to a Ghost fauna from the lower Cretaceous of the Neuquén Basin, Argentina.

PAZOS, PABLO J.; FERNÁNDEZ, DIANA E.; LAZO, DARÍO G.; TUNIK, MAISA A,; AGUIRRE-URRETA, M. BEATRIZ

Buzios

Congreso; Gondwana XIV; 2011

Trace fossils are a tool to refine paleoenvironmental models, to characterize substrates and recognize behavioural patterns in ancient successions sometimes adding comparison with modern analogues. However, due to taphonomical reasons in some cases trace fossils precede unarguably the record of body fossils, enhance the diversity of some fossil groups, supply information about peculiar paleoenvironmental features and constitute a crucial linkage between the classical paleontological and sedimentological points of view in sedimentological interpretations. The Agrio Formation (Late Hauterivian) is a classical unit of the Neuquén Basin that containssome of the most complete ammonoid and nanoplankton biostratigraphic zonations which was recently confirmed as accurate with precise zircon geochronology. The uppermost part of the unit is almost devoid of body fossils, especially of those of value in biostratigraphy. The absence of biostratigraphic markers was subjectof different interpretations mainly based on geological reasons such as erosion produced during the sea level fall related to the overlying Huitrín Formation. Observations in several classical localities of the basin during the last years confirmed that marginal marine environments where regionally widespread in the basin with depositional settings affected by tides, subaerial exposure, meteoric waters and dolomitization. Body fossils diagnostic ofintertidal environments are potamidid gastropods. Palynomorphs have confirmed the abundant continental input with phytoclasts, polen and spores and an almost negligible record of dinoflagellates. In such conditions, in contrast to body fossils, trace fossils are very abundant and assignable to producers never recorded before as body fossils in the unit (or even in the entire basin), and they also provide information useful to interpret exposure in areas considered basinal facies in paleogeographic maps. The most spectacular of them all are theropod tracks recorded on top of oolithic limestones that are affected by  micro-wave ripples confirming that the producers where walking along the coast such as it was documented in Mediterranean basins. Extant xiphosurans are known from the coast of the USA and India and are particularly well documented in marginal marine settings where they go during mating season. Exquisite trackways preserved on ripples prior to exposureand accompanied with Mermia-like trace fossils are the first record of xiphosurans in the basin. Finally, different trace fossils assigned to protobranch bivalves are well known in the unit but ?flower-shaped? expressions of complex behavioural activity of tellinoid bivalves are documented for first time in the basin. Other classical trace fossils documented in the literature in marginal marine settings are also documented but are also present in otherclassical marine deposits of the unit. The ichnofauna here reported enhances the ichnodiversity but also is a window to some inhabitants not documented by body fossil-remains or their trace fossils precede the body fossil remains for several million years.