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 JOSÉ; FERNÁNDEZ, DIANA ELIZABETH; LAZO, DARÍO G.; TUNIK, MAISA; AGUIRRE-URRETA, M. BEATRIZ

Rio de Janeiro

Congreso; Gondwana XIV; 2011

Universidad Federal do Rio de Janeiro

Trace fossils are a tool to refine paleoenvironmental models, to characterize substrates and recognizebehavioural patterns in ancient successions sometimes adding comparison with modern analogues. However, dueto taphonomical reasons in some cases trace fossils precede unarguably the record of body fossils, enhance thediversity of some fossil groups, supply information about peculiar paleoenvironmental features and constitute acrucial linkage between the classical paleontological and sedimentological points of view in sedimentologicalinterpretations. 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 recentlyconfirmed as accurate with precise zircon geochronology. The uppermost part of the unit is almost devoid ofbody 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 fallrelated to the overlying Huitrín Formation. Observations in several classical localities of the basin during the lastyears confirmed that marginal marine environments where regionally widespread in the basin with depositionalsettings affected by tides, subaerial exposure, meteoric waters and dolomitization. Body fossils diagnostic ofintertidal environments are potamidid gastropods. Palynomorphs have confirmed the abundant continental inputwith phytoclasts, polen and spores and an almost negligible record of dinoflagellates. In such conditions, incontrast to body fossils, trace fossils are very abundant and assignable to producers never recorded before asbody fossils in the unit (or even in the entire basin), and they also provide information useful to interpretexposure in areas considered basinal facies in paleogeographic maps. The most spectacular of them all aretheropod tracks recorded on top of oolithic limestones that are affected by micro-wave ripples confirming thatthe producers where walking along the coast such as it was documented in Mediterranean basins. Extantxiphosurans are known from the coast of the USA and India and are particularly well documented in marginalmarine 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, differenttrace fossils assigned to protobranch bivalves are well known in the unit but “flower-shaped” expressions ofcomplex behavioural activity of tellinoid bivalves are documented for first time in the basin. Other classical tracefossils 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 awindow to some inhabitants not documented by body fossil-remains or their trace fossils precede the body fossilremains for several million years.