Multiple observations on the preservation of dinosaur tracks in a lacustrine to fluvial setting (Late Cretaceous, El Molino Formation; Parque Nacional de Toro Toro, Bolivia)

Christian Andreas Meyer ^1,2, José Hugo Heymann ³, Alfonso Alem Riojo ⁴, Ricardo Céspedes Paz ² & Dorothee Hippler ^2,5
1Departement of Environmental Sciences, University of Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland., ²Museo de Historia Natural Alcide d’Orbigny, Avenida Potosí N-1458 Cochabamba, Bolivia., ³Calle Grau 349, Sucre, Bolivia., ⁴Rumi Kipu, Km2 carretera Toro Toro-Cochabamba, Bolivia., ⁵Graz University of Technology, Institute of Applied Geosciences, Rechbauerstrasse 12, 8010 Graz, Austria.

The Upper Cretaceous El Molino Formation (Maastrichtian) in Bolivia is well known for its extensive dinosaur track record. However, the palaeogeographic variability of the dinosaur track record and thus changes in dinosaur biodiversity at the northwestern margin of Gondwana are not yet fully investigated. We therefore aim to study the Nuevo Camino section in the Toro Toro National Park, Bolivia in order to unravel the site-specific track record in relation to ambient depositional, environmental and preservational conditions. Drone-derived orthophotographs, outline drawings and false colour maps in combination with detailed sedimentological logging and thin section analysis of hand relevant specimens complement our approach. We found the Lower Member of the El Molino Formation being made up of 55 m of mixed carbonate-siliciclastic sedimentary rocks. Within this sequence 30 different track layers were identified. They include heavily trampled surfaces as well as surfaces with individual tracks and trackways of medium-sized theropods (FL< 35 cm) and small ankylosaurid trackmakers (FL<35 cm). They are preserved as negative epichnia, positive hypichnia and as underprints. It can be shown that the iconic trackway of Ligabueichnium bolivianum consists of pedes underprints. Macro- and microfacies analysis reveals that tracks occur in a lacustrine to fluvial setting represented by lake shore deposits, on top of abandoned channels, in overbank sediments as well as in palaeosols, with considerable similarities to the Cal Orck’o tracksite in Sucre, Bolivia.