Occurrence of the trace fossil Hillichnus in Upper Cretaceous deposits in association with dinosaur footprints: A case from the Torotoro National Park in Bolivia

Abstract

We present here the occurrence of the scarce, complex trace fossil Hillichnus associated with abundant theropod tracks and locally bird tracks at the Carreras Pampa dinosaur track site in the Torotoro National Park (TTNP), Bolivia. Hillichnus is a complex trace fossil produced by tellinacean bivalves, revealing various behaviours including locomotion, feeding, respiration and defecation. The specimens occur on the surface of a grainstone bed in the Maastrichtian El Molino Formation, characterized by the presence of low-angle planar cross-stratification, ripple cross-lamination, wavy or convoluted lamination and wave ripples. The associated invertebrate trace fossil assemblage mainly consists of superficial sinuous structures of variable size, a variety of holes (vertical burrows?), and radial/rosette trace fossils. The associated vertebrate trace fossil assemblage consists of theropod footprints, many well-preserved theropod swim traces arranged in trackways and bird tracks. The record of Hillichnus, associated to the ichnological assemblage and to sedimentological data, suggests a marginal-marine setting with freshwater influence in shallow water for the Upper Cretaceous of the El Molino Formation at the Carreras Pampa area. Local stratigraphic successions suggest both marine and lacustrine/fluvial environments. The track-bearing bed shows evidence of exposure and is covered by sediments deposited in deeper water. The occurrence of Hillichnus in Bolivia, in association with dinosaur tracks on the same bed, has relevant implications for the characterization of paleoenvironmental conditions during the production of dinosaur tracks at the Carreras Pampa site in the TTNP.