Theropod, avian, pterosaur, and arthropod tracks from the uppermost Cretaceous Las Encinas Formation, Coahuila, northeastern Mexico, and their significance for the end-Cretaceous mass extinction

Two unique localities that combine an unusual diversity of avian, pterosaurian, and dinosaur tracks as well as trails of arthropods were recently discovered by us in uppermost Maastrichtian siliciclastic sediments of the Las Encinas Formation in the Mexican state of Coahuila, ∼40 km north of Saltillo. The trackway assemblages at Amargos and Rancho San Francisco were produced by at least six different types of birds, while trackways of azhdarchoid pterosaurs are rare. Only a single footprint was produced by a nonavian theropod. A diverse ichnofauna of arthropod traces is also present in a different facies. The tetrapod trackway assemblage was deposited during the very latest Maastrichtian, as indicated by an up to 2.5-m-thick unit with abundant smectite spherules attributed to the Chicxulub impact less than 8.5 m stratigraphically up section at Amargos. Sphenodiscus pleurisepta is the last ammonite at Amargos and may have crossed the Cretaceous-Paleogene boundary.