CO-OCCURRENCE OF THERIZINOSAUR AND HADROSAUR FOOTPRINTS IN THE LOWER CANTWELL FORMATION (UPPER CRETACEOUS) OF DENALI NATIONAL PARK, ALASKA: AN ASIAN DINOSAUR TRACK ASSEMBLAGE ON THE NORTH AMERICAN SIDE OF BERINGIA

We report on an association of dinosaur tracks in the Upper Cretaceous lower Cantwell Formation (LCF) of Denali National Park (DENA), central Alaska Range, Alaska that is unique in North America. This report documents the co-occurrence of tracks attributable to hadrosaurids and therizinosaurids, two groups of herbivorous dinosaurs. The LCF is famous as a source of thousands of fossil traces of vertebrates such as fishes, pterosaurs, and avialan and non-avialan dinosaurs. The LCF in DENA consists of numerous upward-fining successions of conglomeratic and pebbly sandstones, cross-stratified and massive sandstones, interbedded sandstones and siltstones, organic-rich siltstones and shales, and rare, thin, bentonites that typically are bounded by thin coal seams. These facies represent fluvial floodplain and distal alluvial fan depositional environments. The study area contains a diverse flora that includes horsetails, ferns, and shoots, cones and seeds of deciduous conifers. Angiosperms are represented by a relatively diverse assemblage of broad-leaved and linear-bladed monocotyledonous and woody dicotyledonous leaf forms. Angiosperm leaves and conifer shoots often form leaf mats pointing to synchronous leaf loss and, therefore, a deciduous habit. This unique vertebrate track co-occurrence is limited to a single locality in a remote part of DENA. The locality is noteworthy because such an association of tracks is not recognized elsewhere in western North America and is more characteristic within correlative rocks in central Asia. While it has long been recognized that Cretaceous Alaska represents the gateway for faunal exchange between these two continental landmasses, previous studies have focused on faunal exchange at the taxon-specific level. The ichnotaxonomic association described in this report instead highlights the Cretaceous Beringian land bridge from a behavioral perspective within the ecosystem. More specifically, this association suggests that parameters defining the continental ecosystem of central Asia were also present in this part of Alaska during the Late Cretaceous.