A large tridactyl trackway from the Upper Cretaceous Hell Creek Formation of North Dakota, likely attributable to an adult Tyrannosaurus rex

Large theropod tracks are uncommon within the Hell Creek Formation with trackways being exceedingly rare. Until recently the tracks and trackways that have been associated with non-Tyrannosaurus tyrannosaurids have not supported a clear comparison with an adult T. rex. Here we describe a newly discovered ∼7 m trackway comprised of four large tridactyl tracks, each ∼1 m in length, from the Upper Cretaceous Hell Creek Formation of North Dakota. The trackway is ∼33 m below the Hell Creek/Fort Union Formation contact and is estimated to be ∼66.5 Ma. The large size and tridactyl nature of the tracks is consistent with an adult Tyrannosaurus rex trackmaker, and numerous T. rex skeletal elements and skeletons have been found locally further strengthening the identification of the trackmaker. Stride lengths and foot lengths indicate a walking speed in the region of 1.5–2 m/s, which is consistent with other large theropod trackways. The tracks are preserved as fine-grained sandstone concretions cemented with calcium carbonate that nucleated around footprint depressions and the resultant footprint casts are harder than the surrounding rock. However, the surfaces of the footprint casts are heavily fractured and retain little anatomical detail beyond size and number of digits. Nevertheless, this is an important data point that marks the first report of a trackway that is likely attributable to an adult Tyrannosaurus rex.