Dinosaur footprints with skin impressions from the Cretaceous of Alberta and Colorado

Cretaceous dinosaur footprints discovered in the J Sandstone of the South Platte Formation (Dakota Group) in Colorado and the St. Mary River Formation of southwestern Alberta exhibit the first reported foot pad skin impressions of large ornithopods. The Canadian tracks occur as sandstone casts preserved in mudstones, whereas the Colorado tracks are natural impressions in a sandstone bed overlain by shale. The South Platte Formation tracks occur as impressions in a widespread "dinoturbated" sandstone bed representing low-gradient, delta plain – coastal plain facies assemblages associated with the upper member of the group, the J Sandstone. Only one of the many iguanodontid trackways exhibits good skin impressions.The St. Mary River Formation palaeoenvironment is interpreted as an anastomosed fluvial system that flowed northeast over a low-gradient floodplain from Montana. Footprints, often preserved in trackways, were left as dinosaurs walked across lake and marsh sediments that were relatively well drained or in various stages of dewatering. The quality of preservation is variable, depending on the properties of the substrate, and only one hadrosaur footprint includes clear casts of skin patches on the bottom of the footprint. Similar track-rich facies assemblages, representing lowland coastal plain and deltaic environments, are found in both the Lower (Gething Formation, British Columbia) and Upper Cretaceous (Mesa Verde, Colorado) successions of western North America. Few substrates of these depositional environments were suitable for the preservation of skeletal remains, so the information derived from tracks is palaeontologically significant.