A Diverse Dinosaur-Bird Footprint Assemblage from the Lance Formation, Upper Cretaceous, Eastern Wyoming: Implications for Ichnotaxonomy

Abstract

A diverse assemblage of dinosaur and bird tracks from Niobrara County, Wyoming, represents the first vertebrate ichnofauna reported from the bone-rich Lance Formation (Maastrichtian, Upper Cretaceous). The ichnofauna includes a hadrosaur track with skin impressions; three theropod track types, including the tetradactyl track Saurexallopus zerbsti (ichnosp. nov.); a tridactyl dinosaur footprint with a fusiform digit III; possible Tyrannosaurus tracks; four distinctive avian ichnites; and invertebrate traces. The footprints are generally well-preserved and so offer a unique insight into the ecology of a small river valley during the Maastrichtian. Saurexallopus zerbsti ichnosp. nov. from the Lance is similar to Saurexallopus lovei recently reported from the Maastrichtian, Harebell Formation, of northwestern Wyoming, but is represented by much better material, facilitating amendment of the ichnogenus. Skeletal equivalents for Saurexallopus are not currently known. Similarly, the tridactyl track with fusiform digit III is similar to footprints reported from the coeval Laramie Formation of Colorado and may also be similar to ichnogenus Ornithomimipus from the Edmonton Group of Alberta (though not necessarily of ornithomimid affinity). The hadrosaurian track with the skin impression is reminiscent of a similar ichnite reported from the Maastrichtian, St. Mary River Formation in Alberta, which is herein named Hadrosauropodus langstoni as part of a reassessment of Cretaceous ornithopod track ichnotaxonomy. Such correlations demonstrate the utility of tracks for local or regional biostratigraphy (palichnostratigraphy) in western North America. It is also clear that tracks add to our knowledge of the composition and distribution of dinosaurian and avian components of Maastrichtian faunas. In particular the bird tracks indicate a diversity of at least four species, one of which was a semi-palmate form, hitherto unknown in the ichnological record and named Sarjeantichnus semipalmatus.