The Mosasaur Prognathodon from the Upper Cretaceous Lewis Shale near Durango, Colorado and distribution of Prognathodon in North America

—We document an incomplete skeleton of the mosasaur Prognathodon overtoni (Williston) from the middle part of the Lewis Shale southwest of Durango, Colorado. The locality’s stratigraphic position below the Huerfanito Bentonite Bed and associated ammonites of the zone of Baculites perplexus Cobban indicate that the mosasaur is of middle Campanian age. In North America, Prognathodon has a remarkably long (essentially all of the Campanian-Maastrichtian) and geographically broad (west to east coasts) distribution. Prognathodon was a large, deep water predator of the Late Cretaceous seas.