The genus is known from the late Aptian to Albian ages of the Early Cretaceous period sediments of western North America, dating between 115 and 108 million years ago. It contains two species, Tenontosaurus tilletti (described by John Ostrom in 1970J. H. Ostrom. 1970. Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana. Peabody Museum Bulletin 35:1-234) and Tenontosaurus dossi (described by Winkler, Murry, and Jacobs in 1997).Winkler, Dale A, Phillip A Murry and Louis L Jacobs. (1997). "A New Species of Tenontosaurus (Dinosauria: Ornithopoda) from the Early Cretaceous of Texas". Journal of Vertebrate Paleontology. Many specimens of T. tilletti have been collected from several geological formations throughout western North America. T. dossi is known from only a handful of specimens collected from the Twin Mountains Formation of Parker County, Texas.
