Tenontosaurus (/tɪˌnɒntoʊˈsɔːrəs/ ti-NON-toh-SOR-əs; meaning "sinew lizard") is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 and 108 million years ago.