 Stegosaurus ( / ˌstɛɡəˈsɔːrəs / ) is a type of armored dinosaur . Their fossil bones have been found in rocks dated to the Late Jurassic period ( Kimmeridgian to early Tithonian ages ) , between 155 and 150 million years ago , in the western United States and Portugal . Several species have been classified in the upper Morrison Formation of the western U.S , though only three are universally recognized ; S. stenops , S. ungulatus and S. sulcatus . The remains of over 80 individual animals of this genus have been found . Stegosaurus would have lived alongside dinosaurs such as Apatosaurus , Diplodocus , Brachiosaurus , Allosaurus and Ceratosaurus ; the latter two may have been predators of it . 
