Stegosaurus, meaning "roof lizard", is a genus 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; Stegosaurus stenops (stenops meaning "narrow-faced"), Stegosaurus ungulatus (ungulatus meaning "hoofed"), and Stegosaurus sulcatus (sulcatus meaning "furrowed"). 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.
The only episode to show a Stegosaurus was "Dinosaurs". The one in this episode was female. Chris said that the iguana reminded him of this dinosaur. Martin agreed, adding that Stegosaurus had huge plates on her back to protect her from enemies.
- Like other dinosaurs in the series, this species is computer animated (due to being extinct), making all individuals of the same species (male and female) look exactly the same.
- The Martinandchrisaurus had the plates of a Stegosaurus.