SPINOSAURUS The "thorn lizard" Spinosaurus was a theropod dinosaur which lived in the Cretaceous Era. It was about 50 feet long, and its fossils have been found in both northern Africa and southern Asia. Spinosaurus had a large sail-fin on its back, but no one knows why. The sail may have been used to catch the warmth of sunlight, to frighten enemies, or to attract a mate. Spinosaurus may have weighed more than 4 tons.