Although Spinosaurus is well-known to dinosaur enthusiasts due to its sail and elongated skull, it is mostly known from remains that have been destroyed, aside from a few more recently discovered teeth and skull elements. Additionally, so far only the skull and backbone have been described in detail, and limb bones have not yet been recovered. Some speculation occurs in arguments among scientists and paleontologists that the Spinosaurus aegyptiacus may have walked on all four limbs, but most are not convinced.
Jaw and skull material published in 2005 show that it had one of the longest skulls of any carnivorous dinosaur, said to be 1.75 meters (5.75 ft) long. The skull had a narrow snout filled with straight conical teeth that lacked serrations. There were six or seven teeth on each side of the very front of the upper jaw, in the premaxilla bones, and another twelve in both maxillae behind them. The second and third teeth on each side were noticeably larger than the rest of the teeth in the premaxilla, creating a space between them and the large teeth in the anterior maxilla; large teeth in the lower jaw faced this space. The very tip of the snout holding those few large anterior teeth was expanded, and a small crest was present in front of the eyes.
The sail of Spinosaurus was formed of very tall neural spines growing on the back vertebrae. These spines were seven to eleven times the height of the vertebrae from which they grew. The spines were slightly longer front to back at the base than higher up, and were unlike the thin rods seen in the pelycosaur finbacks Edaphosaurus and Dimetrodon, the two sailed reptiles from the Permian period.
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