Stegosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Stegosauria > Stegosauridae > Stegosaurinae

Overview: With its double row of kite-shaped plates and its imposing spiked tail, few dinosaurs are as immediately recognizable as Stegosaurus. It’s one of the many dinosaurs described by the famed Othniel C. Marsh – one of the main competitors of the infamous “Bone Wars”. Marsh established Stegosaurus as a genus in 1877, based on a disarticulated partial skeleton. Its generic name means “roofed lizard”, referring to its dorsal plates, which Marsh originally believed laid over its back like shingles, similar to what we see on modern pangolins. It was later recognized that the plates stood erect on the back, though their pattern has been the subject of debate. Some originally thought it had only one row of plates, but we know there were two. In some reconstructions, the plates are paired, but most now agree the two rows ran in a staggered pattern. Stegosaurs often sported a set of spikes on the end of the tail, forming what is often called a “thagomizer”. The spike number varied from genus to genus, but Stegosaurus is usually reconstructed with four spines, pointing out and back, as opposed to pointing up like in some reconstructions or artistic depictions.

Researchers have debated the purpose of the plates and thagomizer. The latter was probably used as a defense against predators. Damage to the pelvic bone of an Allosaurus specimen is likely from an ill fated encounter with a Stegosaurus. The plates have often been seen as defensive structures, but they were situated so far up on the body that they weren’t really much use as such. Instead, it’s likely that they were used to make the animal look larger and more intimidating, or for display. The keratinous coating over the bone may’ve sported striking patterns to catch the eye of the opposite sex. Heat exchange is another potential purpose. Some have speculated that Stegosaurus was able to pump blood into the plates to make them flush, as can be seen in the 1999 BBC documentary series Walking with Dinosaurs, but the discovery of the keratin coating makes this unlikely. One of the most popular misconceptions about Stegosaurus is that it had a second brain in its pelvis. The pelvic region did have an open space, but it probably held a mass to store glycogen to supply the nervous system. Its actual brain was small, but not so tiny as to require a second brain to survive.

Stegosaurus was a primarily low-browsing herbivore, eating plants like ferns and cycads, but some believe it could rear up on its tail to reach into low-hanging branches. Given its size, it was probably able to push down smaller trees. The creature’s snout was long and narrow, the head being notably small for its body size. Stegosaurs tended to have such tiny heads. At the snout tip was a beak, but smaller, leaf-shaped teeth lined the back of the jaws. Stegosaurus is the namesake of its family, the Stegosauridae, as well as the larger thyreophoran clade Stegosauria. Its closest relatives may’ve been genera like Hesperosaurus and Wuerhosaurus. With a length of seven or even eight meters, it was one of the largest known stegosaurs, with only Europe’s Dacentrurus surpassing it. All known fossils of Stegosaurus come from either the Morrison Formation in the United States or Portugal’s Lourinhã Formation. Most are known from the Morrison, which preserves what was then a semi-arid, seasonal floodplain of fern savannas and conifer forests. Stegosaurus lived alongside many famous dinosaurs such as Dryosaurus, Camarasaurus, Brachiosaurus, Diplodocus, Ceratosaurus and Allosaurus.