How did the Stegosaurus defend itself?

The Stegosaurus defended itself primarily with its Thagomizer – two pairs of bony spines at the end of its tail, which it used as a striking weapon against attackers. Its distinctive ones Backplates could have had an additional deterrent effect by making him appear larger and more threatening. Fossil bite marks on Thagomizer spines and a pierced caudal vertebra of an Allosaurus provide direct evidence of this defense mechanism.

The Thagomizer: The actual weapon

At the end of its muscular tail, Stegosaurus bore two pairs of sharp, 60 to 90 centimeter long bone spines. The tail itself was flexible and strong enough to swing these spines sideways with considerable force - similar to a morning star. Paleontologists found evidence that this weapon was actually used: a partially healed stab wound was discovered on an Allosaurus tail vertebra (find UMNH 10781), the shape of which exactly matches the tip of a thagomizer - documented in the Study by Carpenter, Sanders, McWhinney and Wood (2005). Another Research by McWhinney, Rothschild and Carpenter (2001) also showed that around ten percent of 51 Stegosaurus spines examined had healed fractures - a further indication of actual combat use.

Incidentally, the term “thagomizer” does not come from science in the 1800s, but was only coined in 1982 by a cartoon by the illustrator Gary Larson - and has since become established in technical language.

The back plates: protection or signal?

The striking, upright bone plates along the back were long considered armor. Today, research tends to assume that the plates were too thin and poorly supplied with blood to provide effective armor protection. A combined function is more likely: the plates could have served for heat regulation and at the same time acted as an optical signal that made the Stegosaurus appear larger and more intimidating to predators or served for intraspecific communication.

Why did the Stegosaurus need defense at all?

The Stegosaurus lived in the Upper Jurassic around 155 to 150 million years ago together with large predatory dinosaurs such as Allosaurus and Ceratosaurus. Since he moved comparatively slowly and had no escape speed, active close combat defense was essential for survival.

You can find out more about the anatomy, locations and lifestyle of this dinosaur in our detailed Stegosaurus profile.