Stegosaurus had a formidable tail. Studded with four long spikes, this dinosaur's business end would have given Allosaurus and other Jurassic predators plenty of incentive to keep moving. But do we have any evidence that Stegosaurus really used its tail this way?
Among paleontologists, the four-spiked tail of Stegosaurus is called a "thagomizer." It is one of a few terms inspired by one of Gary Larson's beloved "Far Side" cartoons: a caveman points to a slide of a Stegosaurus tail and names the nasty-looking structure in honor of "the late Thag Simmons." Humans and Stegosaurus missed each other by over 140 million years, but the joke was so perfect that paleontologists couldn't help but informally use it.
Whether Stegosaurus—and similarly equipped armored dinosaurs—used their thagomizers as weapons has been a minor point of scientific debate. The spikes certainly look like weapons, but that, by itself, isn't sufficient to tell what their function was. Paleontologists needed some kind of evidence of direct interaction between predator and prey, and in 2005 paleontologists Kenneth Carpenter, Frank Sanders, Lorrie McWhitnney and Lowell Wood reported just that.
Printed in The Carnivorous Dinosaurs, the paper by Carpenter and colleagues looked at several lines of evidence for interactions between Stegosaurus and one of the apex predators of its day, Allosaurus. First, a plate from the neck of a Stegosaurus found in Utah's Cleveland-Lloyd Quarry had a prominent, U-shaped notch taken out of its edge. The front portion of the jaws in Allosaurus corresponded closely to the missing piece, and since the plates were bony rather than carrying any significant amount of flesh, the paleontologists proposed that the missing chunk represented an attack rather than feeding or scavenging.
A second line of indirect evidence came from the Stegosaurus spikes themselves. Out of 51 examined spikes, about ten percent had broken tips with remodeled bone. Stegosaurus were clearly losing the sharp ends of their spikes and surviving for long enough afterward for the bone to start to heal, adding support to the idea that they were being used for defense and were not just for show.
But the most impressive piece of evidence was a single Allosaurus tail vertebra found in the Cleveland-Lloyd Quarry and known as UMNH 10781. You need to know a bit about Allosaurus anatomy to see what's wrong with this bone. Sticking out at an angle from the circular body of the vertebra, there is a wing of bone called the transverse process. In this particular specimen that process is missing a piece of bone measuring about an inch and a half square. This wasn't a break or evidence of damage after death. As with the tail spikes of Stegosaurus, the outside edges of the hole show evidence of remodeled bone, meaning that this Allosaurus was injured and survived for some time after being hurt.
The bite of another theropod doesn't fit the pattern of damage. There are no tell-tale toothmarks, nor is there evidence of crushing. Instead, the damage appears to have been caused by a large, pointed object, and a diagram included in the paper shows how a Stegosaurus tail spike fits the hole perfectly. The Stegosaurus may have even left part of itself behind. While bone around the outer edges of the vertebra shows signs of healing, the wound itself doesn't show the same signs of repair, which led Carpenter and co-authors to suggest that part of the Stegosaurus spike remained lodged in the hole, perhaps just part of the tough outer sheath that would have made the spikes even pointer in life.
Carpenter and colleagues also went a step further in modeling the physics of how Stegosaurus might have used its tail and the damage it could have inflicted. They concluded that the spikes would most likely slash open wounds if the attacking Allosaurus was standing in parallel to the Stegosaurus, but if the predator came in perpendicularly or at another angle the spikes of Stegosaurus were more likely to lodge in the skeleton and break. In these cases both predator and prey would have been injured. Stegosaurus certainly had enough swing to deal out some heavy damage to an attacking Allosaurus, the scientists concluded, but the problem was driving in its spikes with so much force that they might break!
Carpenter, Kenneth; Sanders, Frank; McWhinney, Lorrie A.; and Wood, Lowell (2005). Evidence for predator-prey relationships: Examples for Allosaurus and Stegosaurus The Carnivorous Dinosaurs, 325-350