Tyrannosaurus rex was the top predator in North America during the last days of the dinosaurs, but there’s one dinosaur it may have given a wide berth. That dinosaur is Ankylosaurus magniventris, a low-slung, plant-eating dinosaur covered in bony plates and spikes. Ankylosaurus might not have had the sharp teeth or powerful bite of Tyrannosaurus, but it had its own special weapon: a tail club. In Ankylosaurus and several other species of ankylosaurs, the bones of the end of the tail had evolved into a stiff, interlocking structure, like a baseball bat. Some of the bony plates in its skin had become huge and completely enveloped the tip of the tail. Ankylosaurs also had wide hips, which meant that the muscles that move the tail from side to side were large and powerful.
I wanted to know more about how ankylosaurs might have used their unique tails. The tail club certainly looks like it would have been a formidable weapon, but how hard could an ankylosaur smash its tail club into something? Would a Tyrannosaurus have even noticed getting bopped on the leg by a tail club?
Using information about the size and shape of the tailbones, how large the muscles might have been, and how heavy the knob of bone at the end of the tail was, I calculated a range of impact forces for tail clubs of different sizes. These calculations suggested that the largest ankylosaur tail clubs could have impacted with 10,000 Newtons of force – and maybe much more, depending on how the strength of the tail-swinging muscles is estimated. For comparison, most humans can land a punch with only about 2,000 Newtons.
This study suggests that a tail club may have been a good defensive weapon. A Tyrannosaurus that tried to face off against an Ankylosaurus could have wound up with a broken ankle, a devastating injury for an animal that gets around on two legs. But, an alternative hypothesis is that ankylosaurs could have used their tail clubs against members of their own species to compete for mates or territory, much like how modern-day deer use their antlers. The next questions I’d like to answer as part of my research at the N.C. Museum of Natural Sciences are how and why ankylosaurs evolved such an unusual weapon, since most animals have weapons on their heads and not on their tails. Perhaps understanding the origins of this unique structure might tell us something about how and why weapons evolve in the animal kingdom.
Arbour is the Brimley Postdoctoral Scholar in the Paleontology & Geology Research Lab at the N.C. Museum of Natural Sciences, a joint appointment with the Department of Biological Sciences at N.C. State.