Everyone knows Ankylosaurus. It’s the epitome of the titular group of highly-armored herbivores to which it belonged, and there’s even a case to be made that it’s the best dinosaur. The living tank’s tail club certainly plays into that. It’s difficult to look at that knob of bone and not imagine it smacking into the leg of an attacking tyrannosaur or battering the side of another ankylosaur in a territorial dispute. But how did this famous piece of the dinosaur armament evolve?
North Carolina Museum of Natural Sciences paleontologist Victoria Arbour has quickly become one of the world’s foremost experts on ankylosaurs. In the past she’s helped sort out the number of ankylosaur species and looked at the biomechanical capabilities of those intriguing tail clubs. Now, along with Philip Currie, Arbour has retraced how ankylosaurs went from having flexible tails to stiffened handles supporting great protrusions of bone.
Not all ankylosaurs had tail clubs. The earliest of their kind, as well as the spiky forms called nodosaurids, lacked the famous feature. It was only among a subgroup called ankylosaurids that tail clubs evolved, and, as Arbour and Currie point out, this special appendage had two parts. There was the handle – a series of interlocking vertebrae at the end of the tail – and the knob, which is the actual bony business end.
Up until now, paleontologists have been unsure how the handle and knob evolved. Did the handle come first? The knob? Did they evolve together? A pair of ankylosaurs from Asia solve the puzzle. These two dinosaurs, named Liaoningosaurus paradoxus and Zhongyuansaurus luoyangensis, both had interlocking tail vertebrae that formed early versions of the handle but lacked tail knobs. What’s more, Arbour and Currie note, Liaoningosaurus lived about 122 million years ago and Zhongyuansaurus was shuffling around about 92 million years ago, while the first ankylosaurs with fully-formed tail clubs evolved around 75 million years ago. The handle came first.
The handle-first model makes biomechanical sense. If ankylosaurs evolved a heavy club on a flexible tail, Arbour and Currie write, then it would have been more like a flail than a club, and more likely to injure the ankylosaur wielding it. It was only after the tail became a stiff, bat-like appendage did the terminal club knobs start to grow. Yet, as the paleontologists remind us, ankylosaurs had large osteoderms running along the sides of their tails all the way back to the earliest of their ilk, like Scelidosaurus. Ankylosaurs may have been waving their tails around for defense or display since their earliest days, and we’re just starting to understand the impressive variety of ways they did so.
Arbour, V., Currie, P. 2015. Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of Anatomy. doi: 10.1111/joa.12363