Ankylosaurs were heavily armored herbivorous dinosaurs with bony plates all along their wide backs. They were the tanks of their time, and some even carried a giant club at the end of their tail. Researchers studying the tail clubs of these later “weaponized” forms found that the stiff handle evolved first, and the knob followed. The findings were published in the Journal of Anatomy. 

The distinctive tail clubs of Cretaceous ankylosaurs, such as Ziapelta above, consisted of stiff, interlocking (handle-like) vertebrae with a large, bulbous knob made of enlarged osteoderms, a special bone type that forms in their skin. However, ankylosaurian dinosaurs were around long before these later forms were walloping carnivores like T. rex. Their history can be traced back over 145 million years ago into the Jurassic – but back then they had flexible tails. 

"In order for an ankylosaur to be able to support the weight of a knob and swing it effectively, the tail needs to be stiff, like an ax handle," North Carolina State University’s Victoria Arbour says in a statement. "For that to occur, the vertebrae along the tail had to become less flexible, otherwise the momentum generated by the knob's weight could tear muscle or dislocate vertebrae."

^{Gobisaurus, an ankylosaur with a stiff tail but no knob of bone at the end. Sydney Mohr}

To trace the tail's evolution from supple to stiff, Arbour and colleagues compared Jurassic ankylosaurs to those from the early and late Cretaceous. Some of these early ankylosaurids include 122-million-year-old Liaoningosaurus, 90-million-year-old Gobisaurus (pictured), and 75-million-year-old Pinacosaurus, the earliest specimen with a complete tail club.

"There are three ways the tail could have evolved," Arbour explains. “The knob could have evolved first, in which case you'd see ankylosaurids with osteoderms enveloping the end of the tail, but with the tail remaining flexible. The handle could have evolved first, meaning you would see early ankylosaurids with overlapping or fused tail vertebrae.” Or, the two evolved in tandem.

Based on their comparisons, evolution of the tail club occurred in a stepwise manner: Ankylosaurids evolved handle-like vertebrae before the distal osteoderms enlarged to form a knob. The stiff tails with fused vertebrae were present by the early Cretaceous, and the knob appeared in the late Cretaceous. "While it's possible that some of the species could still have developed the handle and knob in tandem, it seems most likely that the tail stiffened prior to the growth of the osteoderm knob, in order to maximize the tail's effectiveness as a weapon," Arbour adds.

^{A timeline showing the steps in the evolution of ankylosaur tail clubs. Victoria Arbour}