Ankylosaurs were dinosaurs equipped with inbuilt armour. A characteristic anatomical feature of some of them was the giant tail club. The latter, located at the end of the tail, has recently been documented by researchers who suggest that the knob grew less and less flexible over time to culminate into the stiff tail club. The findings are published in the Journal of Anatomy.
An ankylosaur (Ziapelta) with a fully developed tail club. Photo credits: Sydney Mohr.
The tail clubs of the ankylosaurs were made up of stiff interlinked vertebrae ending in a huge bulbous knob. One of them such as the Ziapelta is pictured above. The earliest species of the dinosaurs with a full tail club, the Pinacosaurus, lived 75 millions years ago while their ancestors were on Earth around 145 million years ago, at a time when the latter had flexible tails. How did the evolution of the tail happen? It seems that the development of the knob necessitated the transition to decreasingly-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,” said Victoria Arbour from the North Carolina State University said 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.”
Arbour and her team attempted to trace the evolution of the tail from flexible to stiff by comparing Jurassic ankylosaurs (the latest forms of the dinosaurs) to those living in the Cretaceous period.
The evolution of the tail club occurred step by step. Handle-like vertebrae first developed. Thereafter, the distal osteoderms grew bigger into a knob. By the early Cretaceous time, the stiff tails had fused vertebrae. Only in the late Cretaceous period did the knob appear.
“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,” said Arbour.