Ankylosaurs are a large group of herbivorous armored dinosaurs that lived during the Jurassic and Cretaceous periods. The typical ankylosaur had a wide armored body and a flexible tail. But one group – ankylosaurids – also had a distinctive tail club composed of stiff, interlocking vertebrae (the handle) and large, bulbous osteoderms (the knob) – a special kind of bone formed in the skin that’s unique to armored dinosaurs. According to a new study published in the Journal of Anatomy, the handle arrived first on the scene, and the knob followed.
In this study, Prof Philip Currie from the University of Alberta, Canada, and Dr Victoria Arbour of North Carolina State University and the North Carolina Museum of Natural Sciences compared Jurassic ankylosaurs to those from the early and late Cretaceous, tracing the tail’s evolution from flexible to fearsome.
The paleontologists looked at a number of early ankylosaurids including: Liaoningosaurus which lived 122 million years ago; Gobisaurus, which lived 90 million years ago; and Pinacosaurus, which lived 75 million years ago and is the earliest specimen with a complete tail club, to determine which of three possible evolutionary paths was most likely.
“There are three ways the tail could have evolved,” said Dr Arbour, who is the lead author on the study.
“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,” she added.
“The handle could have evolved first, meaning you would see early ankylosaurids with overlapping or fused tail vertebrae.”
“Or the knob and handle could have evolved in tandem, in which case you’d see ankylosaurids with both structures, but there could have been other differences like shorter handles or smaller knobs.”
“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,” Dr Arbour said.
Victoria Arbour & Philip Currie. Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of Anatomy, published online August 21, 2015; doi: 10.1111/joa.12363