Many dinosaurs were adorned with spikes, horns and plates, but it was the ankylosaurs that took armor to the extreme. These dinosaurs were covered in bony armor from snout to tail-tip, yet, as a new study suggests, there may have been more to some of these structures than just attack and defense.
As reviewed by paleontologists Shoji Hayashi, Kenneth Carpenter, Torsten Scheyer, Mahito Watabe and Daisuke Suzuki in the journal Acta Palaeontologica Polonica, the ankylosaurs can be subdivided into three smaller groups. There was the Polacanthidae (a group with large shoulder spikes and a "shield" over the hips), the Nodosauridae (forms with narrow heads and lacking tail-clubs) and the Ankylosauridae (the classic type with heavy armor over the body and tail-clubs). (There is some debate as to whether the Polacanthidae should be thought of as a distinct group, but since the authors separate it from the others I will follow their lead here.) Members of each group can be distinguished from each other on the basis of features which can be seen with the naked eye, but they are also different at the microscopic level. The arrangement of collagen fibers—one of the chief components of bone—differs in each group, as does the thickness of the bone composing the armor.
The differences in the bony armor of each kind of ankylosaur may help paleontologists determine to which group a specimen belongs based upon fragmentary material, but they may also indicate the different ways in which ankylosaurs used their armor. When the scientists looked at pieces of armor (including spikes and clubs) from several different dinosaurs across the three groups, they found that some of what might be thought to be weaponry was not well suited to the task. The outer layer of bone in the spikes of the polacanthids, for example, was relatively thin, especially in comparison to similar structures from the skeletons of the nodosaurids. This may mean that while the large spikes on the nodosaurids were sturdy enough to be used as weapons, the more fragile spikes of the polacanthids may have played a role primarily in display or regulating body temperature instead.
Additionally, the partial ankylosaurid tail club the researchers examined still showed signs of bone growth even though it appeared to have come from an adult animal. Combined with other recent findings, such as a possible lack of tail clubs among some juvenile ankylosaurids, this may mean that this structure developed later in life and was not initially used as a weapon. Perhaps, the authors hypothesize, developing tail clubs were used by juveniles and young adults for display, but it was not until later that the clubs could also be used for defense. Whatever they were doing, this study confirms that scientists are still learning much about dinosaurs by looking inside their bones.
Hayashi, S. (2010). Function and evolution of ankylosaur dermal armor Acta Palaeontologica Polonica DOI: 10.4202/app.2009.0103