Ankylosaurs were armoured ornithischian dinosaurs whose bodies carried bony elements called osteoderms within the skin. The armour varied among lineages. Some ankylosaurids had broad dorsal plates and a powerful tail club, whereas nodosaurids often carried prominent shoulder spikes and lacked a fully developed club. Fossils preserve these structures in place on rare specimens, but a bony element is only part of the living covering: keratinous sheaths and surrounding skin are less often preserved.
Osteoderms were part of the skin
Osteoderms are bones that form in the dermis rather than as part of the internal skeleton. Ankylosaur specimens preserve rows of larger plates, smaller ossicles and, in some species, spikes. Their position and surface shape help distinguish taxa and reconstruct the body outline. Histology can reveal growth and remodelling, while the arrangement of bones indicates how flexible or continuous the armour may have been.
Armour was not a sealed shell. Plates were distributed across the back and flanks, with smaller elements filling gaps. In living crocodilians, osteoderms interact with skin and connective tissue, but the exact arrangement cannot simply be copied onto dinosaurs. The wider evidence for dinosaur defence includes horns, spikes and other structures that evolved in different groups.
Defence is a strong hypothesis, not a complete answer
Protection from bites is an obvious function for a body covered in bony plates, and the combination of armour and robust body shape is consistent with defence. Yet the presence of a structure does not tell us exactly how it was used. Osteoderms could also contribute to display, species recognition, or other functions, and their roles may have differed by body region and taxon. Claims about temperature control or mineral storage need direct support from anatomy and tissue evidence rather than assumption.
Rare fossils preserve osteoderms with overlying skin, allowing researchers to connect bone shape to the external surface. The ankylosaur Zuul crurivastator, for example, preserves extensive armour and a tail club. Comparisons of healed injuries with the club’s anatomy have raised the possibility that clubs were used in contests between members of the same species, not only against predators. This does not rule out defence; it shows that a weapon-like structure may have had more than one context.
The tail club preserves a mechanical trade-off
A tail club combined enlarged terminal vertebrae with a knob of fused osteoderms. Its shape suggests that it could deliver force, but the effectiveness of a strike depends on tail musculature, movement, target and impact angle. Biomechanical models can estimate stresses under stated assumptions; they cannot directly reveal how often the animal swung its tail or which behaviours were most common.
Healed fractures in the flanks of some ankylosaurids have been compared with the dimensions and position of tail clubs. A match can support club strikes as one explanation, but a lesion can also have other causes. Researchers therefore treat pathologies as clues that strengthen or weaken functional hypotheses, not as a video of a prehistoric fight.
Armour differed across ankylosaurs
Comparing nodosaurids and ankylosaurids prevents a single image from standing for the whole group. Spikes, plates, ossicles and tail clubs appear in different combinations. Their anatomy reflects evolutionary history as well as possible ecological pressures. The fossil record makes the armour visible; its exact behaviour and full range of functions remain partly interpretive.
Frequently asked questions
What were ankylosaur armour plates made of?
They were osteoderms: bones that formed in the skin. Some fossils preserve the plates in position, sometimes with impressions of the overlying skin.
Did all ankylosaurs have a tail club?
No. The best-developed clubs are characteristic of ankylosaurids. Nodosaurids instead often had prominent shoulder spikes and other armour arrangements.
Was ankylosaur armour used only against predators?
Predator defence is a likely role, but injuries and tail-club anatomy also support possible contests within a species. Multiple functions may have been involved.
Can fossils show exactly how an ankylosaur used its tail club?
They preserve the club’s anatomy and sometimes healed injuries. These clues test hypotheses, but cannot record the animal’s behaviour directly.