Palaeopathological evidence for intraspecific combat in ankylosaurid dinosaurs

Ankylosaurid dinosaurs were heavily armoured herbivores with tails modified into club-like weapons. These tail clubs have widely been considered defensive adaptations wielded against predatory theropod dinosaurs. Here we argue instead that ankylosaurid tail clubs were sexually selected structures used primarily for intraspecific combat. We found pathological osteoderms (armour plates) in the holotype specimen of Zuul crurivastator, which are localized to the flanks in the hip region rather than distributed randomly across the body, consistent with injuries inflicted by lateral tail-swinging and ritualized combat. We failed to find convincing evidence for predation as a key selective pressure in the evolution of the tail club. High variation in tail club size through time, and delayed ontogenetic growth of the tail club further support the sexual selection hypothesis. There is little doubt that the tail club could have been used in defence when needed, but our results suggest that sexual selection drove the evolution of this impressive weapon. This changes the prevailing view of ankylosaurs, suggesting they were behaviorally complex animals that likely engaged in ritualized combat for social dominance as in other ornithischian dinosaurs and mammals.