Dinosaur Warfare: Ankylosaur and Theropod Coevolution

Sammanfattning: Ankylosauria is a clade of armoured dinosaurs that, throughout the Mesozoic, demonstrates divergent evolution of defensive traits, between the robust spikes and osteoderms of nodosaurids to the ankylosaurid tail clubs and lightweight armour. One of the longer-standing hypotheses, which is supported by histological data, stipulates that armament was a direct result of a predator-prey relationship between theropods and ankylosaurians. Such a hypothesis predicts that predatory pressures from Theropoda drive the evolution of armament. Here we investigate the coevolutionary hypothesis in a phylogenetic context by searching for reciprocal selection and clade interactions. We undertake two separate analyses. The first is a host-parasite test (ParaFit), which tests, within a phylogenetic framework, the null hypothesis that the evolutionary history of two groups was independent. The second produced principal coordinates from 30 ankylosaurian armour-related traits and was correlated in a linear regression against theropod body mass. The analysis was conducted across 53 theropod species that were sympatric, within a geological formation, with 44 ankylosaur species. The results of the ParaFit test suggest strong evolutionary links between Ankylosauridae and Tyrannosauridae, but not with Nodosauridae. As ankylosaurids replace nodosaurids in Asia during the Mid-Cretaceous this may be representative of local predators escaping from the classical arms race, necessitating a change in prey defensive strategy. The support for this lays in the differences of defensive strategies in Ankylosauria, and the abundance of Nodosauria in Gondwana, outside of the range of Tyrannosauroidea. Results of the trait analysis reveal that changes in theropod mass correlate positively with ankylosaur defensive phenotypic change; the test also demonstrates early ankylosaurs being comparatively under-armoured to their concurrent predators, whilst Late-Cretaceous Ankylosaurini were over-armoured. This study lays the groundwork for investigating coevolution between Ankylosauria and Theropoda within a phylogenetic context, but further investigation of phenotypic changes in Theropoda, and theropod-ankylosaur interactions, will be required to positively identify traits that could have arisen as a specific response to ankylosaur armament.