Adaptive radiation, the product of rapid diversification of an ancestral species into novel adaptive zones, has become pivotal in our understanding of biodiversity. While it has widely been accepted that predators may drive the process of adaptive radiation by creating ecological opportunity (e.g. enemy-free space), the role of predators as selective agents in defensive trait diversification remains controversial. Using phylogenetic comparative methods, we provide evidence for an ‘early burst’ in the diversification of antipredator phenotypes in Cordylinae, a relatively small adaptive radiation of morphologically diverse southern African lizards. The evolution of body armour appears to have been initially rapid, but slowed down over time, consistent with the ecological niche-filling model. We suggest that the observed ‘early burst’ pattern could be attributed to shifts in vulnerability to different types of predators (i.e. aerial versus terrestrial) associated with thermal habitat partitioning. These results provide empirical evidence supporting the hypothesis that predators or the interaction therewith might be key components of ecological opportunity, though the way in which predators influence morphological diversification requires further study.