The origin of birds from non-avian theropod dinosaurs is one of the greatest transitions in evolution. Shortly after diverging from other theropods in the Late Jurassic, Mesozoic birds diversified into two major clades—the Enantiornithes and Ornithuromorpha—acquiring many features previously considered unique to the crown groups along the way. Here we present a comparative phylogenetic study of the patterns and modes of Mesozoic bird skeletal morphology and limb proportions. Our results show that the major Mesozoic avian groups are distinctive in discrete character space, but constrained in morphospace defined by limb proportions. The Enantiornithines, despite being the dominant group of Mesozoic birds, are much less morphologically disparate than their sister clade, the Ornithuromorpha—the clade that gave rise to living birds, showing decoupled disparity and diversity in early avian history. This relatively low disparity suggests that diversification of enantiornithines was characterized in exhausting fine morphologies, whereas ornithuromorphs continuously explored a broader array of morphologies and ecological opportunities. We suggest this clade-specific evolutionary versatility contribute to their contrasting survivorship of the end-Cretaceous mass extinction.

The scripts are used to perform disparate analyses of morphologies of Mesozoic birds