No abstract

Mosasauroidea, prominent marine lizards (Squamata, Toxicofera) of the final 30 million years of the Cretaceous, have been extensively studied for their morphology, ecology and systematics in the past two centuries. However, the relative roles of biological and physical processes as drivers of their morphological diversification remain uncertain. Here we investigate the macroevolution of mosasauroid feeding and locomotory disparity using continuous characters measured from the mandible and forelimb as proxies. Patterns of morphospace occupation demonstrate important roles for innovation and niche partitioning in driving morphological disparity. The early evolution of Mosasauroidea is characterized by large shifts in morphology, especially elongation of the mandibular biting area and hydropedality. The later diversification of derived Mosasaurinae and Plioplatecarpinae is associated with a great expansion of morphospace, attributed to the acquisition of novel feeding and locomotory strategies. Temporally, disparity follows a top‐heavy profile, possibly reflecting opportunism in the wake of the Cenomanian–Turonian anoxic event. The highest levels of disparity are found in the latest Cretaceous, associated with the radiation of derived mosasaurids alongside the persistence of more basal forms. Major morphological innovations are not associated with evolutionary rate shifts, which differentiates them from earlier marine reptiles, and may reflect constant and greater niche occupation in Late Cretaceous oceans. Linear modelling of potential physical drivers indicates a minor role for these processes, suggesting that biological drivers were the primary sculptors of mosasauroid morphological disparity.

It is accepted that non-avian theropod dinosaurs, with their long muscular tails and small forelimbs, had a centre-of-mass close to the hip, while extant birds, with their reduced tails and enlarged wings have their mass centred more cranially. Transition between these states is considered crucial to two key innovations in the avian locomotor system: crouched bipedalism and powered flight. Here we use image-based models to challenge this dichotomy. Rather than a phylogenetic distinction between ‘dinosaurian’ and ‘avian’ conditions, we find terrestrial versus volant taxa occupy distinct regions of centre-of-mass morphospace consistent with the disparate demands of terrestrial bipedalism and flight. We track this decoupled evolution of body shape and mass distribution through bird evolution, including the origin of centre-of-mass positions more advantageous for flight and major reversions coincident with terrestriality. We recover modularity in the evolution of limb proportions and centre-of-mass that suggests fully crouched bipedalism evolved after powered flight.