Limb proportions predict aquatic habits and soft-tissue flippers in extinct amniotes

Among mammals and reptiles, the recurring evolution of fully aquatic forms from land-dwelling ancestors highlights the remarkable powers and implications of natural selection. The most aquatically specialized of these groups have limb morphologies that betray a fully marine lifestyle, but the transitional forms near the base of each lineage have more ambiguous features, making it difficult to determine which fossil species were aquatic. Here, we use a scalable phylogenetic machine-learning pipeline to test previously proposed osteological correlates of interdigital webbing, soft-tissue flippers, and aquatic habits in amniotes. We collect >11,000 original measurements from amniote limbs and use these measurements to train and test phylogenetic logistic regression models that can predict aquatic affinities in extinct species. We then interpret and select among competing predictor models with receiver-operating characteristic analysis. Ultimately, relative hand length makes the best predictions, reconstructing soft-tissue flippers and aquatic habits with >90% accuracy across amniotes and clarifying the aquatic habits of fossil species with historically ambiguous ecologies. Placing these predictions on the phylogenetic tree of amniotes reveals semi-terrestrial habits in mesosaurs and all other sampled stem reptiles, highly/fully aquatic habits in all known ichthyosauromorphs, and multiple independent origins of highly/fully aquatic habits among sauropterygians, mosasaurs, and theropod dinosaurs. Taken more broadly, these results enable a broader comparative assessment of amniote limb proportions that reveals distinct evolutionary landscapes in limb morphometry for highly/fully aquatic vs. semi-terrestrial amniotes, as well as between total-group mammals and reptiles.