How the close dinosaurian relatives of birds evolved gigantic and miniature sizes

An analysis of fossils of non-avialan theropod dinosaurs – a dinosaur clade that includes an array of body sizes – has provided findings that run contrary to expectations regarding the factors that inform the evolution of body size diversity. “Once quantified and analyzed in a phylogenetic framework [like this], we predict that diverse growth strategies will be recognized in other clades,” say the study’s authors. Over evolutionary history, many taxa have evolved very large and very small body sizes, and even closely related species can exhibit widely disparate sizes. The predominant mechanisms underlying the evolution of gigantism and miniaturization is widely considered to be changes to growth rate rather than duration of growth during development. However, despite this assumption, very few studies have explored the evolution of developmental rate and duration across many species within a comparative phylogenetic framework. This is partly because there are few abundantly sampled, long-lived clades containing a diversity of body sizes with which to evaluate these questions. Michael D’Emic and colleagues performed a large-scale phylogenetic comparative analysis examining the developmental strategies underlying the evolution of body size in non-avialan theropod dinosaurs, which can range from tiny (<.5 meters in length) to gigantic (>12 meters in length) in size. Using fossil measurements from 42 non-avialan species, including annually deposited cortical growth marks, D’Emic et al. compiled a comprehensive histological dataset of body size and growth rate. The findings show that, contrary to expectations, changes in growth rate and duration played nearly equal roles in the evolution of body size diversity in non-avialan theropods.