Repeated Evolution of Divergent Modes of Herbivory in Non-avian Dinosaurs

The importance of adaptation [1-4] versus organizational constraints [5-7] in shaping common macroevolutionary trends remains unclear [8]. The fossil record is key to this problem, as it provides data on repetitive trait evolution between lineages [4, 8]. However, quantitative analyses investigating these dynamics with fossil data are rare [8]. Herbivory evolved multiple times within Mesozoic dinosaurs [9, 10], allowing analysis of common phenotypic responses to dietary evolution. Whereas repeated patterns of character acquisition [9] and functional changes [11-13] are observed between some herbivorous dinosaur clades, biomechanical studies resolve significant differences between morphologically similar taxa [12-14]. However, previous biomechanical analyses have not accounted for phylogenetic non-independence (e.g., [13-16]) or been restricted to individual clades (e.g., [11, 12, 16]). Here, we use multivariate analysis of biomechanical characters, within a robust phylogenetic context, to investigate functional pathways to herbivory in a large sample of non-avian dinosaurs. Results demonstrate multiple solutions to herbivory. Notably, two fundamentally different modes are observed to evolve independently multiple times, with morphofunctional changes in the skull co-varying with digestive strategy. These modes distinguish between gut-processing sauropodomorphs and theropods tending toward gracile crania and low bite forces and ornithischian taxa exhibiting character complexes associated with extensive oral processing. Although convergence within these subsets of taxa is common, it is not observed between them due to functional constraints imposed during the early evolution of each group. This highlights the hierarchical nature of evolution, with adaptation driving convergence within regions of morphospace delimited by phylogenetic contingency.