Elevated evolutionary rates of biting biomechanics reveal patterns of extraordinary cranio-dental adaptations in some herbivorous dinosaurs

Adaptation to specialist ecologies is a key innovation that has contributed to the evolutionary success of many vertebrate clades, underpinning the acquisition of diverse skull morphologies. Dinosaurs, which dominated Mesozoic terrestrial faunas, acquired herbivory multiple times, including in clades historically regarded as predominantly carnivorous. The evolution of herbivory in theropod dinosaurs is linked to drastic changes in dental and craniomandibular functional morphology, yet whether such changes occurred more rapidly in herbivorous lineages compared to in carnivorous lineages remains untested in a phylogenetic framework. Here, we infer rates of phenotypic evolution in relative biting edge lengths to test the hypothesis that the acquisition of herbivory is associated with rapid changes in jaw biomechanics. We find elevated rates of biomechanical evolution in theropods with foreshortened and beaked skulls (Oviraptorosauria, Limusaurus), as well as in ceratopsians and Diplodocus. A reduced biting edge length and increased jaw efficiency unites these high-rate lineages, indicating selection for greater efficiency in biting biomechanics. Additionally, we hypothesise that extreme ontogenetic changes within species' lifetimes may be behind some instances of branch-wise elevated rates. Thus, we show how exceptional rates of biomechanical evolution can reveal signatures of adaptations within dinosaur lineages and potentially along ontogenetic sequences.

Methods
Data were collected as measurements from published images using ImageJ, curated in Excel, wrangled in R and analysed in BayesTraits.