Rapid canalisation of mandible structure in Tetrapoda

Evolution of the mandibular structure has played a key role in the taxonomic diversification and niche exploration of tetrapods, from the initial water-to-land transition to the origin and radiation of mammals. While jaw evolution through these key transition points have long attracted attention, no studies of jaw evolution to date capture the breadth of tetrapod diversity and evolutionary history. Here, we reconstruct the compositional evolution of the tetrapod mandible with a comprehensive dataset of 36 traits scored in 3047 species. This sampling spans the earliest eotetrapodiforms to extant amphibians, mammals, and reptiles and birds, capturing immense ecological, developmental, and taxonomic diversity among a near-even split of extant and fossil tetrapods. We find that lower jaw morphology rapidly canalises at the base of each major clade, including, Eotetrapodiformes, Amniota, Synapsida, Sauropsida, and Amphibia, as well as subclades within those groups, and that, as a result, jaw compositional variation is highly phylogenetically structured. We estimate an average of over 100 shifts in three key traits: number of elements (dermal and endochondral bones), number of tooth-bearing elements, and total teeth per hemimandible, with symmetrical rates of gains and losses estimated for the number of elements, and correlated evolution between these three characters. Although novel elements arise occasionally, disparity in jaw composition overwhelmingly decreases as clades canalise rapidly, successively, and pervasively through tetrapod evolutionary history.