Meeting Abstract
The origin of the lower jaw is a key innovation that underpins the adaptive radiation of gnathostomes. The jaw has undergone fundamental changes to its composition and has endured major ecological changes including the transitions from water to land, from land to the air, and from land back to water. Changes in jaw shape and structure may have facilitated the emergence of different feeding behaviors. Here we present an analysis to deduce the timing (geological time) and tempo (evolutionary rates) of lower jaw shape change through gnathostome evolutionary history. We achieve this via an exploration of lower jaw morphospace and an evaluation of the functional and ecological consequences of lower jaw shape variation. Outline analysis and extended eigenshape analysis were used to mathematically quantify variation in lower jaw morphology for four hundred lower jaw specimens spanning three major evolutionary transitions: the origin of Osteichthyes, Amniota and Mammalia. According to Principal Component Analysis (PCA), 22.4% of lower jaw shape variation is attributable to overall length of the dentary bone and/or the configuration of the bones located posteriorly. 16.6% of the variation is attributable to the robustness of the lower jaw, while the relative thickness of the dentary bone and the angle of the bones located posteriorly account for 11.9% of variation. Initial taxonomic group patterns illustrate that fish (including chondrichthyans and osteichthyans) are the most disparate group; birds show little variance in comparison to mammals except for the overall curvature of the mandible and thickness of the (fused) dentary bone. Squamates have less morphological diversity than mammals, but comparable disparity to birds whereas snakes have restricted morphospace occupation. From these results, it is likely that both ecological and functional consequences affect lower jaw shape variation and that transformation in lower jaw shape allowed different feeding behaviors to emerge.
