Meeting Abstract
The origin of the lower jaw is a key innovation that underpins the adaptive radiation of vertebrates. 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. A shift in lower jaw anatomy or rather a transformation in lower jaw shape may have facilitated the emergence of different feeding behaviors. Here we present an analysis to deduce the timing and tempo 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. Three hundred lower jaw specimens were examined; a combination of outline and extended eigenshape analysis were used to quantify variation in lower jaw morphology. Fast Fourier transformation (FFT) and principal component analysis (PCA) were performed on the dataset using the Hangle-2D outline and H-match programs in PAST. Results indicate that principal component (PC) 1 accounts for 73% of lower jaw shape variation. PC1 shape variation describes changes to the relative length of the dentary bone and robustness in the posterior lower jaw. PC2 accounts for 14.4% of lower jaw shape variation and describes differences in jaw depth and slenderness of the dentary bone (i.e. changes to the vertical height of the mandible). Acanthodians and amphibians make relatively minor contributions to overall disparity; acanthodians cluster along the PC1 axis, where the dentary bone is elongated and the back of the jaw is short and amphibians cluster where the back of the jaw is bulky. Archosaurs are distributed mainly along the PC1 axis, but also share a region of morphospace with both stem amniotes and chondrichthyans. From these results, it seems most likely that both ecological and functional consequences affect lower jaw shape variation.
