Meeting Abstract
Sauropterygians were highly successful marine reptiles that survived throughout the whole of the Mesozoic. They exhibited a broad range of body plans, including a large variation in vertebral count ratios, with cervical vertebral counts ranging from 5 to 76, the largest number in any animal. They therefore provide a model system for the evolution of vertebral counts and axial regionalisation in tetrapods, which could shed light on the mechanisms of vertebrate body plan evolution. We present a dataset of axial body plan measurements and vertebral counts from 120 taxa spanning 180 million years of sauropterygian evolution. We use this dataset to test the following explicit hypotheses about sauropterygian evolution using phylogenetic comparative methods including phylogenetic independent contrasts and Bayesian estimation of evolutionary rates: 1) Neck length evolved via somitogenetic (vertebral number) and homeotic (axial regionalisation) effects rather than by differential post-patterning somitic growth. 2) Somitogenetic and homeotic effects were decoupled during sauropterygian evolution. 3) Somitogenetic effects were the dominant generating mechanism of axial body plan change in sauropterygians. Our results show that differential post-patterning growth of somites was not an important driver of macroevolutionary change in sauropterygian body plans, unlike in mammals. Furthermore, background patterns of somitogenetic and homeotic change are decoupled, as has previously been shown in tetrapods. However, the establishment of higher level taxa with novel body plans involved rare, high-magnitude changes to both somitogenesis and homeotic effects, which were highly correlated. Our results demonstrate the importance of heterogeneous statistical models in uncovering the links between the hypothesised developmental drivers of macroevolutionary change in vertebrate body plans.
