EVOLUTION OF AXIAL SKELETAL REGIONALISATION IN VERTEBRATES: INTEGRATING PHYLOGENY, ENVIRONMENT, AND DEVELOPMENTAL MECHANISMS

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Evolutionary diversification of the axial skeleton (i.e. vertebrae and ribs) is a key component of the success of vertebrate animals. "Fishes" are generally thought to possess a two-region axial skeleton, with "precaudal" and "caudal" vertebrae, while tetrapods exhibit regionalisation of their vertebral column into distinct cervical, thoracic, sacral, and caudal regions. Conventional wisdom is, therefore, that the evolutionary transition from water to land coincided with increasing anatomical specialisations of the vertebral column. However, recent discoveries of striking axial skeletal complexity in some living and fossil fish groups suggest an alternative scenario: the primitive presence of a regionalised, complex axial skeleton in vertebrates, with repeated loss or augmentation of regionalisation as adaptation to new environments and ecologies. This project will be the first to explicitly and quantitatively reconstruct the evolutionary history of vertebrate axial skeletal complexity, and to test for environmental and developmental correlates of such complexity.In order to understand the evolution of the vertebral column and its role in diversifications into new environments and ecology, we will combine anatomical and regulatory genetic datasets to undertake a concerted investigation of axial skeletal regionalisation in a broad taxonomic sample of extant and fossil vertebrates. 1) We will quantify axial skeletal regionalization and complexity based on skeletal data derived from Computed Tomography and direct specimen measurement. 2) We will use comparative phylogenetic methods to reconstruct ancestral states for major vertebrate clades and to test for correlations between axial skeletal regionalisation and environmental and ecological factors. 3) We will test for expression of genes that define the amniote cervical-thoracic and thoracic-lumber transitions in embryos of a cartilaginous fish (the skate, Leucoraja erinacea), and we will use fluorescent cell labelling approaches to test whether these genes delineate previously unrecognised axial skeletal regions in this species. By comparing our findings with those from other lineages (e.g. tetrapods), we will be able to infer mechanisms of axial skeletal development at one of the earliest nodes in vertebrate evolutionary history. 4) Finally, we will use evolutionary modelling to combine results of anatomical, evolutionary, and developmental analyses to elucidate the relationships between vertebral morphology and developmental mechanisms in evolutionary transitions into new environments.Debate surrounding the evolution of the vertebrate axial skeleton has lingered for centuries, with current dogma and "textbook scenarios" stemming largely from speculation, and suffering from poor taxon sampling. Our study will integrate anatomical, developmental and environmental data, within a rigorous phylogenetic framework, in order to accurately reconstruct the evolutionary and ecological history of the vertebrate axial skeleton.