Terrestriality as a constraint on vertebral shape in the eastern newt

Eastern newts (Notophthalmus viridescens) are a ubiquitous member of eastern North America's caudate fauna. Unlike most amphibians, which have two major life stages, their life cycle is typically multiphasic. Eastern newts are also polyphenic, possessing alternative life cycle strategies including an aquatic juvenile stage and a facultatively paedomorphic adult stage that is fully aquatic. Because these salamanders occupy different habitats (terrestrial, semi-aquatic, and fully aquatic) throughout their lives, they likely experience various physical forces on their skeletons, providing a unique model to study musculoskeletal changes across development and ecology. We hypothesized that individuals experiencing terrestrial life stages would exhibit vertebral morphologies constrained by the mechanical demands of terrestrial loading, resulting in similar vertebral shapes between terrestrial juveniles and semi-aquatic adults. In contrast, we predicted that fully aquatic phenotypes would exhibit greater morphological disparity due to relaxed gravitational constraints in buoyant environments. Using micro-computed tomography (μCT) and geometric morphometrics (GMM), we quantified vertebral shape and morphological disparity across life stages (terrestrial juvenile, aquatic juvenile, aquatic paedomorph, semi-aquatic adult) and axial regions (cervical, trunk, sacral, caudal). Our findings reveal that terrestrial life stages exhibit constrained vertebral morphology that persists despite subsequent ontogenetic niche shifts. Unexpectedly, aquatic juveniles and aquatic paedomorphs differed significantly despite shared aquatic habitat, suggesting that developmental pathway plays a larger role than current environment in shaping vertebral morphology. Fully aquatic stages displayed greater morphological disparity, particularly in the caudal region, consistent with relaxed gravitational constraints in buoyant environments.