Habitat and complex life cycles promote morphological diversity in salamander limb bones

Salamander evolution featured multiple transitions between water and land that promoted distinct adaptations in limb bones for buoyancy control versus increased load-bearing capacities, respectively. Many extant species spend their entire lives either in water or on land, while others undergo water-land transitions within their lifetime. However, exposure to both environments may impose competing demands that restrict adaptive evolution for a particular habitat. Using a 3D morphological dataset of 133 species spanning the phylogenetic and ecological breadth of salamanders, we find that the external and internal morphology of limb bones is evolutionarily decoupled, which increases the evolvability of limb bones in response to diverse mechanical demands. Terrestrial salamanders have stiffer bones with greater resistance to fracture, while aquatic species have denser bones that are hypothesized to aid in buoyancy regulation. We uncover a functional trade-off between stiffness and density that promotes stiff yet lightweight bones in terrestrial lineages. Released from terrestrial constraints, aquatic paedomorphs have disparate external morphologies, whereas terrestrial direct developers consistently share a rod-like bone shape. Aquatic and terrestrial multiphasic taxa show less morphological divergence than monophasic species living in comparable habitats but are not constrained by their complex life cycle. Multiphasic species have distinct external limb bone shapes that have evolved as fast or faster than monophasic species. Taken together, we propose that the trade-offs imposed by different habitats and complex life cycles increase limb bone diversity by promoting alternate evolutionary pathways.