Approximately 350 million years ago, ancient vertebrates transitioned from their ancestral aquatic home to the terrestrial realm, where they evolved forms with functional capacities, we now take for granted—morphologies that resist gravity and maintain robust locomotion in a three-dimensional, heterogeneous environment. Over the next 100 million years, these terrestrial vertebrates (i.e., tetrapods) would diversify into profoundly different habitats and even re-invade aquatic environments. Among the changes associated with the water-to-land transition was the evolution of multipartite and complex vertebral forms, only one of which remains prominent in all modern tetrapods. Previous workers sought only to describe early vertebral morphologies for osteology-based phylogenetic studies. However, few studies have explored what these early morphologies were capable of functionally, let alone tested potential relationships between their morphology and function experimentally. Thus, the effects of complex vertebral forms on spinal rigidity, correlations to new habitat invasions or reinvasions, and range of motion remains unclear. My dissertation integrates cutting edge methods in paleobiology and biomechanics to answer these critical questions in tetrapod evolution by: (1) investigating links between vertebral diversity and habitat use in early amphibians that straddle the land-water divide (Temnospondyli); (2) developing and validating through modern taxa a new method of osteological range of motion study for ancient taxa ; and (3) combining 3D printing experimental techniques and osteological range of motion to investigate intervertebral joint mechanics in several stem tetrapods. This work has overturned previously held hypotheses of neural spine morphology in stem-amphibians and reptiles and has demonstrated previously undescribed osteological limitations in complex vertebrae.