Dinosaurs are the largest animals to ever walk on the continents and some reached body sizes of up to 70 tons. Observation of their closest living relatives, birds and crocodylians, could never allow for prediction of gigantic dinosaurian forms. Therefore, the fossil record is crucial to understanding the evolutionary changes of these animals through time, including body size trends. The reptile group Archosauria encompasses living and extinct birds and crocodylians as well as non-avian dinosaurs and crocodile relatives called pseudosuchians. My research focuses on studying fossils of extinct archosaurs to determine how the morphology of their skeletons allowed for growth to large body sizes. I am specifically interested in how the vertebral column fits together and how the structures that articulate vertebrae change throughout the phylogeny (i.e. family tree) of Archosauria. Although major body size trends are well known for archosaurs, less research has focused on skeletal features that are associated with the evolution of large body size in that group. I hypothesize that the vertebral column will have these features. To answer the question of how vertebrae morphology is related to body size, I first described eight vertebrae from a large pseudosuchian archosaur Poposaurus langstoni. This animal possesses an accessory articulation between the vertebrae in its trunk region (i.e. between the neck and hips) called the hyposphene-hypantrum articulation. I then surveyed vertebrae from across Archosauria and found a close fit of presence of the articulation with large sizes and that it evolves independently in several archosaur groups. / Master of Science