The endochondral bone entrusted with the task of surrounding and protecting the dorsal nerve cord appears second in evolution, and is mainly thought to appear around the same time as the jaw, as it is found only in gnathostomes [C].

During the renaissance, Leonardo da Vinci made a study of vertebrate anatomy and biology through dissection. This is a drawing from one of his notebooks that illustrates the irregular yet segmental nature of the axial skeleton in contrast to the bones of the limbs and limb girdles.