SANSOM, I. J.*; DONOGHUE, P. C. J.; University of Birmingham, UK; University of Bristol, UK: Evolution and development of the skeleton in the earliest vertebrates

The origin of the vertebrate skeleton is a very simple matter: the most primitive living vertebrates entirely lack mineralised tissues while sharks manifest all of the skeletal systems of vertebrates at once. Thus, it appears, the vertebrate skeleton and its rich diversity of skeletal types arose in a single step, coincident with the origin of jaws. However, there is a problem with this perspective � it entertains just living taxa. If extinct taxa are considered, the pattern of skeletal origin changes dramatically. This is because, in contrast to the living jawless vertebrates, the vast majority of extinct jawless vertebrates possessed a mineralised skeleton in one form or another. Recent analysis of the histological composition of their skeletons, integrated with phylogenetic analyses, has revealed that the vertebrate skeleton did not arise de novo, in a single step. Rather, it appeared piecemeal, the components coinciding with the embryological distinctions between skeletal systems in extant vertebrates. Many of the tissue systems, too, reveal sequential origin. For instance, the neurocranium shows a pattern of sequential increase in complexity, from an unmineralized non-collagenous sheath, through a poorly structured but mineralized cartilage, to one lined by perichondral bone and, finally, permeated by endochondral bone. This phylogenetic pattern, which parallels broadly the development of the neurocranium, occurs fully in taxa intermediate of lampreys and sharks. A similar pattern of increasing mineralization, organization, and association with a collagen fibre-based matrix, is met with in other skeletal systems, and even tissue types such as bone, enameloid and dentine. This meets with the inferred origin of familiar structural proteins involved in biomineralization after the divergence of living jawed and jawless vertebrates.