Evolution of Skeletal Tissues

Abstract

The vertebrate skeletal system is a central subject of research in evolutionary developmental biology. Morphological homologies of vertebrate skeletal elements can be traced in two separate systems: exoskeletons and endoskeletons. This separation is not necessarily linked to differences in histogenesis or to cell lineage origins: homologous bones can be formed through different types of ossification or from cells of different origin. The earliest skeleton that appeared in evolution was an endoskeleton consisting of a notochord and cartilage, likely having evolved through the elaboration of an ancestral developmental mechanism. The exoskeleton consisting of dermal bones first evolved in basal members of the gnathostome lineage, prior to the acquisition of the jaw. From this lineage, a group including jawed vertebrates newly acquired endoskeletal bones. Of the latter group, the chondrichthyans, or “cartilaginous fishes,” underwent a secondary reduction of these bony tissues. Endochondral ossification, in which osteoblasts produce bones inside of a cartilage template, was a new type of ossification and evolved in the osteichthyan lineage. These evolutionary processes based on the fossil record provide a framework for an understanding of the evolutionary developmental biology of vertebrate skeletal tissues based on developmental genetics.