1 Evolutionary Origin of Bone and Cartilage in Vertebrates

Most living vertebrates are characterized by possession of bones and cartilage as major components of the skeletal system, and the origin and evolution of these tissues remain intriguing questions. Classically, Geoffroy Saint-Hilaire (1818) tried to compare arthropods and vertebrates by two types of “inversions.” One is the dorsoventral inversion that brings the ventral nervous system of arthropods to the dorsal side, as seen in vertebrates. The other is the inside-out inversion to transform the arthropod exoskeleton into the vertebrate-type endoskeleton. However, it is misleading to regard the endoskeleton as the major skeleton in vertebrates. Compared with the exoskeleton in arthropods, vertebrates also have exoskeletal elements. Thus, the vertebrate exoskeleton has been compared directly with that of arthropods, as suggested by Patten (1912), Gaskell (1908), and others. However, those ideas have now been refuted by new evidence of phylogenetic relationships among animal phyla, by improved knowledge of skeletal histology and cytology, and by molecular developmental evidence for skeletogenesis. In the modern evolutionary scenario, which is largely based on phylogenetic trees constructed on molecular sequence data, it is generally accepted that vertebrates belong to the deuterostomes together with echinoderms, hemichordates, urochordates, and cephalochordates (amphioxus). This comprises a sister group to the protostomes, consisting of lophotrochozoans and ecdysozoans (Aguinaldo et al. 1997). It is along this phylogenetic tree that the origins of the vertebrate skeleton should be sought, by integrating the fossil evidence (Halstead 1974; Donoghue and Sansom 2002; Hall 2005). Comparative analyses of skeletal development in various living organisms will help guide...