Evolutionary Consequences of Skeletal Differentiation

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Abstract
Some aspects of the differentiation, growth, and morphogenesis of the tissues within the skeleton are discussed and related to the evolution of the vertebrate skeleton. The tissues considered are bone, cartilage, dentine, and enamel. The histology of the skeletal tissues of the Ordovician agnatha is reviewed with the conclusion that the skeletal tissues of the first vertebrates were as diverse and as specialized as are those of present-day vertebrates. Phylogenies of skeletal tissues cannot be established. The trend during evolution appears to have been toward reduction in amount of skeletal tissue and in the number of types of tissues present. The factors which determine when and where a skeletal element develops ontogenetically are reviewed and used to discuss the origin and evolution of jaws, the evolution of membrane bones and the origin of such structures as sesamoid bones. Special importance is attached to epithelial-mesenchymal interactions. The factors which determine what particular skeletal tissue will form at a particular site within the body are reviewed with especial reference to modulation, germ layer derivation, and the role of epigenetic factors. The factors which determine size and shape of the skeleton, both ontogenetically and phylogenetically, are reviewed and the directive role of adjacent tissues emphasized.

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