Four classes of mineralized tissues are found in vertebrates: bone, cartilage, dentine, and enamel. We think of cartilage and bone as skeletal tissues and of enamel and dentine as dental tissues, but enamel and dentine arose evolutionarily together with bone as skeletal tissues in the dermal skeleton (exoskeleton) of early vertebrates. Scales and teeth of sharks are examples of dermal skeletal elements that are still composed of the three ancient components-enamel, dentine, and bone. Cartilage, on the other hand, provided the basis for the second vertebrate skeletal system, the endoskeleton (Smith and Hall, 1990; Hall, 1998a,b). some invertebrate skeletal tissues have surprisingly bone-like features. Examples include chondrocytes interconnected by cell processes in cephalopod cartilages (Cole and Hall, 2004a,b), and the calcium phosphate layer in the shells of brachiopods (Rodland et al., 2003). However, neither bone nor mineralized cartilage have been found in invertebrates. Editors notes: TODO - develops_from