[Evolution of cartilage in the skeletons of ancient anamniotes]

Owing to discoveries of ancient fossils of the Anamnia it was revealed that endo and exoskeletal tissue have an independent history of development. At early evolutional stages, the exoskeletal tissues were connected by various transitions and were at the stage of incomplete differentiation. During evolution variability of these tissues decreased; this resulted in disappearance of aspidin from the tegmentum, and dentin remained only in some primitive groups of fishes (sharks, Latimeria chalumnae, Lepidosteus osseus), while in most vertebrates the exoskeleton was formed exclusively from the osseous tissue. In the endoskeleton the cartilage evolutionally preceded the bone. It is rather difficult to draw any border between the calcified cartilage and the bone in the ancient vertebrates. When an osseous tissue healing occurs at some pathological processes in the vertebrae, a primitive state of the endoskeletal tissue is reproduced. Secondary progressive cartilage development during evolution in the anamnia is considered as a result of negative anabolism and connected with a slowed morphogenesis. Chondrification evidently increases plastic possibilities of the endoskeletal structures.