The increase of skeletal mass is a biomechanical requirement for increasing the body size and the development of post-cranial phosphated skeletons in vertebrates, provided, among other things, biomechanical support for colonization of the earth. In fact, terrestrialization was a gradual process. The fingers were not formed from changes in the rays of the fins having different embryogenic origin. Probably the gradual reduction of fins allowed the increase in Endoskeletal component that produced the fingers; the change may be the result of differences in duration of the various phases of development. The clearest contrast between fish and tetrapods is the evolution of fingers and joints associated with limbs, wrists and ankles, elbows and knees.
