In addition, most vertebrates have a vertebral column and kidneys. Fishes are the most speciose group of vertebrates, with nearly 21,000 identified species. We have learned much about the mechanisms of evolution and natural selection from the study of fishes.
Tetrapods usually live on land and breathe with lungs.
Most of the latter animals are descended from fully terrestrial vertebrates. Order Crossopterygii lobe-finned fishes. Swim bladders present. Tetrapod classification Tetrapods are monophyletic. Besides the following characteristics, the tetrapods are classified by their skull morphology.