From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so they can be classed in groups under groups.

Charles Darwin, Origin of Species, Chapter 13

A tetrapod (name derived from the Greek for, "four-legged") is a vertebrate with four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs and mammals are all tetrapods, and birds and snakes also are tetrapods by descent. The tetrapods are believed to have descended from the Sarcopterygii, or "lobe-finned" fishes, into air-breathing amphibians during the Devonian.

    Superclass Tetrapoda (four-limbed vertebrates)
      Class Amphibia (amphibians)
       Amniota (amniotic embryo)
      Class Sauropsida (reptiles and birds)
        Class Aves (birds)
      Class Synapsida (mammal-like reptiles)
        Class Mammalia (mammals)