- Fish - a vague grouping that includes several distinct classes; the Chondrichthyes (including sharks) and the actinopterygii (ray-finned fishes).
- Amphibians - the first vertebrates to develop true legs and live on the land; most still have to return to the water to breed.
- Reptiles - distinguished from amphibians by their ability to withstand dry conditions and to breed on land by means of shelled eggs. The reptile class is possibly a redundant group according to cladistics.
- Birds - the only class with feathers; their fossil record indicates they evolved from the dinosaurs.
- Mammals - they have fur and suckle their young.