Secondarily aquatic tetrapods

Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least partly aquatic. These animals are called "secondarily aquatic" because although all tetrapods descended from freshwater lobe finned fish (see evolution of tetrapods), their more recent ancestors are terrestrial vertebrates that evolved on land for hundreds of millions of years, and their clades only re-adapted to aquatic environment much later.

Unlike primarily aquatic vertebrates (i.e. fish), secondarily aquatic tetrapods (especially aquatic amniotes), while having appendages such as flippers, dorsal fin and tail fins (flukes) that resemble fish fins due to convergent evolution, still have physiology based on their terrestrial ancestry, most notably their air-breathing respiration via lungs (instead of aquatic respiration via gills) and excretion of nitrogenous waste as urea or uric acid (instead of ammonia like most fish). Nearly all extant aquatic tetrapods are secondarily aquatic, with only larval amphibians (tadpoles) being primarily aquatic with gills, and only some species of paedomorphic mole salamanders (most notably the fully aquatic axolotl) retain the gill-based physiology into adulthood.

Secondary aquatic adaptations of tetrapods tend to develop in early speciation of semi-aquatic animals that venture more and more frequently into water bodies in search of suitable habitats and foraging/hunting for food. As successive generations spend more time in water, natural selection favors those with traits that allow them to fair better in water, hence leading to more specialized aquatic adaptations. Later-generation aquatic tetrapods may evolve to spend most their life in the water, only coming ashore for mating, sleeping or to evade aquatic predators. Finally, some aquatic tetrapods become ultra-specialized aquatic animals who are fully adapted to sleep (while holding breath) and reproduce in water, with some even losing the ability to breathe and stay alive if stranded out of water.