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, 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, secondarily aquatic tetrapods, while having appendages such as flippers, dorsal fin and tail fins that resemble fish fins due to convergent evolution, still have physiology based on their terrestrial ancestry, most notably their air-breathing respiration via lungs and excretion of nitrogenous waste as urea or uric acid. Nearly all extant aquatic tetrapods are secondarily aquatic, with only larval amphibians being primarily aquatic with gills, and only some species of paedomorphic mole salamanders 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 and reproduce in water, with some even losing the ability to breathe and stay alive if stranded out of water.
Marine reptiles
Mesosaurs
Mesosaurs were a group of small aquatic reptiles that lived during the early Permian period, roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having returned to an aquatic lifestyle from more terrestrial ancestors. Most authors consider mesosaurs to have been fully aquatic, although adult animals may have been only semiaquatic.Turtles
Archelon is a type of giant sea turtle dating from the Cretaceous Period, now long extinct. Its smaller cousins survive as the sea turtles of today.Softshell turtles are a taxonomic family of a number of turtle genera that are able to "breathe" underwater with rhythmic movements of their mouth cavity, which contains numerous processes copiously supplied with blood, acting similarly to gill filaments in fish. This enables them to stay under water for prolonged periods. Moreover, the Chinese softshell turtle has been shown to excrete urea while "breathing" underwater; this is an efficient solution when the animal does not have access to fresh water, e.g., in brackish-water environments.
Squamates
Squamata is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians. There are many examples of aquatic squamates, both living and extinct; a secondarily aquatic lifestyle has evolved multiple times.Living at the same time as, but not closely related to, dinosaurs, the mosasaurs resembled crocodiles but were more strongly adapted to marine life. Scientists continue to debate on whether monitor lizards or snakes are the closest living relatives of mosasaurs. Mosasaurs became extinct 66 million years ago, at the same time as the non-avian dinosaurs.
Modern squamates which have made their own adaptions to allow them to spend significant time in the ocean include marine iguanas and sea snakes. Sea snakes are extensively adapted to the marine environment, giving birth to live offspring and are largely incapable of terrestrial activity. The arc of their adaptation is evident by observing the primitive Laticauda genus, which must return to land to lay eggs.
The Annulated sea snake is a species of venomous sea snake that can breathe underwater with help of extensive vascular network across the top of its head to absorb oxygen from the surrounding water.