Broad anatomical descriptors like "tetrapod" and "amphibian" can approximate some members of the stem group, but a few paleontologists opt for more specific terms such as Stegocephali.

Limbs evolved prior to terrestrial locomotion, but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with a semiaquatic lifestyle to exploit food and shelter on land.

These include distinct head and neck structures for feeding and movements, appendicular skeletons (shoulder and pelvic girdles in particular) for weight bearing and locomotion, more versatile eyes for seeing, middle ears for hearing, and more efficient heart and lungs for oxygen circulation and exchange outside water.

The key innovation in amniotes over amphibians is the amnion, which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water.

[12][13] One fundamental subgroup of amniotes, the sauropsids, diverged into the reptiles: lepidosaurs (lizards, snakes, and the tuatara), archosaurs (crocodilians and dinosaurs, of which birds are a subset), turtles, and various other extinct forms.

[20] Other paleontologists use the term stem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes.

[21] The term "fishapod" was popularized after the discovery and 2006 publication of Tiktaalik, an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits.

Overall, the biodiversity of lissamphibians,[23] as well as of tetrapods generally,[24] has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian.

[25] The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic.

Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" (clades) of the tetrapod family tree.

By Aristotle's time, the basic division between mammals, birds and egg-laying tetrapods (the "herptiles") was well known, and the inclusion of the legless snakes into this group was likewise recognized.

In conjunction with robust forelimbs and shoulder girdle, both Tiktaalik and Ichthyostega may have had the ability to locomote on land in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging behind.

[63] These early "stem-tetrapods" included animals such as Ichthyostega,[2] with legs and lungs as well as gills, but still primarily aquatic and poorly adapted for life on land.

[64] When stem-tetrapods reappear in the fossil record in early Carboniferous deposits, some 10 million years later, the adult forms of some are somewhat adapted to a terrestrial existence.

A less popular proposal draws comparisons to the "lepospondyls", an eclectic mixture of various small tetrapods, including burrowing, limbless, and other bizarrely-shaped forms.

All basal amniotes had a small body size, like many of their contemporaries, though some Carboniferous tetrapods evolved into large crocodile-like predators, informally known as "labyrinthodonts".

The latter were the most important and successful Permian land animals, establishing complex terrestrial ecosystems of predators and prey while acquiring various adaptations retained by their modern descendants, the mammals.

Temnospondyls briefly recovered in the Triassic, spawning the large aquatic stereospondyls and the small terrestrial lissamphibians (the earliest frogs, salamanders, and caecilians).

During the Jurassic, one synapsid group (Cynodontia) gave rise to the modern mammals, which survived through the rest of the Mesozoic to later diversify during the Cenozoic.

All these lineages are extinct except for Dipnomorpha and Tetrapoda; from Swartz, 2012:[71] Dipnomorpha (lungfishes and relatives) Kenichthys Rhizodontidae Marsdenichthys Canowindra Koharalepis Beelarongia Gogonasus Gyroptychius Osteolepis Medoevia Megalichthyidae Spodichthys Tristichopterus Eusthenopteron Jarvikina Cabbonichthys Mandageria Eusthenodon Tinirau Platycephalichthys Panderichthys Tiktaalik Elpistostege Elginerpeton Ventastega Acanthostega Ichthyostega Whatcheeriidae Colosteidae Crassigyrinus Baphetidae Tetrapoda Crown tetrapods are defined as the nearest common ancestor of all living tetrapods (amphibians, reptiles, birds, and mammals) along with all of the descendants of that ancestor.

[76] † Embolomeri † Gephyrostegidae † Seymouriamorpha † Microsauria † Lysorophia † Adelospondyli † Aistopoda † Nectridea † Diadectomorpha Crown-group Amniota Cladogram modified after Laurin, How Vertebrates Left the Water (2010).

[77] † Acanthostega † Ichthyostega † Temnospondyli † Embolomeri † Seymouriamorpha † Adelogyrinidae † Aistopoda † Nectridea † Lysorophia Lissamphibia † Diadectomorpha Amniota This hypothesis has batrachians (frogs and salamanders) coming out of dissorophoid temnospondyls, with caecilians out of microsaur lepospondyls.

The evolution of early tetrapod respiration was influenced by an event known as the "charcoal gap", a period of more than 20 million years, in the middle and late Devonian, when atmospheric oxygen levels were too low to sustain wildfires.

The early tetrapod Acanthostega had at least three and probably four pairs of gill bars, each containing deep grooves in the place where one would expect to find the afferent branchial artery.

When the muscles are relaxed, the bony scales spring back into position, generating considerable negative pressure within the torso, resulting in a very rapid intake of air through the spiracle.

[113] According to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a mechanism for relieving respiratory acidosis (acidic blood caused by excess CO2) through compensatory metabolic alkalosis.

The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.

The vomeronasal organ also evolved in the nasal cavity for the first time, for detecting pheromones from biological substrates on land, though it was subsequently lost or reduced to vestigial in some lineages, like archosaurs and catarrhines, but expanded in others like lepidosaurs.

The eye was now exposed to a relatively dry environment rather than being bathed by water, so eyelids developed and tear ducts evolved to produce a liquid to moisten the eyeball.

Only in the early Triassic, about a hundred million years after they conquered land, did the tympanic middle ear evolve (independently) in all the tetrapod lineages.