"Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago).

Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group (clade) exclusive of other tetrapods.

Instead, they consistute an evolutionary grade (a paraphyletic group), ancestral to living tetrapods such as lissamphibians (modern amphibians) and amniotes (reptiles, mammals, and kin).

Particularly the early forms exhibited a lot of variation, yet there are still a few basic anatomical traits that make their fossils very distinct and easily recognizable in the field: Labyrinthodonts were generally amphibian-like in build.

The advanced diadectomorphs from the Late Carboniferous and Early Permian were fully terrestrial with stout skeletons, and were the heaviest land animals of their time.

It is a question of some doubt whether early terrestrial labyrinthodonts had the stapes connected to a tympanum covering their otic notch, and if they had an aerial sense of hearing at all.

[14] When suitable prey swam or walked within reach, the jaw would slam shut, the palatine tusks stabbing the hapless victim.

The existence of a larval stage as the primitive condition in all groups of labyrinthodonts can be fairly safely assumed, in that tadpoles of Discosauriscus, a close relative of the amniotes, are known.

At the close of the Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults.

Temnospondyls appeared in the early Carboniferous and came in all sizes, from small salamander-like Stereospondyli that scurried along the waters edge and undergrowth, to giant, well armoured Archegosauroidea that looked more like crocodiles.

A fossil trackway from Lesotho shows larger forms dragged themselves by the front limbs over slippery surfaces with limited sideways movement of the body, very unlike modern salamanders.

Earliest traces of the land-living forms are fossil trackways from Zachełmie quarry, Poland, dated to 395 million years ago, attributed to an animal with feet very similar to Ichthyostega.

[37][38] By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities.

From their piscine ancestors, they had inherited swim bladders[dubious – discuss] that opened to the esophagus and were capable of functioning as lungs (a condition still found in lungfish and some physostome ray-finned fishes), allowing them to hunt in stagnant water or in waterways where rotting vegetation would have lowered oxygen content.

This is in contrast to the earlier view that fish had first invaded the land—either in search of prey like modern mudskippers, or to find water when the pond they lived in dried out.

[10] They were all carnivorous, initially eating fish and possibly going ashore to feed off washed up carrion of sea animals caught in tidal ponds, only later turning into predators of the large invertebrates of the Devonian at the waters edge.

The proto-tetrapods like from Elginerpeton and Tiktaalik had extremities ending in fin-rays with no clear fingers, primarily suited for movement in open water, but also capable of propelling the animal across sandbanks and through vegetation filled waterways.

While most labyrinthodonts remained aquatic or semi-aquatic, some of the reptile-like amphibians adapted to explore the terrestrial ecological niches as small or medium-sized predators.

They evolved increasingly terrestrial adaptions during the Carboniferous, including stronger vertebrae and slender limbs, and a deeper skull with laterally placed eyes.

They probably had watertight skin, possibly covered with a horny epidermis overlaying small bony nodules, forming scutes, similar to those found in modern caecilians.

To the modern eye, these animals would appear like heavyset, lizards betraying their amphibious nature only by their lack of claws and by spawning aquatic eggs.

Several adaptations to piscivory evolved with some groups having crocodile-like skulls with slender snouts, and presumably had a similar life-style (Archegosauridae, Melosauridae, Cochleosauridae and Eryopidae, and the reptile-like suborder Embolomeri).

[33] All groups developed progressively weaker vertebrae, reduced limb ossification and flatter skulls with prominent lateral line organs, indicating the late Permian/early Triassic temnospondyls rarely if ever left the water.

The "Stockholm school" under Gunnar Säve-Söderbergh and Erik Jarvik argued during much of the 20th century that Amphibia as a whole is biphyletic, based on details of the nasal capsule and cranial nerves.

[56] The cladistic analysis of Gerobatrachus suggested salamanders and frogs evolved from temnospondyl stock and caecilians being the sister group of the reptile-like amphibians, rendering Lissamphibia itself an evolutionary grade relative to the remaining tetrapod classes.

The seymouriamorphs were small to medium-sized animals with stout limbs, their remains are sometimes found in what has been interpreted as dry environments, indicating their skin had a water-tight epidermal horny overlay or even scales as evident in Discosauriscus.

[58] A number of small, fragmentary fossils of possibly diadectomorph affinity has been proposed as the first amniote, including Gephyrostegus,[64] Solenodonsaurus,[61] Westlothiana[65] and Casineria.

[69][70] An alternative name, Stegocephalia was created in 1868 by American paleontologist Edward Drinker Cope, from Greek stego cephalia—"roofed head", and refer to anapsid skull and the copious amounts of dermal armour some of the larger forms evidently had.

[55] Few shared Säve-Söderbergh's view of a biphyletic Amphibia, but his scheme, either with the Lepospondyli as a separate subclass or sunk into Temnospondyli, was continued by Romer in his much used Vertebrate Paleontology of 1933 and later editions,[30] and followed by several subsequent authors with minor variations: Colbert 1969,[73] Daly 1973,[74] Carroll 1988[75] and Hildebrand & Goslow 2001.

This group includes both traditional "labyrinthodonts" as well as more basal tetrapodomorph fish, though its total content is a matter of some uncertainty, as the relationships of these animals are not well understood.