Temnospondyli

Ancestors of modern amphibians adapted to life on land

Temnospondyli (from Greek τέμνειν, temnein 'to cut' and σπόνδυλος, spondylos 'vertebra') or temnospondyls is a diverse ancient order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian and Triassic periods, with fossils being found on every continent. A few species continued into the Jurassic and Early Cretaceous periods, but all had gone extinct by the Late Cretaceous. During about 210 million years of evolutionary history, they adapted to a wide range of habitats, including freshwater, terrestrial, and even coastal marine environments. Their life history is well understood, with fossils known from the larval stage, metamorphosis and maturity. Most temnospondyls were semiaquatic, although some were almost fully terrestrial, returning to the water only to breed. These temnospondyls were some of the first vertebrates fully adapted to life on land. Although temnospondyls are amphibians, many had characteristics such as scales and armour like bony plates that distinguish them from the modern soft bodied lissamphibians (frogs and toads, newts, salamanders and caecilians). Temnospondyls have been known since the early 19th century, and were initially thought to be reptiles. They were described at various times as batrachians, stegocephalians and labyrinthodonts, although these names are now rarely used. Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Temnospondyli means "cut vertebrae", as each vertebra is divided into several parts (intercentrum, paired pleurocentra, neural arch), although this occurs widely among other early tetrapods. Experts disagree over whether temnospondyls were ancestral to modern amphibians (frogs, salamanders and caecilians), or whether the whole group died out without leaving any descendants. Different hypotheses have placed modern amphibians as the descendants of temnospondyls, as descendants of another group of early tetrapods called lepospondyls, or even as descendants of both groups (with caecilians evolving from lepospondyls and frogs and salamanders evolving from temnospondyls). There is further disagreement about a temnospondyl origin of lissamphibians related to whether the modern groups arose from only one group (dissorophoids) or from two different groups (dissorophoids and stereospondyls). The majority of studies place a group of temnospondyls called amphibamiforms as the closest relatives of modern amphibians. Similarities in teeth, skulls and hearing structures link the two groups.

In 2000 Adam Yates and Anne Warren defined the name Temnospondyli as applying to the clade encompassing all organisms that are more closely related to Eryops than to the “microsaur” Pantylus.

Many temnospondyls are much larger than living amphibians, and superficially resemble crocodiles, which has led many taxa to be named with the suffix suchus. The largest taxa, which were predominantly the Mesozoic stereospondyls, had skulls exceeding one meter in length, and the entire animal would have been several meters in length (for reference, the largest living amphibian, Andrias, is about 1.8 meters in body length). Others are smaller and resemble salamanders, in particularly the amphibamiform and micromelerpetid dissorophoids. and the function(s) remains largely unresolved due to the absence of this feature in lissamphibians. Some temnospondyls also exhibit raised tubercles or pustules instead of pits and grooves (e. g., the dissorophoid Micropholis, plagiosaurine plagiosaurids), and the import of this disparity is also unclear. Many temnospondyls also have canal like grooves in their skulls called sensory sulci, the presence of which is used to infer an aquatically inclined lifestyle. The sulci, which usually run around the nostrils and eye sockets, are part of a lateral line system used to detect vibrations in water in modern fish and certain modern amphibians. Many taxa, especially those inferred to have been terrestrial, have an opening at the midline near the tip of the snout called the internarial fenestra / fontanelle; this may have housed a mucus gland used in prey capture. In zatracheids, this opening is greatly enlarged for an unknown purpose. Homologues of most of the bones of temnospondyls are also seen in other early tetrapods, aside from a few bones in the skull, such as interfrontals, internasals and interparietals, that have developed in some temnospondyl taxa. The intertemporal, a bone common in stem tetrapods, is only found in some late Paleozoic taxa like certain edopoids and dvinosaurs. Most temnospondyls have an indentation at the back of the skull called otic notches. It has typically been inferred that this structure supported a typanum for hearing, although there is substantial variation among temnospondyls in the anatomy of this notch such that it may not have served this function in all temnospondyls, and some clades like plagiosaurids and brachyopids lack notches entirely. Recent studies have suggested that these large openings provided additional attachment sites for musculature and that many temnospondyls were capable of retracting their eyeballs through the vacuities, which is observed in modern frogs and salamanders that also have these large palatal openings; there is no evidence for a buccal pump mechanism for respiration. Temnospondyls often have extensive coverings of teeth on their palates, as well as in their jaws, in contrast to modern amphibians. Some of these teeth are so large, they are referred to as tusks or fangs. Although most temnospondyls have monocuspid teeth, the presence of bicuspid and/or pedicellate teeth in some dissorophoids has been cited as evidence for close relatedness to lissamphibians.

Temnospondyls' vertebrae are divided into several segments. In living tetrapods, the main body of the vertebra is a single piece of bone called the centrum, but in temnospondyls, this region was divided into a pleurocentrum and intercentrum. Two primary types of vertebrae are recognized in temnospondyls: stereospondylous and rhachitomous vertebrae. In rhachitomous vertebrae, the intercentra are large and wedge shaped, and the pleurocentra are relatively small blocks that fit between them. Both elements support a spine like neural arch, and well developed interlocking projections called zygapophyses strengthen the connections between vertebrae. The strong backbone and strong limbs of many rhachitomous temnospondyls allowed them to be partially, and in some cases fully, terrestrial. In stereospondylous vertebrae, the pleurocentra have been greatly reduced or lost entirely, with the intercentra enlarged as the main body of the vertebrae. Early concepts of stereospondyl required the pleurocentra to be entirely absent, but newer concepts only require that the intercentrum has become greatly enlarged. Additional types that are less common are the plagiosaurid type in which there is a single enlarged centrum of uncertain homology; and the tupilakosaurid type vertebrae (diplospondyly) in which the pleurocentra and intercentra are the same size and form discs; this occurs in tupilakosaurid dvinosaurs but also at least some brachyopids and several other non temnospondyls. The neural spines tend to be of similar height throughout the presacral region of the trunk, but some temnospondyls exhibit increasing height towards the mid trunk, followed by a decrease in height to produce a more hump backed contour. The most extreme is observed in the dissorophid Platyhystrix, which has greatly elongated neural spines that form a large sail on its back. The function of this sail, like that of the contemporaneous sphenacodontids and edaphosaurids, remains enigmatic, but it is thought to have stiffened the vertebral column in association with the relative terrestriality of this clade. The majority of temnospondyls have presacral counts between 23 and 27, with reduction observed in some amphibamiforms and elongation observed in many dvinosaurs. Caudal length is highly variable, and complete caudal sequences are rare. Based on Eryops, more than 30 caudal positions were possible in some taxa. These elements differ widely in variation across temnospondyls, with such variation attributed to different lifestyles. The interclavicle and clavicles tend to be more lightly built in terrestrial taxa, with little to no ornamentation. In contrast, these elements are massively ossified in the aquatic stereospondyls and are well ornamented in the same fashion as the skull. The cleithrum and scapulocoracoid is more developed in terrestrial taxa, and the coracoid tends not to ossify in aquatic forms such that there is only a much shorter scapula present. The pelvis comprises the ilium, ischium and pubis, the last of which does not always ossify in aquatic forms. The sutural contacts between elements may also be visible, even when all three ossify. The forelimb comprised the typical radius, ulna, humerus and manus. These bones are typically more developed with greater surface area for muscle attachment in taxa inferred to have been terrestrial. Many dissorophoids have long and slender limbs. Historically it has been thought that all temnospondyls had only four fingers, but this has been shown not to be true in at least a few stereospondyls (Metoposaurus, Paracyclotosaurus), and the paucity of complete manuses casts doubt on the sweeping characterization of a four fingered manus as the predominant or plesiomorphic condition. At least in Metoposauridae, there are both taxa with four fingers and taxa with five. The hindlimb comprised the typical tibia, fibula, femur and pes. Relative development is as with the forelimb. All temnospondyls with a known pes have five digits. Unlike modern amphibians, many temnospondyls are covered in small, closely packed scales. The undersides of most temnospondyls are covered in rows of large ventral plates. During early stages of development, they first have only small, rounded scales. Fossils show, as the animals grew, the scales on the undersides of their bodies developed into large, wide ventral plates. The plates overlap each other in a way that allows a wide range of flexibility. Later semiaquatic temnospondyls, such as trematosaurs and capitosaurs, have no evidence of scales. They may have lost scales to make movement easier under water or to allow cutaneous respiration, the absorption of oxygen through the skin. Several groups of temnospondyls have large bony plates (osteoderms) on their backs. One temnospondyl, Peltobatrachus, has armour like plating that covers both its back and underside. The rhytidosteid Laidleria also has extensive plating on its back. Most members of the family Dissorophidae also have armor, although it only covers the midline of the back with one or two narrow rows of plates that tightly articulated with the vertebrae, and osteoderms are also known from a few trematopids. Other temnospondyls, such as Eryops, have been found with small, disc like bony scutes that were in life probably embedded in the skin. All of these temnospondyls were adapted to a terrestrial lifestyle. Armor may have offered protection from predators in the case of Peltobatrachus. A carapace of osteoderms is also seen in plagiosaurids, the only primarily aquatic clade with such extensive ossifications. Plagiosaurids may have inherited their armor from a terrestrial ancestor, as both Peltobatrachus and Laidleria have been considered close relatives of the group. Contrary to older assumptions, more recent studies have argued that the temnospondyls evolved from a terrestrial ancestor (although with aquatic eggs and larvae), and that it was the forms that later returned to water and an aquatic lifestyle which evolved a spine more rigid and stiffer than the terrestrial species.

Very little is known of the soft tissue of temnospondyls because the conditions necessary to preserve such material are uncommon. The most extensive records come from fine grained deposits in the Carboniferous and Permian of Germany; the small bodied and aquatic dissorophoids and the larger stereospondylomorphs are frequently preserved with outlines of soft tissue around the skeleton. Typically preserved features include the outline of the body, external gills, and parts of the eye or stomach. An amphibamiform specimen from the Mazon Creek locality was described as having toepad like features. The holotype specimen of Arenaerpeton supinatus from the Triassic of New South Wales, Australia, displays extensive soft tissue, hinting at the girth of the animal in life. Trace fossils attributed to temnospondyls are fairly common, especially from the Carboniferous through the Triassic. Common ichnogenera include Batrachichnus and Limnopus.

Temnospondyli was named by the German paleontologist Karl Alfred von Zittel in his second edition of Handbuch der Palaeontologie, published in 1888. However, temnospondyl remains have been known since the early part of the 19th century. The naming of these first specimens was disputed. Leopold Fitzinger named the animal Batrachosaurus in 1837. In 1841, the English paleontologist Richard Owen referred to the genus as Labyrinthodon to describe its highly folded or labyrinthine teeth. Owen thought that the name Mastodonsaurus "ought not to be retained, because it recalls unavoidably the idea of the mammalian genus Mastodon, or else a mammilloid form of the tooth and because the second element of the word, saurus, indicates a false affinity, the remains belonging, not to the Saurian, but to the Batrachian order of Reptiles." Owen recognized that the animal was not a "saurian" reptile, yet he also referred Jaeger's Phytosaurus to the genus. Although the two genera have similarly sized conical teeth, Phytosaurus was later found to be a crocodile like reptile. Additional material, including skulls, firmly placed Labyrinthodon as an amphibian. Jaeger also named Salamandroides giganteus in 1828, basing it on partial occiput, or back portion of the skull. In 1833, he described a complete skull of S. giganteus that had the same teeth as his Mastodonsaurus, making it the first known complete skull of a temnospondyl. Because Mastodonsaurus was named first, it has precedence over the other names as a senior subjective synonym. Batrachosaurus is still used as the name of an unrelated brachyopid temnospondyl. Mastodonsaurus and other similar animals were referred to as labyrinthodonts, named like Labyrinthodon for teeth that were highly folded in cross section. Owen's "Labyrinthodon Jaegeri" was later found at Guy's Cliffe, England by paleontologist William Buckland. Other specimens were found in the red sandstone of Warwickshire. As more fossils were uncovered in England, Owen depicted these labyrinthodonts as the "highest" form of batrachian and compared them to crocodiles, which he considered the highest form of reptiles. He also noted the large labyrinthodonts of the Keuper (a unit of rocks that dates to the Late Triassic) were younger than more advanced reptiles in the Magnesian and Zechstein, which are Late Permian in age. Owen used these fossils to counter the notion that reptiles evolved from a sequential progression from early amphibians (what he called "metamorphosed fishes"). In addition to Mastodonsaurus, some of the earliest named genera included Metopias and Rhombopholis in 1842, Zygosaurus in 1848, Trematosaurus in 1849, Baphetes and Dendrerpeton in 1853, Capitosaurus in 1858, and Dasyceps in 1859. Baphetes is now placed as an early tetrapod outside Temnospondyli, and Rhombopholis is now considered a prolacertiform reptile.

Later in the 19th century, temnospondyls were classified as various members of Stegocephalia, a name coined by the American paleontologist Edward Drinker Cope in 1868. Cope placed stegocephalians in the class Batrachia, the name then used for Amphibia. Stegocephalia means "roof headed" in Greek, a reference to the wide, flat heads of temnospondyls and other early tetrapods. During this time, paleontologists considered temnospondyls to be amphibians because they possessed three main features: gill arches in juvenile skeletons, indicating they were amphibious for at least the first part of their lives; ribs that do not connect at the underside of the rib cage; and deep pits in the skull that were interpreted as space for mucous glands. Several suborders of stegocephalians were recognized in the late 19th and early 20th centuries. Animals now regarded as temnospondyls were primarily labyrinthodonts, but some were classified in the Branchiosauria. Branchiosaurs were small bodied and had simple conical teeth, while labyrinthodonts were larger and had complex, folded dentin and enamel in their teeth. Branchiosauria included only a few forms, such as Branchiosaurus from Europe and Amphibamus from North America, that had poorly developed bones, external gills, and no ribs. Some skeletons of Amphibamus were later found with long ribs, prompting its reassignment to Microsauria (although more detailed studies found it to be a temnospondyl). Soft tissue, such as scales and external gills, were found in many well preserved branchiosaur fossils from Germany. In the early 20th century, branchiosaurs would be recognized as larval forms of temnospondyls lacking many of the typical features that define the group, and is no longer recognized as a distinct group. Other animals that would later be classified as temnospondyls were placed in a group called Ganocephala, which was characterized by plate like skull bones, small limbs, fish like scales and branchial arches. Unlike labyrinthodonts, they did not have parietal foramina, small holes in their skulls behind their eye sockets. Archegosaurus, Dendrerpeton, Eryops and Trimerorhachis were placed in this group and were considered to be the most primitive members of Reptilia. Their rhachitomous vertebrae, notochord and lack of occipital condyles (which attached the head to the neck) were features that were also shared with fishes. Thus, they were considered a link between early fishes and more advanced forms such as stegocephalians. Another group was called Microsauria by Cope in 1868. He classified Microsauria as a subgroup of Labyrinthodontia, placing many small, amphibian like animals within it. Among them was Dendrerpeton, once placed in Ganocephala. Dendrerpeton was later placed as a labyrinthodont with other temnospondyls, but confusion existed for many years over the classification of small amphibians. By the end of the 19th century, most of what are today regarded as temnospondyls were placed in the suborder Labyrinthodonta. The American paleontologist Ermine Cowles Case called it Labyrinthodonta vera or "true labyrinthodonts". The names Stegocephalia and Labyrinthodontia were used interchangeably to refer to the order in which it belonged. The labyrinthodontian suborders Microsauria and Branchiosauria, both of which contain temnospondyls, were distinct from Labyrinthodonta. Within Labyrinthodonta were the groups Rhachitomi, Labyrinthodonti and Embolerimi. Members of Rhachitomi, such as Archegosaurus and Eryops, had rhachitomous vertebrae with enlarged intercentra that displaced the pleurocentra. Labyrinthodonti, such as Mastodonsaurus, Trematosaurus and Micropholis, had lost their pleurocentra, and the intercentra made up the entire body of the vertebrae. Embolerimi had intercentra and pleurocentra that were of equal size. Embolomeres are now identified as a separate group of reptiliomorphs or stem group tetrapods, with no particular affinities to temnospondyls. Temnospondyli became a commonly used name at the turn of the 20th century. Paleontologists included both embolomeres and rhachitomes in the group. Cope's Ganocephala and Labyrinthodonta fell out of use. In 1919, British paleontologist D. M. S. Watson proposed that the evolutionary history of these large amphibians could be seen through changes in their vertebrae. Embolomerous forms in the Carboniferous graded into rhachitomous forms in the Permian, and finally into stereospondyls in the Triassic. More importantly, Watson began using the term Labyrinthodontia to refer to these groups. The name Temnospondyli was rarely used in the decades that followed. Swedish paleontologist Gunnar Säve Söderbergh removed embolomeres from the group, narrowing its scope to rhachitomes and stereospondyls. His classification of labyrinthodonts was based heavily on characteristics of the skull rather than the vertebrae. During the Carboniferous, all of the rhachitome clades appeared, including dendrerpetids, edopoids, eryopoids, the various dissorophoid subclades, dvinosaurs and zatracheids. although the former may have appeared earlier and merely be undocumented at present. The vast majority of the Carboniferous records come from the midwestern United States, like the Linton, Five Points and Mazon Creek lagerstätte, and the south central United States where classic redbed formations are found; and from western Europe, particularly the Saar Nahe Basin in Germany and Nýřany in the Czech Republic. The early Permian record of temnospondyls is also concentrated in these regions. Most of the clades from the Late Carboniferous continued to be successful, with a particularly high diversity of dissorophoids.

Whether there are any middle Permian records of temnospondyls is debated as a result of the uncertain age and correlation of different deposits in North America and Russia and the controversy over Olson's Gap.

During the Late Permian, increasing aridity and the diversification of reptiles contributed to a decline in terrestrial temnospondyls, but semiaquatic and fully aquatic temnospondyls continued to flourish, including the large Melosaurus of Eastern Europe. Other temnospondyls, such as archegosaurids, developed long snouts and a close similarity to crocodiles, although they lacked the armor characteristic of the latter group. These temnospondyls included the largest known batrachomorph, the 9 meter long Prionosuchus of Brazil. The stereospondyl record is almost exclusively confined to rhinesuchids.

As temnospondyls continued to flourish and diversify in the Late Permian (260.4–251.0 Mya), a major group called Stereospondyli became more dependent on life in the water. The vertebrae became weak, the limbs small, and the skull large and flat, with the eyes facing upwards. During the Triassic period, these animals dominated the freshwater ecosystems, evolving in a range of both small and large forms. During the Early Triassic (251.0–245.0 Mya) one group of successful long snouted fish eaters, the trematosauroids, even adapted to a life in the sea, the only known batrachomorphs to do so with the exception of the modern crab eating frog. Another group, the capitosauroids, included medium sized and large animals in length, with large and flat skulls that could be over a meter long in the largest forms such as Mastodonsaurus. These animals spent most or all their lives in water as aquatic predators, catching their prey by a sudden opening of the upper jaw and sucking in fish or other small animals. In the Carnian stage of the Late Triassic (237.0–227.0 Mya), capitosauroids were joined by the superficially very similar Metoposauridae. Metoposaurids are distinguished from capitosauroids by the positioning of their eye sockets near the front of their skulls. Another group of stereospondyls, the plagiosaurs, had wide heads and gills, and adapted to life at the bottom of lakes and rivers. By this time, temnospondyls had become a common and widespread component of semiaquatic ecosystems. Some temnospondyls, such as Cryobatrachus and Kryostega, even inhabited Antarctica, which was covered in temperate forests at the time. Triassic temnospondyls were often the dominant semiaquatic animals in their environments. Large assemblages of metoposaurs with hundreds of individuals preserved together have been found in the southwestern United States. They have often been interpreted as mass death events caused by droughts in floodplain environments. Recent studies show these dense assemblages were instead probably the result of currents accumulating dead individuals in certain areas. These environments seem to have had little diversity, as they were inhabited almost exclusively by metoposaurs. Temnospondyls reached a peak diversity during the Early Triassic, and progressively declined throughout the subsequent Middle and Late Triassic, with only 4 members of the Brachyopoidea surviving into the Jurassic and the Cretaceous. Among brachyopoids, the brachyopids Gobiops and Sinobrachyops are known from Middle and late Jurassic deposits across Asia and the chigutisaurid Siderops is known from the Early Jurassic of Australia. The most recent known temnospondyl was the giant chigutisaurid Koolasuchus, known from the Early Cretaceous (Aptian) of Australia. It survived in rift valleys that were too cold in the winter for Crocodylomorphs that normally would have competed with them. Koolasuchus was one of the largest of the brachyopoids, with an estimated weight of .

A study on the fully aquatic Archegosaurus shows that its heat balance, gas exchange, osmoregulation, and digestion were more similar to those of fish than those of modern aquatic amphibians like salamanders.

Although the earliest temnospondyls were primarily semiaquatic, they had the ability to feed on land. Later, eryopoids and dissorophoids, some well adapted to terrestrial life, also fed on land. Some eryopoids became better adapted toward life in water and shifted their diets toward aquatic organisms. The first primarily aquatic feeders were archegosaurs in the Permian. Trematosaurs and capitosaurs became independently aquatic and also returned to this type of feeding. Most aquatic stereospondyls have flattened heads. When feeding, they probably opened their mouths by lifting their skulls instead of lowering their lower jaws. The jaw mechanics of the plagiosaurid Gerrothorax is well known, and is one of the most highly adapted. Gerrothorax is thought to have lifted its skull to around 50 degrees above horizontal through the flexing of the atlanto occipital joint between the occipital condyles of the skull and the atlas vertebra of the neck. As the skull is raised, the quadrate bone pushes forward and causes the lower jaw to protrude outward. Other stereospondyls probably also lifted their skulls, but they are not as well adapted for such movement. D. M. S. Watson was the first to suggest skull lifting as a means of feeding in temnospondyls. He envisioned that Mastodonsaurus, a much larger temnospondyl than Gerrothorax, was able to make the same movement. Paleontologist A. L. Panchen also supported the idea in 1959, suggesting that Batrachosuchus also fed in this way. Earlier aquatic tetrapods and tetrapod ancestors differ from temnospondyls like Phonerpeton in that their skulls were also built to withstand tension. This tension would have been experienced during suction feeding underwater. Temnospondyls like Phonerpeton were among the first tetrapods that were almost exclusively terrestrial and fed by biting.

Temnospondyls, like most modern amphibians, reproduced in aquatic environments. Most temnospondyls probably reproduced through external fertilization. Like most living frogs, female temnospondyls would have laid masses of eggs in water while males released sperm to fertilize them. Several fossils were described from the Early Permian of Texas in 1998 that may be egg masses of dissorophoid temnospondyls. They were the first known fossils of amphibian eggs. The fossils consist of small disks with thin membranes that are probably vitelline membranes and halo like areas surrounding them that are most likely mucous coatings. They are attached to plant fossils, suggesting that these temnospondyls laid eggs on aquatic plants much like modern frogs. The mucous membranes show that the eggs were laid by amphibians, not fish (their eggs lack mucous), but the type of amphibian that laid them cannot be known because no body fossils are preserved with the eggs. The eggs are thought to be from dissorophoids because they are likely to be close relatives of modern amphibians, and probably had similar reproductive strategies. They are also the most common amphibians from the deposit in which the eggs were found. One temnospondyl, the dvinosaur Trimerorhachis, may have brooded young in an area between the gills called the pharyngeal pouch. Small bones belonging to younger Trimerorhachis individuals have been found in these pouches. The living Darwin's Frog is also a mouth brooder and would be the closest modern analogue to Trimerorhachis if it cared for its young in this way. An alternative possibility is that Trimerorhachis was cannibalistic, eating its young like many amphibians do today. If this was the case, the bones of these smaller individuals were originally located in the throat and were pushed into the pharyngeal pouch as the animal fossilized. Body impressions of Early Carboniferous temnospondyls from Pennsylvania suggest that some terrestrial temnospondyls mated on land like some modern amphibians. They reproduced through internal fertilization rather than mating in water. The presence of three individuals in one block of sandstone shows that the temnospondyls were gregarious. The head of one individual rests under the tail of another in what may be a courtship display. Internal fertilization and similar courtship behavior are seen in modern salamanders.

While most types of temnospondyls are distinguished on the basis of features in mature specimens, several are known from juvenile and larval specimens. Metamorphosis is seen in dissorophoids, eryopids and zatrachydids, with aquatic larvae developing into adults capable of living on land. Several types of dissorophoids, such as branchiosaurids, do not fully metamorphose, but retain features of juveniles such as external gills and small body size in what is known as neoteny. Dvinosaurians and the plagiosaurid Gerrothorax also retained gills, although recent studies found that (at least as adults) their gills were internal like those of fish, rather than external like those of salamanders. Temnospondyl larvae are often distinguished by poorly developed bones and the presence of a hyobranchial apparatus, a series of bones that gills would attach to in life. However, some fully mature temnospondyls also possess hyobranchial bones but did not have external gills. A dense covering of scales is also seen in larvae and adults. Major body changes occur in metamorphosis, including the reshaping and strengthening of skull bones, the thickening of postcranial bones, and an increase in body size. Once maturity is reached, most bones have fully formed and growth rate slows. The bones of some temnospondyls like Dutuitosaurus show growth marks, possibly an indication that growth rate varied with the change in seasons. Fossils of temnospondyls like Metoposaurus and Cheliderpeton show that individuals grew larger past maturity. The oldest individuals usually have more pitting on their skulls with deeper sulci. One group of temnospondyls, the Branchiosauridae, is also known from larval specimens. Branchiosaurids like Branchiosaurus and Apateon are represented by many fossils preserving skin and external gills. An entire growth series is exhibited in the wide range of sizes among specimens, but the lack of terrestrially adapted adult forms suggests that these temnospondyls were neotenic. Unlike other temnospondyls, their postcranial skeletons developed quickly but were still partly cartilaginous when fully mature. Adults likely had an aquatic lifestyle similar to juveniles. Recently, large specimens of Apateon gracilis were described with adaptations toward a terrestrial lifestyle, indicating that not all branchiosaurs were neotenic.

Temnospondyls and other early tetrapods have rounded otic notches in the back of the skull that project into the cheek region. In life, the otic notch would have been covered by a membrane called the tympanum, which is seen as a disk like area in living frogs. The tympanum is involved in hearing, and is similar to the ear drum of more advanced tetrapods. It was traditionally thought that the tympanum developed very early in tetrapod evolution as a hearing organ and progressed to form the eardrum of amniotes. Thus, temnospondyls possessed a hearing system supposedly ancestral to that of living amphibians and reptiles. Frogs and all other living tetrapods have a rod like bone called the stapes that aids in hearing by transferring vibrations from the ear drum—or homologous tympanum—to the inner ear. Temnospondyls also have a stapes, which projects into the otic cavity. The stapes likely evolved from the hyomandibula of lobe finned fishes. The positioning of the stapes and the shape of the otic region suggests that the tympani of temnospondyls and frogs are homologous, but the tympani of these amphibians are no longer considered homologous with the hearing systems of reptiles, birds and mammals. Therefore, ear structures in temnospondyls were not ancestral to those of all other tetrapods. Later temnospondyls like Doleserpeton had otic regions adapted to hearing. Doleserpeton has a structure in the inner ear called the perilymphatic duct, which is also seen in frogs and is associated with hearing. Its stapes is also a better transmitter of sound. The hearing system of Doleserpeton and related temnospondyls was able to detect airborne sound and may have been ancestral to that of living amphibians.