page:  = Temnospondyli = 
   
   Temnospondyli ( from Greek <unk> ( <unk> , " to cut " ) and <unk> ( <unk> , " vertebra " ) ) is a diverse subclass of extinct small to giant tetrapods — often considered primitive amphibians — that flourished worldwide during the Carboniferous , Permian , and Triassic periods . A few species continued into the Cretaceous . Fossils have been found on every continent . During about 210 million years of evolutionary history , they adapted to a wide range of habitats , including fresh water , terrestrial , and even coastal marine environments . Their life history is well understood , with fossils known from the larval stage , metamorphosis , and maturity . Most <unk> were semiaquatic , although some were almost fully terrestrial , returning to the water only to breed . These <unk> were some of the first vertebrates fully adapted to life on land . Although <unk> are considered amphibians , many had characteristics , such as scales , claws , and armour @-@ like bony plates , that distinguish them from modern amphibians . 
   <unk> have been known since the early 19th century , and were initially thought to be reptiles . They were described at various times as <unk> , <unk> , and <unk> , 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 . 
   Experts disagree over whether <unk> 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 <unk> , another group of early tetrapods called <unk> , or even as descendants of both groups ( with caecilians evolving from <unk> and frogs and salamanders evolving from <unk> ) . Recent studies place a family of <unk> called the <unk> as the closest relatives of modern amphibians . Similarities in teeth , skulls , and hearing structures link the two groups . 
   
   = = Description = = 
   
   Many <unk> are much larger than living amphibians , and superficially resemble crocodiles . Others are smaller and resemble salamanders . Most have broad , flat heads that are either blunt ( <unk> ) or elongated ( <unk> ) . The skulls are rounded or triangular in shape when viewed from above , and are usually covered in pits and ridges . The rugged surfaces of bones may have supported blood vessels , which could transfer carbon dioxide to the bones to neutralize acidic build up in the blood ( early semiaquatic tetrapods would have had difficulty expelling carbon dioxide from their bodies while on land , and these dermal bones may have been an early solution to the problem ) . Many <unk> also have canal @-@ like grooves in their skulls called sensory sulci . The sulci , which usually run around the nostrils and eye sockets , are part of a lateral line system used to detect vibrations in water . As semiaquatic animals , most <unk> have small limbs with four toes on each front foot and five on each hind foot . Terrestrial <unk> have larger , thicker limbs , and some even have claws . One unusual terrestrial temnospondyl , <unk> , has relatively long limbs for its body , and probably lived as an active runner able to chase prey . 
   <unk> of most of the bones of <unk> are also seen in other early tetrapods , aside from a few bones in the skull , such as <unk> , <unk> , and <unk> , that have developed in some temnospondyl taxa . Most <unk> have tabular horns in the backs of their skulls , rounded projections of bone separated from the rest of the skull by indentations called otic notches ; in some <unk> , such as <unk> , they are pointed and very prominent . Among the most distinguishing features of <unk> are the <unk> vacuities , two large holes in the back of the palate . Another pair of holes , <unk> , are present in front of these vacuities , and connect the nasal passage with the mouth . <unk> often have teeth on their palates , as well as in their jaws . Some of these teeth are so large , they are referred to as tusks . In some <unk> , such as <unk> , tusks in the lower jaw pierce the palate and emerge through openings in the top of the skull . 
   Very little is known of the soft tissue of <unk> . A block of sandstone , described in 2007 from the Early Carboniferous Mauch Chunk Formation of Pennsylvania , included impressions of the bodies of three <unk> . These impressions show , when alive , they had smooth skin , robust limbs with webbed feet , and a ridge of skin on their undersides . Trackways referable to small <unk> have also been found in Carboniferous and Permian rocks . The trackways , called <unk> , are usually found in strata deposited around freshwater environments , suggesting the animals had some ties to the water . 
   Unlike modern amphibians , many <unk> are covered in small , closely packed scales . The undersides of most <unk> 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 <unk> , such as <unk> and <unk> , 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 <unk> have large bony plates on their backs . One temnospondyl , <unk> , has armour @-@ like plating that covers both its back and underside . The temnospondyl <unk> also has extensive plating on its back . Most members of the family <unk> also have armor , although it only covers the midline of the back with two narrow rows of plates . Other <unk> , such as <unk> , have been found with small , disc @-@ like bony scutes that were in life probably embedded in the skin . All of these <unk> were adapted to a terrestrial lifestyle . Armor may have offered protection from predators in the case of <unk> . The scutes may have provided stability for the spine , as they would have limited flexibility and may have been connected by strong ligaments . <unk> such as <unk> and <unk> that may have been at least partly terrestrial also have long neural spines on top of their vertebrae that would have stabilized the spine . Bony scutes are also seen in <unk> , but unlike <unk> , <unk> , <unk> , and <unk> , these animals are thought to have been fully aquatic . <unk> may have inherited their armor from a terrestrial ancestor , as both <unk> and <unk> have been considered close relatives of the group . 
   <unk> ' 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 <unk> , this region was divided into a <unk> and <unk> . Two types of vertebrae are recognized in <unk> : <unk> and <unk> vertebrae . In <unk> vertebrae , the <unk> are large and wedge @-@ shaped , and the <unk> 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 <unk> <unk> allowed them to be partially , and in some cases fully , terrestrial . In <unk> vertebrae , the <unk> have been lost entirely , with the <unk> enlarged as the main body of the vertebrae . This weaker type of backbone indicates the <unk> <unk> spent more time in water . 
   
   = = History of study = = 
   
   Temnospondyli was named by German palaeontologist Karl Alfred von Zittel in his second edition of Handbuch der <unk> , published in 1888 . <unk> remains were known since the early part of the 19th century , however . The earliest described temnospondyl was Mastodonsaurus , named by Georg Friedrich Jaeger in 1828 . Jaeger named Mastodonsaurus from a single tooth , and considered it a reptile . Mastodonsaurus means " breast tooth lizard " after the nipple @-@ like shape of the tip of the tooth . 
   The naming of these first specimens was disputed , however . Leopold Fitzinger named the animal <unk> in 1837 . In 1841 , English palaeontologist Richard Owen referred to the genus as Labyrinthodon to describe its highly folded or labyrinthine teeth . Owen thought the name Mastodonsaurus " ought not to be retained , because it recalls unavoidably the idea of the mammalian genus Mastodon , or else a <unk> 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 <unk> order of Reptiles . " Owen recognized the animal was not a " saurian " reptile , yet he also referred Jaeger 's <unk> to the genus . Although the two genera both have similarly sized conical teeth , <unk> was later found to be a crocodile @-@ like reptile . Additional material , including skulls , firmly placed Labyrinthodon as an amphibian . Jaeger also named <unk> 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 . <unk> is still used as the name of an unrelated <unk> temnospondyl . 
   Mastodonsaurus and other similar animals were referred to as <unk> , named like Labyrinthodon for teeth that were highly folded in cross section . Owen 's " Labyrinthodon <unk> " 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 <unk> as the " highest " form of <unk> and compared them to crocodiles , which he considered the highest form of reptiles . He also noted the large <unk> of the Keuper ( a unit of rocks that dates to the Late Triassic ) were younger than more advanced reptiles in the <unk> and <unk> , 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 <unk> and <unk> in 1842 , <unk> in 1848 , <unk> in 1849 , <unk> and <unk> in 1853 , <unk> in 1858 , and <unk> in 1859 . <unk> is now placed as an early tetrapod outside Temnospondyli , and <unk> is now considered a <unk> reptile . 
   Later in the 19th century , <unk> were classified as various members of <unk> , a name coined by American paleontologist Edward Drinker Cope in 1868 . Cope placed <unk> in the class Batrachia , the name then used for Amphibia . <unk> means " roof @-@ headed " in Greek , a reference to the wide , flat heads of <unk> and other early tetrapods . During this time , palaeontologists considered <unk> 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 <unk> were recognized in the late 19th and early 20th centuries . Animals now regarded as <unk> were primarily <unk> , but some were classified in the <unk> . <unk> were small @-@ bodied and had simple conical teeth , while <unk> were larger and had complex , folded dentin and enamel in their teeth . <unk> included only a few forms , such as <unk> from Europe and <unk> from North America , that had poorly developed bones , external gills , and no ribs . Some skeletons of <unk> were later found with long ribs , prompting its reassignment to <unk> ( although more detailed studies found it to be a temnospondyl ) . Soft tissue , such as scales and external gills , were found in many well @-@ preserved <unk> fossils from Germany . In the early 20th century , <unk> would be recognized as larval forms of <unk> 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 <unk> were placed in a group called <unk> , characterized by plate @-@ like skull bones , small limbs , fish @-@ like scales , and branchial arches . Unlike <unk> , they did not have parietal <unk> , small holes in their skulls behind their eye sockets . <unk> , <unk> , <unk> and <unk> were placed in this group and were considered to be the most primitive members of <unk> . Their <unk> 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 <unk> . 
   Another group called <unk> was named by Cope in 1868 . Cope classified <unk> as a subgroup of Labyrinthodontia , placing many small , amphibian @-@ like animals within it . Among them were <unk> , once placed in <unk> . <unk> was later placed as a <unk> with other <unk> , 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 <unk> were placed in the suborder <unk> . American paleontologist Ermine Cowles Case called it <unk> vera or " true <unk> " . The names <unk> and Labyrinthodontia were used interchangeably to refer to the order in which it belonged . The <unk> suborders <unk> and <unk> , both of which contain <unk> , were distinct from <unk> . Within <unk> were the groups <unk> , <unk> , and <unk> . Members of <unk> , such as <unk> and <unk> , had <unk> vertebrae with enlarged <unk> that displaced the <unk> . <unk> , such as Mastodonsaurus , <unk> , and <unk> , had lost their <unk> , and the <unk> made up the entire body of the vertebrae . <unk> had <unk> and <unk> that were of equal size . <unk> are now identified as <unk> distantly related to <unk> . 
   In 1888 , von Zittel divided <unk> among three taxa : Lepospondyli , Temnospondyli , and <unk> . He placed <unk> in Lepospondyli , a group which he characterized as having simple , spool @-@ shaped vertebral centra . Temnospondyli included forms with the centra divided into <unk> and <unk> . All members of <unk> had amphicoelous centra composed only of the <unk> . Cope objected to von Zittel 's classification , considering the vertebrae of <unk> and <unk> indistinguishable because each had a simple spool shape . He continued to use <unk> and <unk> ( which he alternatively referred to as <unk> ) to distinguish animals based on the absence or presence of occipital condyles . 
   Temnospondyli became a commonly used name at the turn of the century . Paleontologists included both <unk> and <unk> in the group . Cope 's <unk> and <unk> 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 . <unk> forms in the Carboniferous graded into <unk> forms in the Permian , and finally into <unk> 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 <unk> @-@ <unk> removed <unk> from the group , narrowing its scope to <unk> and <unk> . His classification of <unk> was based heavily on characteristics of the skull rather than the vertebrae . 
   American paleontologist Alfred Romer brought the name Temnospondyli back into use in the later 20th century . <unk> @-@ <unk> used the name Labyrinthodontia in a strict sense ( sensu stricto ) to refer to <unk> and <unk> , excluding <unk> . Romer agreed with this classification , but used the name Temnospondyli to avoid confusion with Labyrinthodontia in its wider sense ( sensu lato ) . Unlike modern temnospondyl classification , however , Romer included the primitive <unk> in the group . 
   
   = = Evolutionary history = = 
   
   
   = = = Carboniferous and Early Permian = = = 
   
   <unk> first appeared in the Early Carboniferous around 330 million years ago ( Mya ) . During the Carboniferous , <unk> included basal medium @-@ sized forms such as <unk> or large semiaquatic forms such as <unk> . Other , more derived <unk> , such as the <unk> , were smaller and more terrestrial . They resembled salamanders , and some taxa , such as the genus <unk> , even retained external gills like the modern @-@ day axolotl . During the latest Carboniferous and Early Permian around 300 Mya , several groups , such as the <unk> and <unk> evolved strong , robust limbs and vertebrae and became adapted to life on land while others such as the <unk> , developed into large semiaquatic predators . The <unk> , a group of small aquatic <unk> , evolved from terrestrial ancestors in the Late Carboniferous . 
   
   = = = Late Permian = = = 
   
   During the Late Permian , increasing aridity and the diversification of reptiles contributed into a decline in terrestrial <unk> , but semiaquatic and fully aquatic <unk> continued to flourish , including the large <unk> of Eastern Europe . Other <unk> , such as <unk> , developed long snouts and a close similarity to crocodiles , although they lacked the armor characteristic of the latter group . These <unk> included the largest known amphibian , the 9 @-@ m @-@ long <unk> of Brazil . 
   
   = = = Mesozoic = = = 
   
   As <unk> continued to flourish and diversify in the Late Permian ( 260 @.@ 4 - 251 @.@ 0 Mya ) , a major group called <unk> 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 <unk> , even adapted to a life in the sea , the only known amphibians to do so with the exception of the modern crab @-@ eating frog . Another group , the <unk> , included medium- and large @-@ sized animals 2 @.@ 3 to 4 m ( 7 @.@ 5 to 13 @.@ 1 ft ) 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 ( 228 @.@ 0 - 216 @.@ 5 Mya ) , <unk> were joined by the superficially very similar <unk> . <unk> are distinguished from <unk> by the positioning of their eye sockets near the front of their skulls . Another group of <unk> , the <unk> , had wide heads with external gills , and adapted to life at the bottom of lakes and rivers . By this time , <unk> had become a common and widespread component of semiaquatic ecosystems . Some <unk> , such as <unk> and <unk> , even inhabited Antarctica , which was covered in temperate forests at the time . 
   Triassic <unk> were often the dominant semiaquatic animals in their environments . Large assemblages of <unk> 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 <unk> . 
   The Triassic @-@ Jurassic extinction event around 199 @.@ 6 Mya led to the extinction of most Mesozoic <unk> . The <unk> survived , as well as a few <unk> and <unk> . While the latter two groups soon became extinct , <unk> persisted and grew to large sizes during the Jurassic . Among <unk> , the <unk> flourished in China and the <unk> became common in Gondwana . The most recent known temnospondyl was the giant <unk> <unk> , known from the Early Cretaceous of Australia . It survived in rift valleys that were too cold in the winter for crocodiles that normally would have competed with them . <unk> was one of the largest of the <unk> , with an estimated weight of 500 kg ( 1 @,@ 100 lb ) . 
   
   = = Classification = = 
   
   Originally , <unk> were classified according to the structure of their vertebrae . Early forms , with complex vertebrae consisting of a number of separate elements , were placed in the suborder <unk> , and large Triassic aquatic forms with simpler vertebrae were placed in the suborder <unk> . With the recent growth of phylogenetics , this classification is no longer viable . The basic <unk> condition is found in many primitive tetrapods , and is not unique to one group of <unk> . Moreover , the distinction between <unk> and <unk> vertebrae is not entirely clear . Some <unk> have <unk> , <unk> , and <unk> vertebrae at different points in the same vertebral column . Other taxa have intermediate morphologies that do not fit into any category . <unk> is no longer recognized as a group , but <unk> is still considered valid . Below is a simplified taxonomy of <unk> showing currently recognized groups : 
   Class Amphibia 
   Order Temnospondyli 
   Superfamily <unk> 
   Family <unk> ( <unk> ) 
   Family <unk> 
   Family <unk> 
   Suborder <unk> 
   Superfamily <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Superfamily <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Clade <unk> 
   Clade <unk> 
   Superfamily <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> ( placement is uncertain ) 
   Family <unk> 
   Suborder <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Clade <unk> 
   Superfamily <unk> ( <unk> ) 
   Family <unk> 
   Family <unk> 
   Family <unk> 
   Infraorder <unk> 
   Superfamily <unk> 
   Superfamily <unk> 
   Superfamily <unk> 
   Superfamily <unk> 
   Superfamily <unk> 
   
   = = = Phylogeny = = = 
   
   In one of the earliest phylogenetic analyses of the group , Gardiner ( 1983 ) recognized five characteristics that made Temnospondyli a clade : a bone at the back of the skull , the <unk> , is connected to another bone on the underside of the skull , the pterygoid ; large openings called <unk> vacuities are present between the <unk> ; the stapes ( a bone involved in hearing ) is connected to the <unk> and projects upward ; the <unk> , a bone in the pectoral girdle , is thin ; and part of the vertebra called the <unk> attaches to the neural arch . Additional features were given by Godfrey et al . ( 1987 ) , including the contact between the <unk> and <unk> at the back of the skull , small projections ( uncinate processes ) on the ribs , and a pelvic girdle with each side having a single iliac blade . These shared characteristics are called synapomorphies . 
   <unk> are placed as basal tetrapods in phylogenetic analyses , with their exact positioning varying between studies . Depending on the classification of modern amphibians , they are either included in the crown group <unk> or the stem of <unk> . Crown @-@ group tetrapods are descendants of the most recent common ancestor of all living tetrapods and stem tetrapods are forms that are outside the crown group . Modern amphibians have recently been suggested as descendants of <unk> , which would place them within crown <unk> . Below is a cladogram from <unk> et al . ( 2003 ) placing Temnospondyli within crown <unk> : 
   Other studies place modern amphibians as the descendants of <unk> and place <unk> in a more basal position within the stem of <unk> . Below is a cladogram from Laurin and Reisz ( 1999 ) placing Temnospondyli outside crown <unk> : 
   Most phylogenetic analyses of temnospondyl interrelationships focus on individual families . One of the first broad @-@ scale studies of temnospondyl phylogeny was conducted by paleontologist Andrew Milner in 1990 . A 2007 study made a " <unk> " of all temnospondyl families , combining the family @-@ level trees of previous studies . The following cladogram is modified from <unk> et al . ( 2007 ) : 
   1 Temnospondyli , 2 <unk> , 3 <unk> , 4 <unk> , 5 <unk> , 6 <unk> , 7 <unk> , 8 <unk> , 9 <unk> , 10 <unk> , 11 <unk> , 12 <unk> , 13 <unk> , 14 <unk> 
   The most basal group of <unk> is the superfamily <unk> . <unk> have several primitive or plesiomorphic features , including a single occipital condyle and a bone called the <unk> that is absent in other <unk> . <unk> include the Late Carboniferous genus <unk> and the family <unk> . <unk> has also been included in <unk> , and is the oldest known temnospondyl family . <unk> <unk> is the oldest species , having been present over 330 million years ago during the <unk> stage of the Early Carboniferous . Recent analyses place <unk> outside <unk> in a more derived position . Other primitive <unk> include <unk> and <unk> . <unk> and <unk> , both described in 2005 from Niger , are also primitive yet come from the Late Permian . They are almost 40 million years younger than other basal <unk> , implying a long ghost lineage of species that are not yet known in the fossil record . 
   In 2000 , paleontologists Adam Yates and Anne Warren produced a revised phylogeny of more derived <unk> , naming several new clades . Two major clades were <unk> and <unk> . <unk> includes the <unk> that were once called <unk> and includes two subfamilies , the <unk> and the <unk> . <unk> include small , mostly terrestrial <unk> that may be the ancestors of modern amphibians . <unk> include larger <unk> like <unk> . The second major clade , <unk> , includes most Mesozoic <unk> , as well as some Permian groups . Within <unk> are the superfamily <unk> and the most derived <unk> , the <unk> . 
   Yates and Warren also named <unk> , a clade of small aquatic <unk> from the Carboniferous , Permian , and Triassic . They placed <unk> within <unk> , but more recent studies disagree on their position . For example , a 2007 study places them even more basal than <unk> , while a 2008 study keeps them as basal <unk> . 
   Within <unk> , Yates and Warren erected two major clades : <unk> and <unk> . <unk> include large semiaquatic <unk> like Mastodonsaurus with flat heads and eyes near the back of the skull . <unk> include a diversity of <unk> , including large marine <unk> , aquatic <unk> , <unk> that survived into the Cretaceous , and <unk> with eyes near the front of their heads . In 2000 , paleontologists Rainer Schoch and Andrew Milner named a third major clade of <unk> , the <unk> . This group included more primitive <unk> that could not be placed in either <unk> or <unk> , and included groups like <unk> , <unk> , and <unk> . While <unk> and <unk> are still widely used , <unk> is not often supported as a true clade in recent analyses . <unk> and <unk> are now grouped with <unk> , but <unk> are still considered to be a primitive family of <unk> . 
   A new phylogeny of <unk> was offered by paleontologist Rainer Schoch in 2013 . It supported many of the clades that were found by Yates and Warren , but it did not find support for their division of derived <unk> into <unk> and <unk> . <unk> were found to be more closely related to <unk> than to <unk> , which were grouped with <unk> . The clade including <unk> and <unk> was named <unk> . In addition , Schoch named the clade containing all <unk> except <unk> <unk> and reinstated the name <unk> for the clade containing all <unk> except <unk> and <unk> . Below is the cladogram from Schoch 's analysis : 
   
   = = = Relationship to modern amphibians = = = 
   
   Modern amphibians ( frogs , salamanders , and caecilians ) are classified in Lissamphibia . <unk> appear to have arisen in the Permian . Molecular clock estimates place the first <unk> in the Late Carboniferous , but the first member of Batrachia ( frogs and salamanders , but not caecilians ) is estimated to have appeared in the Middle Permian using the same technique . Using fossil evidence , there are three main theories for the origin of modern amphibians . 
   One is that they evolved from <unk> <unk> . Another is that they evolved from <unk> , most likely the <unk> . A third hypothesis is that caecilians descended from <unk> and frogs and salamanders evolved from <unk> . 
   Recently , the theory that <unk> were the ancestors of all lissamphibians has gained wide support . The skull morphology of some small <unk> has been compared to those of modern frogs and salamanders , but the presence of bicuspid , pedicellate teeth in small , paedomorphic or immature <unk> has been cited as the most convincing argument in favor of the temnospondyl origin of lissamphibians . Seen in lissamphibians and many <unk> <unk> , pedicellate teeth have calcified tips and bases . During the development of most tetrapods , teeth begin to <unk> at their tips . <unk> normally proceeds downward to the base of the tooth , but calcification from the tip stops abruptly in pedicellate teeth . <unk> resumes at the base , leaving an area in the center of the tooth <unk> . This pattern is seen in living amphibians and fossils . 
   The <unk> family <unk> is thought to be most closely related to Lissamphibia . In 2008 , an <unk> called <unk> <unk> was named from Texas and was nicknamed the " <unk> " for its frog @-@ like head and salamander @-@ like body . It was thought to be the most closely related temnospondyl to lissamphibians and was placed as the sister taxon of the group in a phylogenetic analysis . Another species of <unk> called <unk> annectens is now thought to be even more closely related to lissamphibians . Unlike <unk> , <unk> was known since 1969 , and the presence of pedicellate teeth in its jaws has led some paleontologists to conclude soon after its naming that it was a relative of modern amphibians . It was first described as a " <unk> " , and the specific name annectens means " connecting " in reference to its inferred transitional position between <unk> and lissamphibians . The structure of its tympanum , a disk @-@ like membrane that functions like an ear drum , is similar to that of frogs and has also been used as evidence for a close relationship . Other features including the shape of the palate and the back of the skull , the short ribs , and the smooth skull surface also point to it being a closer relative of lissamphibians than is <unk> . Below is a cladogram modified from <unk> and Bolt ( 2010 ) showing the relationships of <unk> , <unk> , and Lissamphibia : 
   
   = = Paleobiology = = 
   
   
   = = = Feeding = = = 
   
   Although the earliest <unk> were primarily semiaquatic , they had the ability to feed on land . Later , <unk> and <unk> , some well adapted to terrestrial life , also fed on land . Some <unk> became better adapted toward life in water , and shifted their diets toward aquatic organisms . The first primarily aquatic feeders were <unk> in the Permian . <unk> and <unk> became independently aquatic and also returned to this type of feeding . 
   Most aquatic <unk> 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 <unk> <unk> is well known , and is one of the most highly adapted . <unk> is thought to have lifted its skull to around 50 ° above horizontal through the flexing of the <unk> @-@ 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 <unk> probably also lifted their skulls , but they are not as well adapted for such movement . <unk> Watson was the first to suggest skull lifting as a means of feeding in <unk> . He envisioned that Mastodonsaurus , a much larger temnospondyl than <unk> , was able to make the same movement . Paleontologist A.L. Panchen also supported the idea in 1959 , suggesting that <unk> also fed in this way . At the time it was thought that these <unk> lifted their heads with strong jaw muscles , but it is now thought that they used larger muscles in the neck that were attached to the large pectoral girdle . <unk> , a close relative of <unk> , also has a <unk> skeleton that muscles may have attached to . <unk> has very small teeth and a large area for muscle attachment behind the skull , suggesting that it could suction feed by rapidly opening its mouth . 
   Unlike semiaquatic <unk> , terrestrial <unk> have skulls that are adapted for biting land @-@ living prey . The sutures between the bones of the skull in the <unk> <unk> are able to withstand a high degree of compression . <unk> forces would have been experienced when biting down on prey . Earlier aquatic tetrapods and tetrapod ancestors differ from <unk> like <unk> in that their skulls were also built to withstand tension . This tension would have been experienced during suction feeding underwater . <unk> like <unk> were among the first tetrapods that were almost exclusively terrestrial and fed by biting . 
   
   = = = Reproduction = = = 
   
   <unk> , like all amphibians , reproduced in aquatic environments . Most <unk> probably reproduced through external fertilization . Like most living frogs , female <unk> 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 <unk> <unk> . They were the first known fossils of amphibian eggs . The fossils consist of small disks with thin membranes that are probably <unk> membranes and halo @-@ like areas surrounding them that are most likely mucous coatings . They are attached to plant fossils , suggesting that these <unk> 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 <unk> 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 <unk> <unk> , may have brooded young in an area between the gills called the pharyngeal pouch . Small bones belonging to younger <unk> individuals have been found in these pouches . The living Darwin 's Frog is also a mouth <unk> and would be the closest modern analogue to <unk> if it cared for its young in this way . An alternative possibility is that <unk> 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 <unk> from Pennsylvania suggest that some terrestrial <unk> 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 <unk> 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 . 
   
   = = = Growth = = = 
   
   While most types of <unk> are distinguished on the basis of features in mature specimens , several are known from juvenile and larval specimens . Metamorphosis is seen in <unk> , <unk> , and <unk> , with aquatic larvae developing into adults capable of living on land . Several types of <unk> do not fully metamorphose , but retain features of juveniles such as gills and small body size in what is known as neoteny . <unk> and the <unk> <unk> were also neotenic because they retained gills , but they are only known from adult specimens . 
   <unk> larvae are often distinguished by poorly developed bones and the presence of a <unk> apparatus , a series of bones that gills would attach to in life . However , some fully mature <unk> also possess <unk> 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 . 
   <unk> like <unk> are known from both large adult specimens and small larvae , showing an extreme change in body shape . In these species , the shape and proportions of skull bones change in the early stages of development . The ornamentation on the surface of the skull roof also develops at this time . Small , regularly spaced pits are the first to form , followed by larger ridges . As development continues , the external gills disappear . Small teeth that once covered the palate are lost . The postcranial skeleton does not develop at the same rate as the skull , with ossification ( the replacement of cartilage by bone ) happening more slowly . Vertebrae and limb bones are poorly developed , ribs and fingers are absent in the early stages , and the <unk> and ischium are entirely absent through most of development . Once maturity is reached , most bones have fully formed and growth rate slows . The bones of some <unk> like <unk> show growth marks , possibly an indication that growth rate varied with the change in seasons . Fossils of <unk> like <unk> and <unk> show that individuals grew larger past maturity . The oldest individuals usually have more pitting on their skulls with deeper sulci . 
   One group of <unk> , the <unk> , is also known from larval specimens . <unk> like <unk> and <unk> 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 <unk> adapted adult forms suggests that these <unk> were neotenic . Unlike other <unk> , 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 <unk> gracilis were described with adaptations toward a terrestrial lifestyle , indicating that not all <unk> were neotenic . 
   While most <unk> are aquatic in early stages of life , most <unk> appear to have been terrestrial in their juvenile stage . Like other Mesozoic <unk> , adult <unk> were adapted to a semiaquatic lifestyle . Their bones are not highly developed for movement on land . The cross @-@ sectional thickness of limb bones in adult <unk> shows that they could not withstand the stress of terrestrial locomotion . Juvenile individuals have bones that are thick enough to withstand this stress , and could probably move about on land . To maintain a terrestrial lifestyle , a temnospondyl 's limb bones would have to thicken with positive allometry , meaning that they would grow at a greater rate than the rest of the body . This is not the case in <unk> , meaning that as their bodies grew larger they became less adapted toward a terrestrial lifestyle . 
   
   = = = Hearing = = = 
   
   <unk> 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 ear drum of amniotes . Thus , <unk> 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 . <unk> also have a stapes , which projects into the otic cavity . The stapes likely evolved from the <unk> of lobe @-@ finned fishes . The positioning of the stapes and the shape of the otic region suggests that the tympani of <unk> 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 <unk> were not ancestral to those of all other tetrapods . 
   The ability of the tympanum and stapes to effectively transmit vibrations is called impedance matching . Early tetrapods like <unk> have thick stapes with poor impedance matching , so it is now thought that they were not used for hearing . Instead , these thick stapes may have functioned to support the tissue that covers the otic notch . Early <unk> like <unk> could not hear airborne sound but would have been able to detect vibration in the ground . Later <unk> like <unk> had otic regions adapted to hearing . <unk> has a structure in the inner ear called the <unk> 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 <unk> and related <unk> was able to detect airborne sound and may have been ancestral to that of living amphibians . 
   
   