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Unlike modern amphibians , many <unk> are covered in small , closely packed scales . The <unk> of most <unk> are covered in rows of large ventral plates . During early stages of development , they first have only small , rounded scales . <unk> show , as the animals grew , the scales on the <unk> 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 <unk> <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 <unk> <unk> , the absorption of oxygen through the skin .
Several groups of <unk> have large bony plates on their backs . One <unk> , <unk> , has armour @-@ like plating that covers both its back and underside . The <unk> <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 <unk> 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 <unk> 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 . <unk> <unk> 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 <unk> , the main body of the <unk> is a single piece of bone called the <unk> , 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 <unk> projections called <unk> 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 = =
<unk> was named by German <unk> Karl Alfred von <unk> in his second edition of <unk> der <unk> , published in 1888 . <unk> remains were known since the early part of the 19th century , however . The earliest described <unk> was <unk> , named by Georg Friedrich <unk> in 1828 . <unk> named <unk> from a single tooth , and considered it a reptile . <unk> means " breast tooth lizard " after the <unk> @-@ like shape of the tip of the tooth .
The naming of these first specimens was disputed , however . Leopold <unk> named the animal <unk> in 1837 . In 1841 , English <unk> Richard Owen referred to the genus as <unk> to describe its highly folded or <unk> teeth . Owen thought the name <unk> " ought not to be retained , because it recalls <unk> the idea of the mammalian genus <unk> , or else a <unk> form of the tooth ... and because the second element of the word , <unk> , indicates a false affinity , the remains belonging , not to the <unk> , but to the <unk> order of <unk> . " Owen recognized the animal was not a " <unk> " reptile , yet he also referred <unk> '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 <unk> as an amphibian . <unk> also named <unk> <unk> in 1828 , basing it on partial <unk> , or back portion of the skull . In 1833 , he described a complete skull of S. <unk> that had the same teeth as his <unk> , making it the first known complete skull of a <unk> . Because <unk> was named first , it has precedence over the other names as a senior <unk> synonym . <unk> is still used as the name of an unrelated <unk> <unk> .
<unk> and other similar animals were referred to as <unk> , named like <unk> for teeth that were highly folded in cross section . Owen 's " <unk> <unk> " was later found at Guy 's <unk> , 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 <unk> , which he considered the highest form of reptiles . He also noted the large <unk> of the <unk> ( a unit of rocks that dates to the Late <unk> ) were younger than more advanced reptiles in the <unk> and <unk> , which are Late <unk> in age . Owen used these fossils to counter the notion that reptiles evolved from a <unk> progression from early amphibians ( what he called " <unk> fishes " ) .
In addition to <unk> , 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 <unk> outside <unk> , 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 <unk> Cope in 1868 . Cope placed <unk> in the class <unk> , the name then used for <unk> . <unk> means " roof @-@ headed " in Greek , a reference to the wide , flat heads of <unk> and other early <unk> . During this time , <unk> 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 <unk> glands .
Several <unk> 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 <unk> 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 <unk> to <unk> ( although more detailed studies found it to be a <unk> ) . 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 <unk> 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 <unk> arches . Unlike <unk> , they did not have parietal <unk> , small holes in their skulls behind their eye <unk> . <unk> , <unk> , <unk> and <unk> were placed in this group and were considered to be the most primitive members of <unk> . Their <unk> vertebrae , <unk> , and lack of <unk> <unk> ( 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 <unk> of <unk> , 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 <unk> <unk> Case called it <unk> <unk> or " true <unk> " . The names <unk> and <unk> were used <unk> to refer to the order in which it belonged . The <unk> <unk> <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 <unk> , <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 <unk> divided <unk> among three taxa : <unk> , <unk> , and <unk> . He placed <unk> in <unk> , a group which he characterized as having simple , <unk> @-@ shaped <unk> <unk> . <unk> included forms with the <unk> divided into <unk> and <unk> . All members of <unk> had <unk> <unk> composed only of the <unk> . Cope objected to von <unk> 's classification , considering the vertebrae of <unk> and <unk> indistinguishable because each had a simple <unk> 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 <unk> <unk> .
<unk> became a commonly used name at the turn of the century . <unk> 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 <unk> graded into <unk> forms in the <unk> , and finally into <unk> in the <unk> . More importantly , Watson began using the term <unk> to refer to these groups . The name <unk> was rarely used in the decades that followed . Swedish paleontologist <unk> <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 <unk> brought the name <unk> back into use in the later 20th century . <unk> @-@ <unk> used the name <unk> in a strict sense ( <unk> <unk> ) to refer to <unk> and <unk> , excluding <unk> . <unk> agreed with this classification , but used the name <unk> to avoid confusion with <unk> in its wider sense ( <unk> <unk> ) . Unlike modern <unk> classification , however , <unk> included the primitive <unk> in the group .
= = <unk> history = =
= = = <unk> and Early <unk> = = =
<unk> first appeared in the Early <unk> around 330 million years ago ( <unk> ) . During the <unk> , <unk> included basal medium @-@ sized forms such as <unk> or large <unk> forms such as <unk> . Other , more derived <unk> , such as the <unk> , were smaller and more terrestrial . They resembled <unk> , and some taxa , such as the genus <unk> , even retained external gills like the modern @-@ day <unk> . During the latest <unk> and Early <unk> around 300 <unk> , 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 <unk> predators . The <unk> , a group of small aquatic <unk> , evolved from terrestrial ancestors in the Late <unk> .
= = = Late <unk> = = =
During the Late <unk> , increasing <unk> and the <unk> of reptiles contributed into a decline in terrestrial <unk> , but <unk> and fully aquatic <unk> continued to <unk> , including the large <unk> of Eastern Europe . Other <unk> , such as <unk> , developed long <unk> and a close similarity to <unk> , although they lacked the armor characteristic of the latter group . These <unk> included the largest known amphibian , the 9 @-@ m @-@ long <unk> of Brazil .
= = = <unk> = = =
As <unk> continued to <unk> and <unk> in the Late <unk> ( 260 @.@ 4 - 251 @.@ 0 <unk> ) , 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 <unk> period , these animals dominated the freshwater ecosystems , evolving in a range of both small and large forms . During the Early <unk> ( 251 @.@ 0 - 245 @.@ 0 <unk> ) one group of successful long @-@ <unk> fish @-@ <unk> , 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 <unk> 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 <unk> . 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 <unk> stage of the Late <unk> ( 228 @.@ 0 - 216 @.@ 5 <unk> ) , <unk> were joined by the superficially very similar <unk> . <unk> are distinguished from <unk> by the positioning of their eye <unk> 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 <unk> ecosystems . Some <unk> , such as <unk> and <unk> , even inhabited Antarctica , which was covered in temperate forests at the time .
<unk> <unk> were often the dominant <unk> animals in their environments . Large <unk> 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 <unk> 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 <unk> @-@ Jurassic extinction event around 199 @.@ 6 <unk> led to the extinction of most <unk> <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 <unk> . The most recent known <unk> 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 <unk> 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 <unk> aquatic forms with simpler vertebrae were placed in the suborder <unk> . With the recent growth of <unk> , this classification is no longer viable . The basic <unk> condition is found in many primitive <unk> , 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 <unk> 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 <unk>
Order <unk>
<unk> <unk>
Family <unk> ( <unk> )
Family <unk>
Family <unk>
<unk> <unk>
<unk> <unk>
Family <unk>
Family <unk>
Family <unk>
Family <unk>
<unk> <unk>
Family <unk>
Family <unk>
Family <unk>
<unk> <unk>
<unk> <unk>
<unk> <unk>
Family <unk>
Family <unk>
Family <unk> ( placement is uncertain )
Family <unk>
<unk> <unk>
Family <unk>
Family <unk>
Family <unk>
Family <unk>
<unk> <unk>
<unk> <unk> ( <unk> )
Family <unk>
Family <unk>
Family <unk>
<unk> <unk>
<unk> <unk>
<unk> <unk>
<unk> <unk>
<unk> <unk>
<unk> <unk>
= = = Phylogeny = = =
In one of the earliest phylogenetic analyses of the group , <unk> ( 1983 ) recognized five characteristics that made <unk> a clade : a bone at the back of the skull , the <unk> , is connected to another bone on the underside of the skull , the <unk> ; large openings called <unk> <unk> are present between the <unk> ; the <unk> ( a bone involved in hearing ) is connected to the <unk> and projects upward ; the <unk> , a bone in the pectoral <unk> , is thin ; and part of the <unk> called the <unk> <unk> 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 ( <unk> processes ) on the ribs , and a pelvic <unk> with each side having a single <unk> blade . These shared characteristics are called <unk> .
<unk> are placed as basal <unk> 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 <unk> are descendants of the most recent common ancestor of all living <unk> and stem <unk> 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 <unk> 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 <unk> and <unk> ( 1999 ) placing <unk> outside crown <unk> :
Most phylogenetic analyses of <unk> interrelationships focus on individual families . One of the first broad @-@ scale studies of <unk> phylogeny was conducted by paleontologist Andrew <unk> in 1990 . A 2007 study made a " <unk> " of all <unk> families , combining the family @-@ level trees of previous studies . The following cladogram is modified from <unk> et al . ( 2007 ) :
1 <unk> , 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 <unk> features , including a single <unk> <unk> and a bone called the <unk> that is absent in other <unk> . <unk> include the Late <unk> genus <unk> and the family <unk> . <unk> has also been included in <unk> , and is the oldest known <unk> family . <unk> <unk> is the oldest species , having been present over 330 million years ago during the <unk> stage of the Early <unk> . 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 <unk> , are also primitive yet come from the Late <unk> . 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 <unk> 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 <unk> <unk> , as well as some <unk> groups . Within <unk> are the superfamily <unk> and the most derived <unk> , the <unk> .
<unk> and Warren also named <unk> , a clade of small aquatic <unk> from the <unk> , <unk> , and <unk> . 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> , <unk> and Warren erected two major clades : <unk> and <unk> . <unk> include large <unk> <unk> like <unk> 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 <unk> <unk> and Andrew <unk> 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 <unk> <unk> in 2013 . It supported many of the clades that were found by <unk> 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 , <unk> 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 <unk> 's analysis :
= = = Relationship to modern amphibians = = =
Modern amphibians ( frogs , <unk> , and <unk> ) are classified in <unk> . <unk> appear to have arisen in the <unk> . Molecular clock estimates place the first <unk> in the Late <unk> , but the first member of <unk> ( frogs and <unk> , but not <unk> ) is estimated to have appeared in the Middle <unk> 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 <unk> descended from <unk> and frogs and <unk> evolved from <unk> .
Recently , the theory that <unk> were the ancestors of all <unk> has gained wide support . The skull morphology of some small <unk> has been compared to those of modern frogs and <unk> , but the presence of <unk> , <unk> teeth in small , <unk> or immature <unk> has been cited as the most convincing argument in favor of the <unk> origin of <unk> . <unk> in <unk> and many <unk> <unk> , <unk> teeth have <unk> tips and bases . During the development of most <unk> , teeth begin to <unk> at their tips . <unk> normally proceeds downward to the base of the tooth , but <unk> from the tip stops abruptly in <unk> 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 <unk> . In 2008 , an <unk> called <unk> <unk> was named from Texas and was nicknamed the " <unk> " for its frog @-@ like head and <unk> @-@ like body . It was thought to be the most closely related <unk> to <unk> and was placed as the sister taxon of the group in a phylogenetic analysis . Another species of <unk> called <unk> <unk> is now thought to be even more closely related to <unk> . Unlike <unk> , <unk> was known since 1969 , and the presence of <unk> 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 <unk> means " connecting " in reference to its inferred transitional position between <unk> and <unk> . The structure of its <unk> , 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 <unk> than is <unk> . Below is a cladogram modified from <unk> and <unk> ( 2010 ) showing the relationships of <unk> , <unk> , and <unk> :