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'''Tetrapoda''' ([[Bahasa Yunani|Yunani]] τετραποδη ''tetrapoda'', [[Bahasa Latin|Latin]] ''[[quadrupedal]]'', "berkaki-empat") adalah [[hewan]] [[vertebrata]] yang berkaki empat, kaki atau seperti kaki. [[Amfibia]], [[reptil]], [[dinosaurus]], [[burung|unggas]], dan [[mamalia]] merupakan bagian dari tetrapoda, dan bahkan [[ular]] yang tidak memiliki kaki juga merupakan tetrapoda menurut keturunan. Tetrapoda awal berasal dari [[Sarcopterygii]] atau [[ikan]] bersirip lobus.
==Referensi==
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== Evolusi ==
[[Berkas:Fishapods.png|thumb|left|400px|In [[Devonian|Late Devonian]] [[vertebrate]] speciation, descendants of [[Pelagic zone|pelagic]] [[Sarcopterygii|lobe-finned fish]] — like ''[[Eusthenopteron]]'' — exhibited a sequence of adaptations:<Br>
•''[[Panderichthys]]'', suited to muddy shallows;<Br>
•''[[Tiktaalik]]'' with limb-like fins that could take it onto land;<Br>
•Early tetrapods in weed-filled swamps, such as;<Br>
•''[[Acanthostega]]'', which had feet with eight digits,<Br>
•''[[Ichthyostega]]'' with limbs.<Br>
Descendants also included pelagic lobe-finned fish such as [[coelacanth]] species.]]
=== Devonian tetrapods ===
Research by [[Jennifer A. Clack]] and her colleagues showed that the earliest tetrapods, such as ''[[Acanthostega]]'', were wholly aquatic and quite unsuited to life on land. This overturned the earlier view that fish had first invaded the land — either in search of prey (like modern [[mudskipper]]s) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.
====Life in the swamps====
The first tetrapods are now thought to have [[evolution|evolved]] in shallow and [[swamp]]y [[Fresh water|freshwater]] [[habitat (ecology)|habitats]], towards the end of the Devonian, a little more than 365 million years ago. By the late Devonian, land [[plant]]s had stabilized freshwater habitats, allowing the first [[wetland]] [[ecosystem]]s to develop, with increasingly complex [[Food chain|food web]]s that afforded new opportunities. [http://news.softpedia.com/news/How-did-fish-grow-legs-15424.shtml]
Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.
====Execretion in tetrapods====
The common ancestor of all present [[Gnathostomata|gnathostomes]] lived in freshwater, and later migrated back to the sea. To deal with the much higher salinity in sea water, they evolved the ability to turn the [[nitrogen]] waste product [[ammonia]] into harmless [[urea]], storing it in the body to make the blood as salty as the sea water without poisoning the organism. This is the system currently found in [[cartilaginous fishes]] and the first [[bony fishes]]. [[Ray-finned fishes]] later returned to freshwater and lost this ability, while the [[fleshy-finned fishes]] retained it. Since the blood of ray-finned fishes contain more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they did not recover their old trick of turning ammonia to urea, and they had to evolve salt excreting glands instead. [[Lungfish]]es do the same when they are living in water, making ammonia and no urea, but when the water dries up and they are forced to burrow down in the mud, they switch to urea production. Like cartilaginous fishes, the [[coelacanth]] can store urea in its blood, as can the only known amphibians that can live for long periods of time in salt water (the [[toad]] ''[[Cane Toad|Bufo marinus]]'' and the [[frog]] ''[[Fejervarya raja|Rana cancrivora]]''). These are traits they have inherited from their ancestors.
If early tetrapods lived in freshwater, and if they lost the ability to produce urea and used ammonia only, they would have to evolve it from scratch again later. Not a single species of all the ray-finned fishes living today has been able to do that, so it is not likely the tetrapods would have done so either. [[Terrestrial animal]]s that can only produce ammonia would have to drink constantly, making a life on land impossible (a few exceptions exist, as some terrestrial [[woodlice]] can excrete their nitrogenous waste as ammonia gas). This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater (unless they first migrated into freshwater habitats and then migrated onto land so shortly after that they still retained the ability to make urea), although some species never left, or returned to, the water could of course have adapted to freshwater lakes and rivers.
====Lobe-finned fishes====
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid [[Sarcopterygii|sarcopterygian]]"), with a two-lobed [[brain]] in a flattened [[skull]]. As a shallow water dweller, it sported a dorsoventrally flatttened body, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and [[bone]]s. The "[[living fossil]]" coelacanth is a related marine lobe-finned fish without these shallow-water adaptations).
Even more closely related was ''[[Gorgonasus]]'' and ''[[Panderichthys]]'', which even had a [[choana]].
These fishes used their fins as [[paddle]]s in shallow-water habitats choked with plants and [[detritus]].<ref>Monash University. "West Australian Fossil Find Rewrites Land Mammal Evolution." ScienceDaily 19 October 2006. 11 March 2009 <http://www.sciencedaily.com /releases/2006/10/061019093718.htm></ref>
Their fins could also have been used to attach themselves to plants or similar while they were laying in ambush for prey. The universal tetrapod characteristics of front [[Limb (anatomy)|limb]]s that bend backward at the [[Elbow-joint|elbow]] and hind limbs that bend forward at the [[knee]] can plausibly be traced to early tetrapods living in shallow water.
====Lungs====
It is now clear that the common ancestor of the bony fishes had a primitive air-breathing [[lung]] (later evolved into a [[Gas bladder|swim bladder]] in most ray-finned fishes). This suggests that it evolved in warm shallow waters, the kind of habitat the lobe finned fishes were living and made use of their simple lung when the oxygen level in the water became too low.
The lungfishes are now considered as being the closest living relatives of the tetrapods, even closer than the coelacanth.
Fleshy lobe fins supported on bones rather than ray-stiffened fins seems to have been an original trait of the bony fishes ([[Osteichthyes]]). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned ([[Actinopterygii]]) fishes evolved their fins in the opposite direction. The most primitive group of the ray-fins, the [[bichir]]s, still have fleshy frontal fins.
====Fossil early tetrapods====
Nine [[Genus|genera]] of Devonian tetrapods have been described, several known mainly or entirely from lower [[jaw]] material. All of them were from the European-North American [[supercontinent]], which comprised [[Europe]], [[North America]] and [[Greenland]]. The only exception is a single [[Gondwana]]n genus, ''[[Metaxygnathus]]'', which has been found in [[Australia]].
The first Devonian tetrapod identified from [[Asia]] was recognized from a [[fossil]] jawbone reported in 2002. The [[China|Chinese]] tetrapod ''[[Sinostega|Sinostega pani]]'' was discovered among fossilized tropical plants and lobe-finned fish in the red [[sandstone]] sediments of the [[Ningxia|Ningxia Hui]] Autonomous Region of northwest China. This finding substantially extended the geographical range of these animals and has raised new questions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time.
[[Berkas:Eusthenopteron BW.jpg|thumb|right|230px|''[[Eusthenopteron]]'']]
[[Berkas:Panderichthys BW.jpg|thumb|right|230px|''[[Panderichthys]]'']]
[[Berkas:Tiktaalik BW.jpg|thumb|right|230px|''[[Tiktaalik]]'']]
[[Berkas:Acanthostega BW.jpg|thumb|right|230px|''[[Acanthostega]]'']]
[[Berkas:Ichthyostega BW.jpg|thumb|right|230px|''[[Ichthyostega]]'']]
[[Berkas:Hynerpeton BW.jpg|thumb|right|230px|''[[Hynerpeton]]'']]
[[Berkas:Tulerpeton12DB.jpg|thumb|right|230px|''[[Tulerpeton]]'']]
[[Berkas:Crassigyrinus BW.jpg|thumb|right|230px|''[[Crassigyrinus]]'']]
[[Berkas:Diadectes BW.jpg|thumb|right|230px|''[[Diadectes]]'']]
These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early [[Carboniferous]] deposits, some 20 million years later. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the [[mud]].
Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their [[Metamorphosis (biology)|metamorphosis]] had what it took to make use of what land had to offer. Already adapted to breathe air and move around in shallow waters near land as a protection (just as modern fish (and amphibians) often spent the first part of their life in the comparative safety of shallow waters like [[mangrove]] forests), two very different niches partially overlapped each other, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous while the other was much safer and much less crowded, offering less competition over resources. The terrestrial niche was also a much more challenging place for primary aquatic animals, but because of the way evolution and the selection pressure works, those juveniles who could take advantage of this would be rewarded. Once they gained a small foothold on land, thanks to their preadaptations and being at the right place at the right time, favourable variations in their descendants would gradually result in continuing evolution and diversification.
At this time the abundance of invertebrates crawling around on land and near water, in moist soil and wet litter, offered a food supply. Some were even big enough to eat small tetrapods, but the land was free from dangers common in the water.
It is plausible that at first adults would be too heavy and slow and have greater needs for large prey. Small juveniles would be much lighter, faster and could subsist on relatively small invertebrates. Modern [[mudskipper]]s are said to be able to snap insects in flight while on land, and the early juvenile tetrapods might also have shown formidable abilities.{{Citation needed|date=November 2008}}
====From water to land====
Initially making only tentative forays onto land, tetrapods adapted to terrestrial environments over time and spent longer periods away from the water, while also spending a longer part of their juvenile stage on land before returning to the water for the rest of their life. It is also possible that the adults started to spend some time on land (as the skeletal modifications in early tetrapods such as ''[[Ichthyostega]]'' suggests) but only to bask in the sun close to the water's edge, not to hunt or move around. The first true tetrapods that were adapted to terrestrial locomotion were small. Only later did they increase in size.
The fully grown kept most of the anatomical adaptations from their juvenile stage, giving them modified limbs and other traits associated with a terrestrial lifestyle. To be successful adults they first had to be successful juveniles. The adults of some of the smaller species were in that case probably able to move on land too when sufficiently evolved.
If some sort of [[neoteny]] or dwarfism occurred, making the animals sexually mature and fully grown while still living on land, they would only need to visit water to drink and reproduce.
===Carboniferous tetrapods===
Until the 1990s, there was a 30 million year gap in the fossil record between the late Devonian tetrapods and the reappearance of tetrapod fossils in recognizable mid-[[Carboniferous]] [[amphibian]] lineages. It was referred to as "[[Romer's gap|Romer's Gap]]", after the [[Paleontology|palaeontologist]] who recognized it.
During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. [[Ear]]s, [[skull]]s and [[vertebra]]l columns all underwent changes too. The number of digits on [[hand]]s and feet became standardized at five, as lineages with more digits died out. The very few tetrapod fossils found in the "gap" are all the more precious.
The transition from an aquatic lobe-finned fish to an air-breathing amphibian was a momentous occasion in the evolutionary history of the [[vertebrate]]s. For an animal to live in a [[gravity]]-neutral, aqueous environment and then invade one that is entirely different required major changes to the overall body plan, both in form and in function. ''[[Eryops]]'' is an example of an animal that made such adaptations. It retained and refined most of the traits found in its fish ancestors. Sturdy [[Limb (anatomy)|limb]]s supported and transported its body while out of water. A thicker, stronger [[backbone]] prevented its body from sagging under its own weight. Also, by utilizing vestigial fish jaw bones, a rudimentary ear was developed, allowing ''Eryops'' to hear airborne [[sound]].
By the [[Visean]] age of mid-Carboniferous times the early tetrapods had radiated into at least three main branches. Recognizable basal-group tetrapods are representative of the [[Temnospondyli|temnospondyls]] (e.g. ''[[Eryops]]'') [[Lepospondyli|lepospondyls]] (e.g. ''[[Diplocaulus]]'') and [[anthracosauria|anthracosaurs]], which were the relatives and ancestors of the [[Amniote|Amniota]]. Depending on whichever authorities one follows, [[lissamphibia|modern amphibians]] (frogs, [[salamander]]s and [[caecilian]]s) are derived from either temnospondyls or lepospondyls (or possibly both, although this is now a minority position). The first amniotes are known from the early part of the [[Pennsylvanian|Late Carboniferous]], and during the [[Triassic]] counted among their number the earliest [[mammal]]s, [[turtle]]s, and [[crocodile]]s ([[lizard]]s and [[bird]]s appeared in the [[Jurassic]], and [[snake]]s in the [[Cretaceous]]). As living members of the tetrapod clan — that is of the tetrapod "crown-group" — these varied tetrapods represent the [[Phylogenetics|phylogenetic]] end-points of these two divergent lineages. A fourth [[Carboniferous]] group, the [[Loxommatidae|baphetid]]s, which are thought to be related to temnospondyls, left no modern survivors.
===Permian tetrapods===
In the [[Permian]] period, as the separate tetrapod lineages each developed in their own way, in the term "tetrapoda" becomes less useful. In addition to temnospondyl and anthracosaur clades among the early "amphibia" (labyrinthodonts), there were two important divergent clades of amniotes, the [[Sauropsida]] and the [[Synapsida]], of which the latter were the most important and successful Permian animals. Each of these lineages, however, remains grouped with the tetrapoda, just as ''Homo sapiens'' could be considered a very highly-specialized kind of ''lobe-finned fish''.
===Living tetrapods===
The beginning of the [[Mesozoic]] saw a major turnover in fauna following the [[Permian–Triassic extinction event]]. Many of the once large and diverse groups died out or were greatly reduced. A small group of reptiles, the [[diapsid]]s, began to deversify during the Triassic, notably the [[dinosaur]]s. By the late Mesozoic, many large tetrapod groups that first appeared during the Paleozoic such as temnospondyls and anthracosaurs had gone extinct. All current major groups of sauropsids evolved during the Mesozoic, with [[birds]] first appearing in the [[Jurassic]] as a derived clade of [[theropod]] dinosaurs. Many groups of [[synapsid]]s such as [[anomodontia]]ns and [[therocephalia]]ns that once comprised the dominant terrestrial fauna of the Permian also became extinct during this time, but during the Triassic, one group ([[Cynodontia]]) gave rise to the descendant taxon [[Mammalia]], which survived through the Mesozoic to later diversify into the dominant terrestrial fauna during the Cenozoic.
Following the great faunal turnover at the end of the Mesozoic, only three categories of living [[crown group]] tetrapods were left, all of which also include many [[extinct]] groups:
* [[Lissamphibia]] : Modern [[frog]]s and [[toad]]s, [[salamander|newts and salamanders]], and [[caecilian]]s
* [[Sauropsida]] : [[Turtle]]s, [[lepidosauria]]ns ([[tuatara]]s, [[lizard]]s, [[amphisbaenian]]s and [[snake]]s), [[bird]]s, and [[crocodilia]]ns
* [[Synapsida]] : [[Mammal]]s
==Classification==
Tetrapods were originally classified by means of [[Linnean taxonomy]], but currently their taxonomy is more frequently being evaluated [[Cladistics|cladistically]].
===Linnaean classification===
Traditional classification has the tetrapods classed into four classes based on gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits.
<ref>Romer, A.S. (1949): ''The Vertebrate Body.'' W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)</ref> Note that [[snake]]s and other legless reptiles are considered tetrapods because they are descended from ancestors who had a full complement of limbs. Similar considerations apply to [[caecilians]] and [[aquatic animal|aquatic]] mammals:
* Class [[Amphibia]] (Amphibians)
* Class [[Reptilia]] (Reptiles)
* Class [[Aves]] (birds)
* Class [[Mammalia]] (mammals)
This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, has come under critique from [[cladistics]]. Reptiles are [[paraphyletic]], as they have given rise to another group ([[bird]]s) that is traditionally not considered to be a type of reptile. [[Basal (phylogenetics)|basal]] non-mammalian [[synapsid]]s (classically called "mammal-like reptiles") are excluded from the traditional classifications as they were not true mammals nor were they reptiles. Thus some authors have argued for a new classification based purely on [[phylogeny]], disregarding the anatomy and physiology (see below).
===Phylogenetic classification===
All early tetrapods and tetrapodomorphs that were not amphibians in the strict phylogenetic sense, nor amniotes, were once placed together in the [[paraphyletic]] group [[Labyrinthodontia]]. Labyrinthodonts were distinguished mainly by their complex dentine infolding [[tooth]] structure, a feature shared with crossopterygian fish. The labyrinthodonts were divided into the [[Ichthyostegalia]] (another paraphyletic assemblage of primitive tetrapods and kin, such as ''[[Ichthyostega]]''), the [[Temnospondyli]] (possibly members of [[Amphibia]]), and the [[Anthracosauria]] (close relatives of [[amniote]]s). The main difference between the three groups was based on their respective vertebral structures. The Anthracosauria had small pleurocentra, which grew and fused, becoming the true [[centrum]] in later vertebrates. In contrast, the Temnospondyli had a conservative vertebral column in which the pleurocentra remained small in primitive forms, vanishing entirely in the more advanced ones. The intercentra are large and form a complete ring. Temnospondyli is thought to have been the sister group of [[Anthracosauria]], which would eventually give rise to amniotes.
===Tetrapod groups===
[[Berkas:Pederpes22small.jpg|thumb|300px|''[[Pederpes]] finneyae'']]
[[Berkas:Lyddekerina1db.jpg|thumb|300px|''[[Lyddekerina]] huxleyi'']]
[[Berkas:Benthosuchus2DB2small.jpg|thumb|300px|''[[Benthosuchus]] sushkini'']]
A partial taxonomy of the tetrapods:
* Phylum [[Chordata]]
*** Class '''[[Sarcopterygii]]'''
**** '''Subclass [[Tetrapodomorpha]]'''
***** ''[[Eusthenopteron]]''
***** ''[[Panderichthys]]''
***** ''[[Tiktaalik]]''
***** ''[[Ventastega]]''
** '''Superclass Tetrapoda'''
***** Family [[Elginerpeton]]tidae
***** Family [[Acanthostega|Acanthostegidae]]
***** Family [[Ichthyostega|Ichthyostegidae]]
***** ''[[Hynerpeton]]''
***** Family [[Tulerpeton]]
***** Family [[Crassigyrinus|Crassigyrinidae]]
***** Family [[Loxommatidae]]
***** Family [[Colosteidae]]
***** Family [[Whatcheeriidae]]
***** Family [[Diadectes]]
*** [[Batrachomorpha]] (directly above, below, or redundant to Amphibia)
*** '''Class [[Amphibian|Amphibia]]''' — Amphibians
**** Subclass [[Lepospondyli]]
**** Subclass [[Temnospondyli]]
**** Subclass [[Lissamphibia]] — frogs, salamanders
*** '''Superorder [[Reptiliomorpha]]''' contains among others:
**** Series [[Amniote|Amniota]], which contains among others:
***** Class [[Reptile|Reptilia]] — Reptiles
***** Class [[Bird|Aves]] — Birds
***** Class [[Synapsida]] — Mammal-like reptiles
***** Class [[Mammalia]] — Mammals
===Phylogeny===
[[Cladogram]] modified after Ruta, Jeffery, & Coates (2003).<ref name=RJC03>{{citejournal |last=Ruta |first=M. |coauthors=Jeffery, J. E.; and Coates, M. I. |year=2003 |title=A supertree of early tetrapods |journal=Proceedings of the Royal Society B |volume=270 |pages=2507-2516}}</ref>
{{clade|style=font-size:90%;line-height:100%
|label1='''Tetrapoda'''
|1={{clade
|1=''[[Acanthostega]]''
|2={{clade
|1=''[[Ichthyostega]]''
|2={{clade
|1=''[[Hynerpeton]]''
|2={{clade
|1=''[[Tulerpeton]]''
|2={{clade
|1=[[Whatcheeriidae]]
|2={{clade
|1=''[[Crassigyrinus]]''
|2={{clade
|1=''[[Caerorhachis]]''
|2={{clade
|1={{clade
|1={{clade
|1=[[Gephyrostegidae]]
|2={{clade
|1=[[Eoherpetontidae]]
|2=[[Embolomeri]]}} }}
|2={{clade
|1=[[Seymouriamorpha]]
|label2= Crown group Tetrapoda
|2={{clade
|1={{clade
|1=Crown [[Amniota]]
|2={{clade
|1=[[Diadectomorpha]]
|2=Crown Amniota}} }}
|label2= [[Lepospondyli]]
|2={{clade
|1={{clade
|1=[[Nectridea]]
|2={{clade
|1=[[Adelospondyli]]
|2=[[Aistopoda]]}} }}
|label2= [[Microsauria]]
|2={{clade
|1=[[Tuditanomorpha]]
|2={{clade
|1=[[Microbrachomorpha]]
|2={{clade
|1=[[Lysorophidae]]
|2={{clade
|1=Microbrachomorphs
|2={{clade
|1=Crown [[Lissamphibia]]*
|2=Tuditanomorphs}} }} }} }} }} }} }} }} }}
|2={{clade
|1=[[Loxommatidae|Baphetidae]]
|label2= [[Temnospondyli]]
|2={{clade
|1=[[Colosteidae]]
|2={{clade
|1=[[Edopoidea]]
|2={{clade
|1=[[Trimerorhachoidea]]
|2={{clade
|1={{clade
|1=[[Eryopoidea]]
|2=[[Dissorophoidea]]}}
|2={{clade
|1=[[Archegosauroidea]]
|2={{clade
|1=[[Rhinesuchidae]]
|2={{clade
|1={{clade
|1=[[Rhytidosteidae]]
|2={{clade
|1=[[Chigutisauridae]]
|2={{clade
|1=[[Plagiosauridae]]
|2=[[Brachyopidae]]}} }} }}
|2={{clade
|1=[[Mastodonsauroidea]]
|2={{clade
|1=[[Metoposauroidea]]
|2=[[Trematosauroidea]]
}} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} *<u>Note</u>: The origin of the subclass Lissamphibia, to which all extant amphibians belong, is disputed. This cladogram is the result of one analysis conducted by Ruta, Jeffery, & Coates (2003) that placed Lissamphibia within Lepospondyli, with the latter clade being within the [[crown group]] Tetrapoda. A second analysis by the authors placed Lissamphibia within Temnospondyli, thus placing Lepospondyli outside crown group Tetrapoda and Temnospondyli within. Another prevailing theory not represented by either of the cladograms is a [[diphyletic]] grouping of Lissamphibia with both Lepospondyli and Temnospondyli, with [[gymnophiona]]ns belonging to the former clade and [[anura]]ns and [[caudata]]ns belonging to the latter. }}
==Anatomical features of early tetrapods==
The tetrapod's ancestral fish must have possessed similar traits to those inherited by the early tetrapods, including internal nostrils (to separate the breathing and feeding passages) and a large fleshy [[fin]] built on bones that could give rise to the tetrapod limb. The rhipidistian [[crossopterygian]]s fulfill every requirement for this ancestry. Their [[palate|palatal]] and jaw structures were identical to those of early tetrapods, and their [[dentition]] was identical too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. The crossopterygian paired fins were smaller than tetrapod limbs, but the skeletal structure was very similar in that the crossopterygian had a single proximal bone (analogous to the [[humerus]] or [[femur]]), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the [[carpus]] / [[tarsus (skeleton)|tarsus]] and [[hand|phalanges]] of a hand.
The major difference between crossopterygians and early tetrapods was in relative development of front and back [[skull]] portions; the snout is much less developed than in most early tetrapods and the post-orbital skull is exceptionally longer than an amphibian's.
A great many kinds of early tetrapods lived during the [[Carboniferous]] period. Therefore, their ancestor would have lived earlier, during the [[Devonian]] period. Devonian [[Ichthyostegids]] were the earliest of true tetrapods, with a skeleton that is directly comparable to that of rhipidistian ancestors. Early [[temnospondyl]]s (Late Devonian to Early [[Mississippian]]) still had some ichthyostegid features such as similar skull bone patterns, labyrinthine tooth structure, the fish skull-hinge, pieces of [[gill]] structure between the cheek and shoulder, and the [[vertebral column]]. They had, however, lost several other fish features such as the fin rays in the [[tail]].
In order to propagate in the terrestrial [[natural environment|environment]], certain challenges had to be overcome. The animal's body needed additional support, because [[buoyancy]] was no longer a factor. A new method of [[Respiration (physiology)|respiration]] was required in order to extract [[Earth's atmosphere|atmospheric]] [[oxygen]], instead of oxygen dissolved in water. A means of [[animal locomotion|locomotion]] would need to be developed to traverse distances between waterholes. Water retention was now important since it was no longer the living [[matrix (biology)|matrix]], and it could be lost easily to the environment. Finally, new sensory input systems were required if the animal was to have any ability to function reasonably while on land.
===Skull===
The most notable characteristics that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the [[orbit (anatomy)|orbital vacuities]] were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull. The [[lacrimal bone]] was not in contact with the frontal anymore, having been separated from it by the prefrontal bone. Also of importance is that the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck.
A diagnostic character of temnospondyls is that the [[tabular bones]] (which formed the posterior corners of the skull-table) were separated from the respective left and right [[parietal bone|parietals]] by a [[sutural junction]] between the [[postparietals]] and [[supratemporals]]. Also at the rear of the skull, all bones dorsal to the [[cleithrum]] were lost.
The lower jaw of, for example, ''[[Eryops]]'' resembled its crossopterygian ancestors in that on the outer surface lay a long [[dentary]] that bore teeth. There were also bones below the dentary on the jaw: two [[splenial]]s, the [[angulary]] and the [[surangular]]. On the inside were usually three [[coronoid]]s that bore teeth and lay close to the dentary. On the upper jaw was a row of marginal labyrinthine teeth, located on the [[maxilla]] and [[premaxilla]]. In ''Eryops'', as in all early amphibians, the teeth were replaced in waves that traveled from the front of the jaw to the back in such a way that every other tooth was mature, and the ones in between were young.
===Dentition===
The "labyrinthodonts" had a peculiar tooth structure from which their name was derived and, although not exclusive to the group, the labyrinthine dentition is a useful indicator as to proper classification. The important feature of the tooth is that the [[tooth enamel|enamel]] and [[dentine]] were folded in such a way as to form a complicated corrugated pattern when viewed in cross section. This infolding resulted in strengthening of the tooth and increased wear resistance. Such teeth survived for 100 Ma, first among crossopterygian fish, then stem reptiles. Modern amphibians no longer have this type of dentition but rather [[pleurodont]] teeth, in fewer numbers of the whole group.
===Sensory organs===
There is a [[density]] difference between air and water that causes [[odor|smells]] (certain chemical compounds detectable by [[chemoreceptor]]s) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory [[apparatus]] was designed for aquatic detection.
Fish have a [[lateral line]] system that detects [[pressure]] fluctuations in the water. Such pressure is non-detectable in air, but grooves for the lateral line sense organs were found on the skull of labyrinthodonts, suggesting a partially aquatic [[habitat (ecology)|habitat]]. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates. The [[olfaction|olfactory]] [[epithelium]] would also have to be modified in order to detect airborne [[odor]]s.
In addition to the lateral line organ system, the eye had to change as well. This change came about because the [[refractive index]] of light differs between air and water, so the [[focal length]] of the [[lens (anatomy)|lens]] was altered in order to properly function. The eye was now exposed to a relatively dry environment rather than being bathed by water, so [[eyelid]]s developed and [[tear duct]]s evolved to produce a liquid, moistening the eyeball.
===Hearing===
The balancing function of the middle ear was retained from the fish ancestry, but delicate air [[oscillation|vibrations]] could not set up [[pulsation]]s through the skull in order for it to function a proper auditory [[Organ (anatomy)|organ]]. Typical of most labyrinthodonts, the [[spiracular gill pouch]] was retained as the [[otic notch]], closed in by the [[Tympanal organ|tympanum]], a thin, tight [[biological membrane|membrane]].
The [[hyomandibula]] of fish migrated upwards from its jaw supporting position, and was reduced in size to form the [[stapes]]. Situated between the tympanum and braincase in an air-filled cavity, the stapes was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes became an important element in an [[Impedance mismatch|impedance]] matching system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibian [[Lineage (evolution)|lineages]].
In order for the impedance matching ear to work, certain conditions had to be met. The stapes must have been perpendicular to the tympanum, small and light enough to reduce its [[inertia]] and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz [[frequency|frequencies]], the footplate of the stapes is 1/20th the area of the tympanum. However, in early amphibians the stapes was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears that only high intensity, low frequency sounds could be detected, with the stapes more probably being used to support the braincase against the cheek.
===Girdles===
The [[pectoral girdle]] of early tetrapods such as ''Eryops'' was highly developed, with a larger size for both increased [[muscle]] attachment to it and to the limbs. Most notably, the shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The crossopterygian ''cleithrum'' was retained as the [[clavicle]], and the interclavicle was well-developed, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could have grown ventrally in such a way as to form a broad chest plate, although such was not the case in ''Eryops''. The upper portion of the girdle had a flat, [[scapular blade]], with the [[glenoid cavity]] situated below performing as the [[articulation]] surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.
The [[pelvis|pelvic]] girdle also was much larger than the simple plate found in fishes, accommodating more muscles. It extended far dorsally and was joined to the backbone by one or more specialized sacral [[rib]]s. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was the [[ilium (bone)|ilium]], while the broad ventral plate was composed of the [[pubis (bone)|pubis]] in front and the [[ischium]] in behind. The three bones met at a single point in the center of the pelvic triangle called the ''acetabulum'', providing a surface of articulation for the femur.
The main strength of the ilio-sacral attachment of ''Eryops'' was by [[ligament]]s, a condition structurally, but not [[phylogenetically]], intermediate between that of the most primitive embolomerous amphibians and early reptiles. The condition that is more usually found in higher vertebrates is that [[cartilage]] and fusion of the sacral ribs to the blade of the ilium are utilized in addition to ligamentous attachments.
===Limbs===
The humerus was the largest bone of the arm, its head articulating with the glenoid cavity of the pectoral girdle, distally with the [[radius (bone)|radius]] and [[ulna]]. The radius resided on the inner side of the forearm and rested directly under the humerus, supporting much of the weight, while the ulna was located to the outside of the humerus. The ulna had a head, which muscles pulled on to extend the limb, called the [[olecranon]] that extended above the edge of the humerus.
The radius and the ulna articulated with the [[carpus]], which was a [[proximal]] row of three elements: the [[radiale]] underlying the radius, the [[ulnare]] underneath the ulna and an intermedium between the two. A large central element was beneath the last and may have articulated with the radius. There were also three smaller centralia lying to the radial side. Opposite the head of each [[toe]] lay a series of five [[distal]] [[carpal]]s. Each [[digit]] had a first segment, the [[metacarpal]], lying in the palm region.
The pelvic limb bones were essentially the same as in the pectoral limb, but with different names. The [[Analogy|analogue]] to the humerus was the femur, which was longer and slimmer. The two lower arm bones corresponded to the [[tibia]] and [[fibula]] of the hind leg, the former being the innermost and the latter the outermost bones. The tarsus is the hind version of the carpus and its bones correspond as well.
===Feeding===
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were fang-like palatal teeth that, when coupled with the gape, suggests an inertial feeding habit. This is when the amphibian would grasp the [[prey]] and, lacking any chewing mechanism, toss the head up and backwards, throwing the prey farther back into the mouth. Such feeding is seen today in the crocodile and [[alligator]].
The [[tongue]] of modern adult amphibians is quite fleshy and attached to the front of the lower jaw, so it is reasonable to speculate that it was fastened in a similar fashion in primitive forms, although it was probably not specialized like it is in a frog.
It is taken that early tetrapods were not very active, thus a [[predator]]y [[lifestyle]] was probably not the norm. It is more likely that it fed on fish either in the water or on those that became stranded at the margins of [[lake]]s and swamps. Also abundant at the time was a large supply of terrestrial [[invertebrates]], which may have provided a fairly adequate food supply.
===Respiration===
Modern amphibians [[Respiration (physiology)|breathe]] by inhaling air into [[lungs]], where oxygen is absorbed. They also breathe through the moist lining of the mouth and [[skin]]. ''Eryops'' also inhaled, but its ribs were too closely spaced to suggest that it did this by expanding the [[rib cage]]. More likely, it breathed by [[buccal pumping]] in which it opened its mouth and nostrils, depressed the [[hyoid apparatus]] to expand the oral cavity, closed its mouth and nostrils finally and elevated the floor of the mouth to force air back into the lungs — in other words, it gulped then swallowed. It probably exhaled by contraction of the elastic [[biological tissue|tissue]] in the lung walls. Other special respiratory methods probably existed.
===Circulation===
Early tetrapods most likely had a three-chambered [[heart]], as do modern amphibians and reptiles, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enters by separate atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated blood. The spiral valve is essential to keeping the mixing of the two types of blood to a minimum, enabling the animal to have higher metabolic rates, and be more active than otherwise.
===Locomotion===
In typical early tetrapod posture the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near [[right angle]]. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal's [[physical strength|strength]] was used to just lift its body off the ground for walking, which was probably slow and difficult. With this sort of posture, it could only make short broad strides. This has been confirmed by fossilized footprints found in Carboniferous [[rock (geology)|rock]]s.
Ligamentous attachments within the limbs were present in ''Eryops'', being important because they were the precursor to bony and cartilaginous variations seen in modern terrestrial animals that use their limbs for locomotion.
Of all body parts, the spine was the most affected by the move from water to land. It now had to resist the bending caused by body weight and had to provide mobility where needed. Previously, it could bend along its entire length. Likewise, the paired appendages had not been formerly connected to the spine, but the slowly strengthening limbs now transmitted their support to the axis of the body.
== See also ==
* [[Geologic timescale]]
* [[Prehistoric life]]
* [[Body form]]
==References==
{{Refimprove|date=July 2009}}
{{reflist}}
==ExternalPranala linksluar==
{{External links|date=July 2009}}
*[http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.800.html#Osteolepiformes Cladistic analysis of osteolepiform Sarcopterygians]
*[http://www.origins.tv/darwin/tetrapods.htm "The evolution of tetrapods and the closing of Romer's Gap."]
*[http://news.softpedia.com/news/Did-our-ancestors-breathe-through-their-ears-16611.shtml "Did Our Ancestors Breathe through Their Ears?"]
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