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Tetrapods are vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s having four limbs
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....

. Amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s and mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s are all tetrapods; even snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 from the lobe-finned fishes
Sarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...

 in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

. They are now a dominant part of the terrestrial fauna, representing all known larger land animals. Some groups have even returned to an aquatic existence, including the largest animal known, the blue whale
Blue Whale
The blue whale is a marine mammal belonging to the suborder of baleen whales . At in length and or more in weight, it is the largest known animal to have ever existed....

.

Biodiversity


Tetrapoda includes four classes: amphibians, reptiles, mammals and birds. Overall the biodiversity of tetrapods has grown exponentially over time from a single amphibian group sometime in the Devonian to many thousands of species today. The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown so has the number of niches tetrapods occupied. The first tetrapods lived in waterside habitats and fed primarily on fish, today the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.

Evolution



Origin


The evolution of the first tetrapods marked the moment when the two basic forms of vertebrates, fishes and tetrapods, diverged. This transition, from a body plan for breathing and navigating in water and a body plan enabling the animal to move on land, involved a series of changes taking place throughout most of the 56.8 million years that make up the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period. While it is one of the most profound evolutionary changes known, it is also one of the best understood, largely thanks to a number of amazing fossil finds in the late 20th century combined with improved phylogenetic analysis.

"The Age of Fishes"



The Devonian period is traditionally known as the "Age of Fishes", marking the diversification of numerous extinct and modern major fish groups. Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period. The early types resembled their cartilaginous forefathers
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 in many aspects of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton
Axial skeleton
The axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column...

.

They did however have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms: With the excaption of a pair of spiracles, the gills did not open singly to exterior like in sharks, rather they were hidden behind a bony operculum
Operculum (fish)
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

. The gill chamber was bound posteriorly by a stout cleithrum bone
Cleithrum
The cleithrum is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key ", by analogy with "clavicle" from Latin clavicula = "little key".In modern fishes, the cleithrum is a...

, also functioning as anchoring for the pectoral fins. As part of the overall armour of rhomboid
Rhomboid
Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are oblique.A parallelogram with sides of equal length is a rhombus but not a rhomboid....

 cosmin scales, the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 had a full cover of dermal bone
Dermal bone
A dermal bone - bony structures derived from intramembranous ossification that form components of the vertebrate skeleton including the skull, jaws, gills, fins and exoskeleton. In contrast to endochondral bone, dermal bone does not form from cartilage first and then calcify...

, constituting a skull roof
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...

 over the otherwise shark-like cartilaginous inner cranium
Endocranium
For internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...

. Importantly, they also had a swim bladder/lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

, a feature lacking in all other fishes.

Lungs before land


The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. In its primitive form, the air bladder was open to the, a condition called physostome
Physostome
Physostomes are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal. This allows the gas bladder to be filled or emptied via the mouth. In contrast, fishes without any connection to their gas bladder are called Physoclisti. The physostome fish encompass the...

 and still found in many fish. The primary function is not entirely certain. One consideration is buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

. The heavy scale armour of the early bony fishes would certainly weigh the animals down. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

. Another factor is oxygen consumption. Ambient oxygen was relatively low in the early Devonian, possibly about half of modern values. The partial pressure of oxygen is necessarily much higher in air than in water, and vertebrates are active animals with high energy requirement compared to invertebrates of similar sizes. The Devonian saw increasing oxygen levels, opening up ecological niches as active, large bodied animals for groups able to exploit aerial oxygen. Particularly in tropical swampland habitats, atmospheric oxygen is much more stable, and may have prompted a reliance of lungs rather than gills for primary oxygen uptake. In the end, both buoyancy and breathing may have been important, some modern physostome fishes do indeed use their bladders for both.

In order to function in gas exchange, lungs need to be supplied by blood. In cartilaginous fishes and teleosts
Teleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

, the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 pumps blood forward through the ventral aorta which splits up in a series of paired aortic arches, each corresponding to a gill arch. The aortic arches then merge to form a dorsal aorta
Dorsal aorta
Each primitive aorta receives anteriorly a vein—the vitelline vein—from the yolk-sac, and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aorta....

 supplying the body with oxygenated blood. In lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

es, bowfin
Bowfin
The Bowfin, Amia calva, is the last surviving member of the order Amiiformes , and of the family Amiidae...

 and bichir
Bichir
The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

s, the swim bladder is supplied by paired pulmonary arteries
Pulmonary artery
The pulmonary arteries carry deoxygenated blood from the heart to the lungs. They are the only arteries that carry deoxygenated blood....

 branching off from the hindmost (6th) aortic arch. The same basic pattern is found in the lungfish Protopterus
Protopterus
The African lungfishes are the genus Protopterus and constitute the four species of lungfish found in Africa. Protopterus is the sole genus in the family Protopteridae.-Description:...

and in terrestrial salamanders, and was likely the pattern found in the tetrapods' immediate forefathers as well as the first tetrapods. In most other bony fishes the swim bladder is supplied by the dorsal aorta.

Skull morphology


The tetrapods have their root in the early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 tetrapodomorph fish
Tetrapodomorpha
Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish...

. Primitive tetrapods developed from an osteolepid tetrapodomorph lobe-finned fish (sarcopterygian-crossopterygian), with a two-lobed brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 in a flattened skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

. The coelacanth group represent marine sarcopterygians that never acquired these shallow-water adaptations. The sarcopterygians apparently took two different lines of descent and are accordingly separated into two major groups: the Dipnomorpha (including the Dipnoi or lungfishes) and the Tetrapodomorpha (which include extinct lines of lobe-finned fishes that evolved into the Tetrapoda).

From fins to feet


The oldest known tetrapodomorph is Kenichthys
Kenichthys
Kenichthys was an ancient tetrapodomorph, a lineage that included tetrapods. It was one of the most primitive tetrapodomorphs. This specific genus is important because it contains a primitive choana on its upper lip, giving it a cleft palate-like structure...

from China, dated at around 395 million years old. Two of the earliest tetrapodomorphs, dating from 380 Ma, were Gogonasus
Gogonasus
Gogonasus was a lobe-finned fish known from 3-dimensionally preserved 380 million-year-old fossils found from the Gogo Formation in Western Australia. It lived in the late Devonian period, on what was once a 1400 kilometre coral reef off the Kimberley coast surrounding the north-west of Australia...

and Panderichthys
Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...

. They had a choana
Choana
Choana is the posterior nasal aperture.The choanae are separated by the vomer.- Boundaries :It is the opening between the nasal cavity and the nasopharynx....

 and used their fins as paddle
Paddle
A paddle is a tool used for pushing against liquids, either as a form of propulsion in a boat or as an implement for mixing.-Materials and designs:...

s in shallow-water habitats choked with plants and detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...

.
Their fins could also have been used to attach themselves to plants or similar while they were lying in ambush for prey. The universal tetrapod characteristics of front limb
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....

s that bend backward at the elbow and hind limbs that bend forward at the knee
Knee
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...

 can plausibly be traced to early tetrapods living in shallow water.

It has been suggested that the evolution of the tetrapod limb from lobe-finned fishes is related to the loss of the proteins actinodin 1 and actinodin 2, which are involved in fish fin development

Denizens of the swamp


The first tetrapods are thought to have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 in coastal and brackish marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 environments, and in shallow and swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...

y freshwater
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 habitats
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

. Formerly, the timing was thought to be towards the end of the Devonian. In 2010, this belief was challenged by the discovery of the oldest known tetrapod tracks, preserved in marine tidal flat sediments of the southern coast of Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...

, now Świętokrzyskie (Holy Cross) Mountains
Swietokrzyskie Mountains
Świętokrzyskie Mountains , are a mountain range in central Poland, in the vicinity of the city of Kielce. The mountain range consists of a number of separate ranges, the highest of which is Łysogóry . The two highest peaks are Łysica at 612 meters and Łysa Góra at 593 meters...

 of Poland. They were made during the Eifelian stage of the Middle Devonian. The tracks are dated to about 395 million years ago, 18 million years earlier than the oldest known tetrapod body fossils. Some tracks show digits, indicating that the animal had the ability to walk on land. Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward. This type of motion would have been impossible in tetrapodomorphs such as Tiktaalik
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...

. The animal that produced the tracks is estimated to have been up to 2.5 metres (8.2 ft) long with footpads up to 26 centimetres (10.2 in) wide, although most tracks are only 15 centimetres (5.9 in) wide. The new finds suggest that the first tetrapods lived as scavengers on the tidal flats, feeding on washed up animals and marine creatures stranded by the tide.

Devonian tetrapods


Research by Jennifer A. Clack
Jennifer A. Clack
Jennifer Alice Clack, FRS, is an English paleontologist, an expert in the field of evolutionary biology. She studies the "fish to tetrapod" transition— the origin, evolutionary development and radiation of early tetrapods and their relatives among the lobe-finned fishes...

 and her colleagues showed that the earliest tetrapods, such as Acanthostega
Acanthostega
Acanthostega is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto...

, were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskipper
Mudskipper
Mudskippers are members of the subfamily Oxudercinae , within the family Gobiidae . They are completely amphibious fish, fish that can use their pectoral fins to walk on land...

s) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.

By the late Devonian, land plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s had stabilized freshwater habitats, allowing the first wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....

 ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s to develop, with increasingly complex food web
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

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.
Excretion in tetrapods

The common ancestor of all present gnathostomes
Gnathostomata
Gnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates...

 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
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 waste product ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 into harmless urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

, 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 (acanthodians). Ray-finned fishes (Actinopterygii) later returned to freshwater and lost this ability, while the fleshy-finned fishes (Sarcopterygii) retained it. Since the blood of ray-finned fishes contains 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
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

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
Coelacanth
Coelacanths are members of an order of fish that includes the oldest living lineage of Sarcopterygii known to date....

 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
Toad
A toad is any of a number of species of amphibians in the order Anura characterized by dry, leathery skin , short legs, and snoat-like parotoid glands...

 Bufo marinus
Cane Toad
The Cane Toad , also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad which is native to Central and South America, but has been introduced to various islands throughout Oceania and the Caribbean...

and the frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

 Rana cancrivora
Fejervarya raja
Fejervarya raja is a species of frog in the Ranidae family.It is found in Malaysia and Thailand.Its natural habitats are coastal saline lagoons, urban areas, and canals and ditches. Adults can survive in salt water with salinity as high as 2.8%, and tadpoles can survive salinities as high as...

). 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
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

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.
Lungs

It is now clear that the common ancestor of the bony fishes (Osteichthyes) had a primitive air-breathing lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

  -- later evolved into a swim bladder
Gas bladder
The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...

 in most actinopterygians (ray-finned fishes). This suggests that crossopterygians evolved in warm shallow waters, using their simple lung when the oxygen level in the water became too low.

Fleshy lobe-fins supported on bones rather than ray-stiffened fins seems to have been an ancestral trait of all bony fishes (Osteichthyes
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned fishes (Actinopterygii
Actinopterygii
The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...

) evolved their fins in a different direction. The most primitive group of actinopterygians, the bichir
Bichir
The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

s, still have fleshy frontal fins.
Fossil early tetrapods

Nine genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of Devonian tetrapods have been described, several known mainly or entirely from lower jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

 material. All but one were from the European-North American supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...

, which comprised Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

. The only exception is a single Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

n genus, Metaxygnathus
Metaxygnathus
Metaxygnathus is an extinct genus of tetrapodomorph lobe-finned fishs found in Late Devonian deposits of New South Wales, Australia .It is only known from a lower jawbone.-References:...

, which has been found in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

.

The first Devonian tetrapod identified from Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

 was recognized from a fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 jawbone reported in 2002. The Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 tetrapod Sinostega pani
Sinostega
Sinostega is an extinct genus of early tetrapod from the Late Devonian of China.The fossil was discovered in the Ningxia Hui Autonomous Region, northwest China, and consist of a fragmentary lower jawbone measuring 7 cm in length. It is the first Devonian tetrapod to be found in Asia.-References:*...

was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 sediments of the Ningxia Hui
Ningxia
Ningxia, formerly transliterated as Ningsia, is an autonomous region of the People's Republic of China. Located in Northwest China, on the Loess Plateau, the Yellow River flows through this vast area of land. The Great Wall of China runs along its northeastern boundary...

 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.









These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 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
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

.

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 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
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...

 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
Mudskipper
Mudskippers are members of the subfamily Oxudercinae , within the family Gobiidae . They are completely amphibious fish, fish that can use their pectoral fins to walk on land...

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.
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
Ichthyostega
Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...

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
Neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

 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
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

 lineages. It was referred to as "Romer's Gap
Romer's gap
Romer's Gap is an example of a gap in the fossil record used in the study of evolution. Such gaps represent a period from which excavators have found no or very few fossils. Romer's gap is named after paleontologist Dr...

", after the palaeontologist
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 who recognized it.

During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. Ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

s, skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

s and vertebral columns all underwent changes too. The number of digits on hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

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
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

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
Eryops
Eryops meaning "drawn-out face" because most of its skull was in front of its eyes is a genus of extinct, semi-aquatic amphibian found primarily in the Lower Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United...

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
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....

s supported and transported its body while out of water. A thicker, stronger backbone
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

 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
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

.

By the Visean
Viséan
The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from 345.3 ± 2.1 to 328.3 ± 1.6 Ma...

 age of mid-Carboniferous times the early tetrapods had radiated into at least three main branches. Recognizable basal-group tetrapods are representative of the temnospondyls
Temnospondyli
Temnospondyli is a diverse order of 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...

 (e.g. Eryops
Eryops
Eryops meaning "drawn-out face" because most of its skull was in front of its eyes is a genus of extinct, semi-aquatic amphibian found primarily in the Lower Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United...

) lepospondyls
Lepospondyli
Lepospondyli are a group of small but diverse Carboniferous to early Permian tetrapods. Six different groups are known, the Acherontiscidae, Adelospondyli, Aïstopoda, Lysorophia, Microsauria and Nectridea, and between them they include newt-like, eel- or snake-like, and lizard-like forms, along...

 (e.g. Diplocaulus
Diplocaulus
Diplocaulus is an extinct genus of leponspondyl amphibian from the Permian period of North America.- Description :...

) and anthracosaurs
Anthracosauria
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon.-Various definitions:...

, which were the relatives and ancestors of the Amniota
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

. Depending on whichever authorities one follows, modern amphibians
Lissamphibia
The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...

 (frogs, salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s and caecilian
Caecilian
The caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade Apoda. They are mostly...

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 Late Carboniferous
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...

, and during the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 counted among their number the earliest mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s (lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s appeared in the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, and snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

), and a fourth Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 group, the baphetid
Loxommatidae
Baphetidae is an extinct family of early tetrapods. Baphetids were large labyrinthodont predators of the Late Carboniferous period of Europe. Fragmentary remains from the Early Carboniferous of Canada have been tentatively assigned to the group...

s, which are thought to be related to temnospondyls, left no modern survivors.

Amphibians and reptiles were affected by the Carboniferous Rainforest Collapse
Carboniferous Rainforest Collapse
The Carboniferous Rainforest Collapse was an extinction event that occurred around 305 million years ago in the Carboniferous period). Vast coal forests covered the equatorial region of Euramerica...

 (CRC), an extinction event that occurred ~300 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Several large groups, labyrinthodont amphibians were particularly devastated, while the first reptiles fared better, being ecologically adapted to the drier conditions that followed. Amphibians must return to water to lay eggs, in contrast, reptiles - whose amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

 eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land - were better adapted to the new conditions. Reptiles invaded new niches at a faster rate and began diversifying their diets, developing herbivory and carnivory, previously only having been insectivores and piscivores.

Permian tetrapods


In the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 period, as the separate tetrapod lineages each developed in their own way, 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
Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...

 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.

The end of the Permian saw a major turnover in fauna during the Permian–Triassic extinction event. There was a protracted loss of species, due to multiple extinction pulses. Many of the once large and diverse groups died out or were greatly reduced.

Mesozoic tetrapods


Life on Earth seemed to recover quickly after the Permian extinctions, but this was mostly in the form of disaster taxa
Pioneer organism
A pioneer organism is an organism that populates a region after a natural disaster or any other event that may have caused most life in that area to disappear. Common pioneer organisms include lichens and algae. Mosses usually follow lichens in colonization, but cannot serve as pioneer organisms...

, such as the hardy Lystrosaurus
Lystrosaurus
Lystrosaurus was a genus of Late Permian and Early Triassic Period dicynodont therapsids, which lived around 250 million years ago in what is now Antarctica, India, and South Africa...

; specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction; and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.

A small group of reptiles, the diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

s, began to diversify during the Triassic, notably the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s. By the late Mesozoic, the large Laborynthodont
Labyrinthodontia
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...

 groups that first appeared during the Paleozoic such as temnospondyls and reptile-like amphibians
Reptiliomorpha
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to...

 had gone extinct. All current major groups of sauropsids evolved during the Mesozoic, with birds first appearing in the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 as a derived clade of theropod dinosaurs. Many groups of synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

s such as anomodontians and therocephalia
Therocephalia
Therocephalians are an extinct suborder of carnivorous eutheriodont therapsids that lived from the middle and late Permian into the Triassic 265.0—245.0 Ma existing for approximately ....

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.

Extant (Living) tetrapods


Following the great faunal turnover at the end of the Mesozoic, only six major groups of tetrapods were left, all of which also include many extinct groups:
  • Lissamphibia
    Lissamphibia
    The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...

     : frog
    Frog
    Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

    s and toad
    Toad
    A toad is any of a number of species of amphibians in the order Anura characterized by dry, leathery skin , short legs, and snoat-like parotoid glands...

    s, newts and salamanders
    Salamander
    Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

    , and caecilian
    Caecilian
    The caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, making them the least familiar order of amphibians. All extant caecilians and their closest fossil relatives are grouped as the clade Apoda. They are mostly...

    s
  • Testudines : turtle
    Turtle
    Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

    s and tortoise
    Tortoise
    Tortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...

    s
  • Lepidosauria
    Lepidosauria
    The Lepidosauria are reptiles with overlapping scales. This subclass includes Squamata and Sphenodontidae. It is a monophyletic group and therefore contains all descendents of a common ancestor. The squamata includes snakes, lizards, tuataras, and amphisbaenia. Lepidosauria is the sister taxon...

     : tuatara
    Tuatara
    The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...

    s, lizard
    Lizard
    Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

    s, amphisbaenians and snake
    Snake
    Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

    s
  • Crocodilia
    Crocodilia
    Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

     : crocodile
    Crocodile
    A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

    s, alligator
    Alligator
    An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

    s, caimans and gharial
    Gharial
    The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...

    s
  • Neornithes : modern birds
  • Mammalia : mammals

Classification


Tetrapods were originally classified by means of Linnean taxonomy, but currently their taxonomy is more frequently being evaluated cladistically
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

.

Linnaean classification


Traditional classification has the tetrapods classed into four classes based on gross anatomical
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 and physiological
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 traits.
Note that snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

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
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 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
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

. The earliest tetrapods are grouped under Class Amphibia, despite several of the groups would have been more closely related to Amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s than to modern day amphibians
Lissamphibia
The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...

. Reptiles too form a paraphyletic group, as they have given rise to (bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s that are traditionally not considered to be a type of reptile. Basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 non-mammalian synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

s ("mammal-like reptiles") traditionally also sort under Class Reptilia as a separate subclass. They are however not among in the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 reptiles, and while the early groups may have been reptile-like in biology, the later ones was decidedly not reptile-like, nor were they true mammals. 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
Labyrinthodontia
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...

. Labyrinthodonts were distinguished mainly by their complex dentine infolding tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 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
Ichthyostega
Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...

), the Temnospondyli
Temnospondyli
Temnospondyli is a diverse order of 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...

 (possibly members of Amphibia), and the Anthracosauria
Anthracosauria
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon.-Various definitions:...

 (close relatives of amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

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
Centrum
Centrum means center in Latin.Centrum may refer to:* The central portion of a vertebra*Centrum , a Washington state performing arts organization* Centrum , metro station in Warsaw, Poland...

 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
Anthracosauria
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon.-Various definitions:...

, which would eventually give rise to amniotes. A molecular study has shown that lungfish are the closest living relatives of tetrapods.

Tetrapod groups





A partial taxonomy of the tetrapods:
  • Phylum Chordata
      • Class Sarcopterygii
        Sarcopterygii
        The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...

        • Subclass Tetrapodomorpha
          Tetrapodomorpha
          Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish...

          • Eusthenopteron
            Eusthenopteron
            Eusthenopteron is a genus of prehistoric lobe-finned fish which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a strictly aquatic animal...

          • Panderichthys
            Panderichthys
            Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...

          • Tiktaalik
            Tiktaalik
            Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...

          • Ventastega
            Ventastega
            Ventastega was a basal tetrapod that lived during the Famennian subdivision of the Late Devonian period approximately 374.5 to 359.2 million years ago, though Ventastega origins as a tetrapod lineage are probably seated in the preceding Frasnian period of the Late Devonian when a surge of...

    • Superclass Tetrapoda
          • Family Elginerpeton
            Elginerpeton
            Elginerpeton is a monotypic genus of early tetrapod, the fossils of which were recovered from Scat Craig, Scotland, in rocks dating to the late Devonian Period...

            tidae
          • Family Acanthostegidae
            Acanthostega
            Acanthostega is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto...

          • Family Ichthyostegidae
            Ichthyostega
            Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...

          • Hynerpeton
            Hynerpeton
            Hynerpeton was a basal carnivorous tetrapod that lived in the lakes and estuaries of the Late Devonian period around 360 million years ago. Like many primitive tetrapods, it is sometimes referred to as an "amphibian", though it is not a true member of the modern Lissamphibia...

          • Family Tulerpeton
            Tulerpeton
            Tulerpeton is a fossil of an extinct genus of Devonian labyrinthodont that was found in the Tula Region of Russia at a site named Andreyevka...

          • Family Crassigyrinidae
            Crassigyrinus
            Crassigyrinus is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous of Scotland and possibly Greer, West Virginia. The type specimen was originally described as Macromerium scoticum and lacked a complete skull...

          • Family Loxommatidae
            Loxommatidae
            Baphetidae is an extinct family of early tetrapods. Baphetids were large labyrinthodont predators of the Late Carboniferous period of Europe. Fragmentary remains from the Early Carboniferous of Canada have been tentatively assigned to the group...

          • Family Colosteidae
            Colosteidae
            The Colosteidae are a family of amphibian tetrapods that lived in the Carboniferous period. They appear to be a fairly primitive group, a sister group to the reptile-like amphibians and the other temnospondylans....

          • Family Whatcheeriidae
            Whatcheeriidae
            Whatcheeriidae was a family of tetrapods which lived in the Mississippian sub-period, a subdivision of the Carboniferous period. It contains the genera Pederpes and Whatcheeria...

          • Family Diadectidae
            Diadectidae
            Diadectidae is an extinct family of large diadectomorph reptiliomorphs. Diadectids lived in North America and Europe during the Late Carboniferous and Early Permian. They were the first herbivorous tetrapods, and also the first fully terrestrial animals to attain large sizes. Footprints indicate...

      • Batrachomorpha
        Batrachomorpha
        ‫‬‭‮‪‫‬‭‮Batrachomorpha is a name traditionally given to recent and extinct amphibians that are not related to reptiles. It most often includes the extinct groups Temnospondyli and Lepospondyli.-Origin of the term:...

         (directly above, below, or redundant to Amphibia)
      • Class Amphibia
        Amphibian
        Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

        — Amphibians
        • Subclass Lepospondyli
          Lepospondyli
          Lepospondyli are a group of small but diverse Carboniferous to early Permian tetrapods. Six different groups are known, the Acherontiscidae, Adelospondyli, Aïstopoda, Lysorophia, Microsauria and Nectridea, and between them they include newt-like, eel- or snake-like, and lizard-like forms, along...

        • Subclass Temnospondyli
          Temnospondyli
          Temnospondyli is a diverse order of 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...

        • Subclass Lissamphibia
          Lissamphibia
          The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...

           — frogs, salamanders
      • Superorder Reptiliomorpha
        Reptiliomorpha
        Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to...

        contains among others:
        • Series Amniota
          Amniote
          The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

          , which contains among others:
          • Class Reptilia
            Reptile
            Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

             — Reptiles
          • Class Aves
            Bird
            Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

             — Birds
          • Class Synapsida — Mammal-like reptiles
          • Class Mammalia — Mammals

Phylogeny


Cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 modified after Ruta, Jeffery, & Coates (2003).

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
Fin
A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...

 built on bones that could give rise to the tetrapod limb. The rhipidistian crossopterygians fulfill every requirement for this ancestry. Their palatal
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...

 and jaw structures were identical to those of early tetrapods, and their dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...

 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
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

 or femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

), 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
Carpus
In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

 / tarsus
Tarsus (skeleton)
In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...

 and phalanges
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

 of a hand.

The major difference between crossopterygians and early tetrapods was in relative development of front and back skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 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
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period. Therefore, their ancestor would have lived earlier, during the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period. Devonian Ichthyostega
Ichthyostega
Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...

 were the earliest of true tetrapods, with a skeleton that is directly comparable to that of rhipidistian ancestors. Early temnospondyls (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
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 structure between the cheek and shoulder, and the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

. They had, however, lost several other fish features such as the fin rays in the tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

.

In order to propagate in the terrestrial environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

, certain challenges had to be overcome. The animal's body needed additional support, because buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

 was no longer a factor. A new method of respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

 was required in order to extract atmospheric
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, instead of oxygen dissolved in water. A means of locomotion
Animal locomotion
Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

 would need to be developed to traverse distances between waterholes. Water retention was now important since it was no longer the living matrix
Matrix (biology)
In biology, matrix is the material between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular 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 orbital vacuities
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...

 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
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...

 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 parietals
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....

 by a sutural junction between the postparietals and supratemporals. Also at the rear of the skull, all bones dorsal to the cleithrum
Cleithrum
The cleithrum is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key ", by analogy with "clavicle" from Latin clavicula = "little key".In modern fishes, the cleithrum is a...

 were lost.

The lower jaw of, for example, Eryops
Eryops
Eryops meaning "drawn-out face" because most of its skull was in front of its eyes is a genus of extinct, semi-aquatic amphibian found primarily in the Lower Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United...

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
Splenial
The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....

s, the angulary and the surangular. On the inside were usually three coronoids that bore teeth and lay close to the dentary. On the upper jaw was a row of marginal labyrinthine teeth, located on the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

 and premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

. 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 enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...

 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
Pleurodont
Pleurodont is a formation of the teeth that are fused by their sides to the inner surface of the jaw bones.This formation is common in the order Squamata.-External links:* *...

 teeth, in fewer numbers of the whole group.

Sensory organs


The difference in density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 between air and water causes smells
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

 (certain chemical compounds detectable by chemoreceptors) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory apparatus
Apparatus
Apparatus may refer to:*Technical term for body of the Soviet and post-Soviet governments *Machine*Equipment*Critical apparatus, the critical and primary source material that accompanies an edition of a text-See also:*Fire apparatus...

 was designed for aquatic detection.

Fish have a lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 system that detects pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 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
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates. The olfactory
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

 epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 would also have to be modified in order to detect airborne odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

s.

In addition to the lateral line organ system, the eye had to change as well. This change came about because the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 of light differs between air and water, so the focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 of the lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 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
Eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

s developed and tear ducts 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 vibrations
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

 could not set up pulsations through the skull in order for it to function a proper auditory organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

. Typical of most labyrinthodonts, the spiracular gill pouch was retained as the otic notch
Otic notch
Otic notches are invagination in the posterior margin of the skull roof, one behind each orbit. Such notches are found in labyrinthodonts and some of their immediate ancestors, but not their reptilian descendants...

, closed in by the tympanum
Tympanal organ
A tympanal organ is a hearing organ in insects, consisting of a membrane stretched across a frame backed by an air sac and associated sensory neurons...

, a thin, tight membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

.

The hyomandibula
Hyomandibula
The hyomandibula, commonly referred to as hyomandibular [bone] , and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays a role in suspending the jaws and/or operculum...

 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 matching system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibian lineages
Lineage (evolution)
An evolutionary lineage is a sequence of species, that form a line of descent, each new species the direct result of speciation from an immediate ancestral species. Lineages are subsets of the evolutionary tree of life. Lineages are often determined by the techniques of molecular systematics.-...

.

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
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

 and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, 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
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 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
Cleithrum
The cleithrum is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key ", by analogy with "clavicle" from Latin clavicula = "little key".In modern fishes, the cleithrum is a...

 was retained as the clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...

, and the interclavicle
Interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. Monotremes, although part of the mammalian class, do have interclavicles...

 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
Glenoid cavity
The glenoid cavity is a shallow pyriform, articular surface, which is located on the lateral angle of the scapula. It is directed laterally and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest.This cavity forms the...

 situated below performing as the articulation
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...

 surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.

The pelvic
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 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
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

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
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...

, while the broad ventral plate was composed of the pubis
Pubis (bone)
In vertebrates, the pubic bone is the ventral and anterior of the three principal bones composing either half of the pelvis.It is covered by a layer of fat, which is covered by the mons 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
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...

, providing a surface of articulation for the femur.

The main strength of the ilio-sacral attachment of Eryops
Eryops
Eryops meaning "drawn-out face" because most of its skull was in front of its eyes is a genus of extinct, semi-aquatic amphibian found primarily in the Lower Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United...

was by ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

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
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 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
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...

 and ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

. 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
Olecranon
The olecranon is a large, thick, curved bony eminence of the forearm that projects behind the elbow.It is situated at the upper end of the ulna, one of the two bones in the forearm...

 that extended above the edge of the humerus.

The radius and the ulna articulated with the carpus
Carpus
In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

, 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
Toe
Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being digitigrade. Humans, and other animals that walk on the soles of their feet, are described as being plantigrade; unguligrade animals are those that walk on hooves at the tips of...

 lay a series of five distal carpals. Each digit
Digit (anatomy)
A digit is one of several most distal parts of a limb, such as fingers or toes, present in many vertebrates.- Names:Some languages have different names for hand and foot digits ....

 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 analogue
Analogy
Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...

 to the humerus was the femur, which was longer and slimmer. The two lower arm bones corresponded to the tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....

 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
Alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

.

The tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

 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, suggesting that they were not predatory. It is more likely that it fed on fish either in the water or on those that became stranded at the margins of lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

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 breathe
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

 by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

, known as cutaneous respiration. Eryops also inhaled, but its ribs were too closely spaced to suggest that it did this by expanding the rib cage
Rib cage
The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....

. More likely, it breathed by buccal pumping
Buccal pumping
Buccal pumping is a method of respiration in which the animal moves the floor of the mouth in a rhythmic manner that is externally apparent.This method has several stages. These will be described for an animal starting with lungs in a deflated state: First, the glottis is closed, and the...

 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 tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 in the lung walls. Other special respiratory methods probably existed.

Circulation


Early tetrapods most likely had a three-chambered heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

, 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
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...

. 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 strength
Physical strength
Physical strength is the ability of a person or animal to exert force on physical objects using muscles. Increasing physical strength is the goal of strength training.-Overview:...

 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
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

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
  • Hexapod
  • Octopod

External links


Devonian tetrapods


Carboniferous tetrapods