{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}
{{Otheruses4|four-limbed vertebrates|the structure|Tetrapod (structure)}}
{{Taxobox
| name = Tetrapods
| fossil_range = {{fossil range|365|0}}Late
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
to Recent
| image = Varanus komodoensis.JPG
| image_width = 200px
| image_caption = A
Komodo dragonThe Komodo dragon is a species of lizard that inhabits the islands of Komodo, Rinca, Flores, and Gili Motang in Indonesia. A member of the monitor lizard family , it is the largest living species of lizard, growing to an average length of and weighing around...
| regnum =
AnimalAnimals are a major group of mostly 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...
ia
| phylum = Chordata
| subphylum = Vertebrata
| superclassis =
Tetrapoda
| superclassis_authority = Broili, 1913
| subdivision_ranks =
ClassesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
| subdivision =
- Amphibia
- Aves
- Mammalia
- Reptilia
- Synapsida
}}
Tetrapods (
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
τετραποδη
tetrapoda,
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
quadrupedQuadrupedalism is a form of land animal locomotion using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet"...
, "four-footed") are
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
animalAnimals are a major group of mostly 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...
s having four feet,
legŁęg may refer to the following places in Poland:*A former name for the town of Ełk *Łęg, Lower Silesian Voivodeship *Łęg, Kuyavian-Pomeranian Voivodeship *Łęg, Łódź Voivodeship...
s or leglike
appendageAn appendage in the broadest sense is an additional or subsidiary part existing on, or added to, something which can generally still function if the appendage has never existed or is later provided or grown, or will still perform a primary function if the appendage is removed.- Biological context...
s.
AmphibianAmphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
s,
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s,
dinosaur{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}{{Taxobox|name = Dinosaurs|fossil_range = {{Fossil range|230|65|earliest=230|latest=0|PS=
Descendant taxon Aves survives to present.}}|image = field_dinos_2.jpg...
s/
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s, and
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s are all tetrapods, and even the limbless
snakeSnakes 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 are tetrapods by descent. The earliest tetrapods radiated from the
SarcopterygiiSarcopterygii - Crossopterygii is traditionally the class of fleshy-finned, lobe-finned fishes, consisting of lungfish, coelacanths and all tetrapods.-Characteristics:Sarcopterygians - crossopterygians are bony fish with fleshy,...
, or lobe-finned
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
.
Evolution
Devonian tetrapods
Research by
Jennifer A. ClackJennifer 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
AcanthostegaAcanthostega is an extinct tetrapod 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 overturned the earlier view that fish had first invaded the land — either in search of prey (like modern
mudskipperMudskippers are members of the subfamily Oxudercinae , within the family Gobiidae . They are completely amphibious fish, fish that can use its 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.
Life in the swamps
The first tetrapods are now thought to have
evolvedIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
in shallow and
swampA swamp is a wetland featuring temporary or permanent inundation of large areas of land by shallow bodies of water. A swamp generally has a substantial number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types...
y
freshwaterFresh 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...
habitatsA habitat is an ecological or environmental area that is inhabited by a particular animal and plant species...
, towards the end of the Devonian, a little more than 365 million years ago. By the late Devonian, land
plantPlants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants,...
s had stabilized freshwater habitats, allowing the first
wetlandA wetland is an area of land whose soil is saturated with moisture either permanently or seasonally. Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes, and bogs, among others. The water found in wetlands can be saltwater, freshwater,...
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s to develop, with increasingly complex
food webFood chains describe the eating relationships between species within an ecosystem or a particular living place. Many types of food chains or webs are applicable depending on habitat or environmental factors...
s that afforded new opportunities.
http://news.softpedia.com/news/How-did-fish-grow-legs-15424.shtml
Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.
Execretion in tetrapods
The common ancestor of all present
gnathostomesGnathostomata is the group of vertebrates with jaws. The term derives from Greek γνάθος "jaw" + στόμα "mouth"....
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
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
waste product
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
into harmless
ureaUrea or carbamide is an organic compound with the chemical formula
2CO. The molecule has two amine residues 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. Ray-finned fishes later returned to freshwater and lost this ability, while the fleshy-finned fishes retained it. Since the blood of ray-finned fishes contain more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they did not recover their old trick of turning ammonia to urea, and they had to evolve salt excreting glands instead.
LungfishLungfish 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
coelacanthCoelacanth is the common name for an order of fish that includes the oldest living lineage of gnathostomata 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
toadA toad can refer to a number of species of amphibians in the order Anura. A distinction is often made between frogs and toads by their appearance, prompted by the convergent adaptation among so-called "toads" to dry habitats. Many "toads" have leathery skin for better water retention, and brown...
Bufo marinusThe cane toad , also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad native to Central and South America. It is a member of the subgenus Chaunus of the genus Bufo, which includes many different true toad species found throughout Central and South America...
and the
frogFrogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...
Rana cancrivoraFejervarya 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 animalTerrestrial 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 . Terrestrial animals evolved from marine animals...
s that can only produce ammonia would have to drink constantly, making a life on land impossible (a few exceptions exist, as some terrestrial woodlice can excrete their nitrogenous waste as ammonia gas). This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater (unless they first migrated into freshwater habitats and then migrated onto land so shortly after that they still retained the ability to make urea), although some species never left, or returned to, the water could of course have adapted to freshwater lakes and rivers.
Lobe-finned fishes
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid
sarcopterygianSarcopterygii - Crossopterygii is traditionally the class of fleshy-finned, lobe-finned fishes, consisting of lungfish, coelacanths and all tetrapods.-Characteristics:Sarcopterygians - crossopterygians are bony fish with fleshy,...
"), with a two-lobed
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
in a flattened
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
. As a shallow water dweller, it sported a dorsoventrally flatttened body, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and
boneBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s. The "
living fossilLiving fossil is an informal term for any living species of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities...
" coelacanth is a related marine lobe-finned fish without these shallow-water adaptations).
Even more closely related was
Gorgonasus and
PanderichthysPanderichthys is a 90–130 cm long fish from the Devonian period 380 million years ago, of Latvia. It has a large tetrapod-like head. Panderichthys exhibits transitional features between lobe-finned fishes and early tetrapods such as Acanthostega...
, which even had a
choanaChoana is the posterior nasal aperture.The choanae are separated by the vomer.- Boundaries :It is the opening between the nasal cavity and the nasopharynx.It is therefore not a structure but a space bounded as follows:...
.
These fishes used their fins as
paddleA 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
detritusDetritus 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 laying in ambush for prey. The universal tetrapod characteristics of front
limbA 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
kneeThe knee joint joins the thigh with the leg and consists of two articulations: one between the femur and tibia, and one between the femur and patella. It is the largest and most complicated joint in the human body. The knee is a mobile trocho-ginglymus , which permits flexion and extension as well...
can plausibly be traced to early tetrapods living in shallow water.
Lungs
It is now clear that the common ancestor of the bony fishes had a primitive air-breathing
lungThe lung or pulmonary system is the essential respiration organ in 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 in the chest on either side of the heart...
(later evolved into a
swim bladderThe gas 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.-Species:Gas bladders are only found in ray-finned fish...
in most ray-finned fishes). This suggests that it evolved in warm shallow waters, the kind of habitat the lobe finned fishes were living and made use of their simple lung when the oxygen level in the water became too low.
The lungfishes are now considered as being the closest living relatives of the tetrapods, even closer than the coelacanth.
Fleshy lobe fins supported on bones rather than ray-stiffened fins seems to have been an original trait of the bony fishes (
OsteichthyesOsteichthyes , also called bony fish, are a taxonomic group of fish that includes the ray-finned fish and lobe-finned fish . The split between these two classes occurred about 450 million years ago....
). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned (
ActinopterygiiThe Actinopterygii constitute the class of the ray-finned 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 the class Sarcopterygii...
) fishes evolved their fins in the opposite direction. The most primitive group of the ray-fins, the
bichirThe 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
generaIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
of Devonian tetrapods have been described, several known mainly or entirely from lower
jawThe 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 of them were from the European-North American
supercontinentIn 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
EuropeEurope 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 water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
,
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and
GreenlandGreenland is an autonomous constituent country within the Kingdom of Denmark located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago...
. The only exception is a single
GondwanaGondwana , originally Gondwanaland, is the name given to a southern precursor-supercontinent. Its final joining occurred between ca. 570 and 510 Ma ago, joining East Gondwana to West Gondwana. It later separated from Laurasia 180- during the breakup of the Pangaea supercontinent that existed about...
n genus,
MetaxygnathusMetaxygnathus is an extinct genus of tetrapodomorph found in Late Devonian deposits of New South Wales, Australia .It is only known from a lower jawbone.-References:...
, which has been found in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
.
The first Devonian tetrapod identified from
AsiaAsia is the world's largest and most populous continent, located in the eastern and northern hemispheres. It covers 8.6% of the earth's total surface area and with approximately 4 billion people, it hosts 60% of the world's current human population.Asia is traditionally defined as part of the...
was recognized from a
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
jawbone reported in 2002. The
ChineseChina is a cultural region, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
tetrapod
Sinostega paniSinostega 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
sandstoneSandstone 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 color, but the most common colors are tan, brown, yellow,...
sediments of the
Ningxia HuiNingxia , full name Ningxia Hui Autonomous Region , is a Hui autonomous region of the People's Republic of China, located on the northwest Loess highland, the Yellow River flows through a vast area of its 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
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
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
mudMud is a liquid or semi-liquid mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as siltstone or solid, mudrock lutites. When geological deposits of mud are formed in estuaries the resultant layers are...
.
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
mangroveMangroves are trees and shrubs that grow in saline coastal habitats in the tropics and subtropics – mainly between latitudes N and S. The saline conditions tolerated by various species range from brackish water, through pure seawater , to water of over twice the salinity of ocean seawater,...
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
mudskipperMudskippers are members of the subfamily Oxudercinae , within the family Gobiidae . They are completely amphibious fish, fish that can use its 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.{{Citation needed|date=November 2008}}
From water to land
Initially making only tentative forays onto land, tetrapods adapted to terrestrial environments over time and spent longer periods away from the water, while also spending a longer part of their juvenile stage on land before returning to the water for the rest of their life. It is also possible that the adults started to spend some time on land (as the skeletal modifications in early tetrapods such as
IchthyostegaIchthyostega is an early tetrapod genus that lived in the Upper Devonian period, 367-362.5 million years ago. It was a labyrinthodont that represents an intermediate form between fish and amphibians. 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
neotenyNeoteny , also called juvenilization, is the retention, by adults in a species, 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 animal or organism is slowed or delayed...
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-
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
amphibianAmphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
lineages. It was referred to as "
Romer's GapRomer's Gap is an example of a gap in the fossil record. It is named after paleontologist Dr. Alfred Romer. The fossil record is used in the study of evolution. A fossil record gap is a period from which excavators have found very few fossils....
", after the
palaeontologistPaleontology
[from Greek: παλαιός "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought"] is the study of prehistoric life, including 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.
EarThe ear is the organ that detects sound. The vertebrate ear shows a common biology from fish to humans, with variations in structure according to order and species. It not only acts as a receiver for sound, but plays a major role in the sense of balance and body position...
s,
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
s and
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
l columns all underwent changes too. The number of digits on
handThe hands are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm of a primate. They are the chief organs for physically manipulating the environment, used for both gross motor skills and fine motor skills...
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
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
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.
EryopsEryops 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 States...
is an example of an animal that made such adaptations. It retained and refined most of the traits found in its fish ancestors. Sturdy
limbA 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
backboneBackbone may mean:* Vertebral column, of a vertebrate organism* Backbone chain, in polymer chemistry, the framework of the molecule* Backbone Entertainment, a video game development company* Backbone network, the top level of a hierarchical network...
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
soundSound is a travelling wave which 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.- Perception of sound...
.
By the
ViseanThe 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
temnospondylsTemnospondyli are an important and extremely diverse taxon of small to giant primitive amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous...
(e.g.
EryopsEryops 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 States...
)
lepospondylsLepospondyli are a group of small but diverse Carboniferous to early Permian amphibians. Six different clades 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.
DiplocaulusDiplocaulus is an extinct genus of leponspondyl amphibian from the Permian period of North America.Diplocaulus had a stocky, salamander-like body, but was relatively large, reaching up to in length. Its most distinctive features were the long protrusions on the sides of its skull, giving the head...
) and
anthracosaursAnthracosauria refers to a group 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....
, which were the relatives and ancestors of the
AmniotaThe amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include mammals, birds and reptiles, 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. In humans,...
. Depending on whichever authorities one follows,
modern amphibiansThe 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...
(frogs,
salamanderSalamander is a common name of approximately 500 species of amphibians. They are typically characterized by their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant species are grouped together as the Urodela...
s and
caecilianThe caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, which makes them the least explored order of amphibians, and widely unknown.-Anatomy:...
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 CarboniferousThe Pennsylvanian is in the ICS geologic timescale the youngest subperiod or upper subsystem of the Carboniferous period. It lasted from roughly to Ma . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the...
, and during the
TriassicThe Triassic is a geologic period and system that extends from about 251 to 199 Ma . 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
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s,
turtleTurtles 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
crocodileA crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all members of the order Crocodilia: i.e...
s (
lizardLizards are a very large and widespread group of squamate reptiles, with nearly 5,000 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s and
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s appeared in the
JurassicThe Jurassic is a geologic period and system that extends from about Ma to Ma, 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
snakeSnakes 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
CretaceousThe Cretaceous , Latin language for "chalky", 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 on the Jurassic period and is followed by the Paleogene period of the...
). As living members of the tetrapod clan — that is of the tetrapod "crown-group" — these varied tetrapods represent the
phylogeneticIn biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
end-points of these two divergent lineages. A fourth
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
group, the
baphetidThe Baphetids or Loxommatids were large tetrapod 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.
Permian tetrapods
In the
PermianThe Permian
[The 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...]
period, as the separate tetrapod lineages each developed in their own way, in the term "tetrapoda" becomes less useful. In addition to temnospondyl and anthracosaur clades among the early "amphibia" (labyrinthodonts), there were two important divergent clades of amniotes, the
SauropsidaSauropsida is a group of amniotes that includes all existing reptiles, dinosaurs, and birds. The Sauropsida is distinguished from Theropsida , more commonly called Synapsida, which includes mammals....
and the Synapsida, of which the latter were the most important and successful Permian animals. Each of these lineages, however, remains grouped with the tetrapoda, just as
Homo sapiens could be considered a very highly-specialized kind of
lobe-finned fish.
Living tetrapods
The beginning of the
MesozoicThe Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the "Mesozoic" was "Secondary" The Mesozoic Era is one of three geologic eras of the...
saw a major turnover in fauna following the Permian–Triassic extinction event. Many of the once large and diverse groups died out or were greatly reduced. A small group of reptiles, the
diapsidDiapsids 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 deversify during the Triassic, notably the
dinosaur{{Otheruses}}{{pp-semi-protected|small=yes}}{{Otheruses}}{{pp-semi-protected|small=yes}}{{Taxobox|name = Dinosaurs|fossil_range = {{Fossil range|230|65|earliest=230|latest=0|PS=
Descendant taxon Aves survives to present.}}|image = field_dinos_2.jpg...
s. By the late Mesozoic, many large tetrapod groups that first appeared during the Paleozoic such as temnospondyls and anthracosaurs had gone extinct. All current major groups of sauropsids evolved during the Mesozoic, with birds first appearing in the
JurassicThe Jurassic is a geologic period and system that extends from about Ma to Ma, 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
synapsidSynapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals"...
s such as anomodontians and
therocephaliaTherocephalians 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.
Following the great faunal turnover at the end of the Mesozoic, only three categories of living
crown groupA crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all members, and all of that ancestor's descendants...
tetrapods were left, all of which also include many extinct groups:
- 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...
: Modern frogFrogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...
s and toadA toad can refer to a number of species of amphibians in the order Anura. A distinction is often made between frogs and toads by their appearance, prompted by the convergent adaptation among so-called "toads" to dry habitats. Many "toads" have leathery skin for better water retention, and brown...
s, newts and salamandersSalamander is a common name of approximately 500 species of amphibians. They are typically characterized by their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant species are grouped together as the Urodela...
, and caecilianThe caecilians are an order of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, which makes them the least explored order of amphibians, and widely unknown.-Anatomy:...
s
- Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles, dinosaurs, and birds. The Sauropsida is distinguished from Theropsida , more commonly called Synapsida, which includes mammals....
: TurtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s, lepidosauriaThe Lepidosauria are reptiles with overlapping scales. They include the tuataras, lizards, snakes and amphisbaenians. Lepidosaurians are the most successful of modern reptiles.Lepidosauria is a superorder of Sauropsida and comprises the orders :* Squamata...
ns (tuataraThe 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, lizardLizards are a very large and widespread group of squamate reptiles, with nearly 5,000 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, amphisbaenians and snakeSnakes 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), birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s, and crocodiliaCrocodilia 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...
ns
- Synapsida : Mammal
Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s
Classification
Tetrapods were originally classified by means of Linnean taxonomy, but currently their taxonomy is more frequently being evaluated
cladisticallyCladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
.
Linnaean classification
Traditional classification has the tetrapods classed into four classes based on gross
anatomicalAnatomy 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
physiologicalPhysiology is the science of the functioning of living systems. It is a subcategory of biology...
traits.
Note that
snakeSnakes 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
aquaticAn 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
cladisticsCladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
. Reptiles are paraphyletic, as they have given rise to another group (
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s) that is traditionally not considered to be a type of reptile.
basalIn 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
synapsidSynapsids , also known as theropsids , are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. The non-mammalian members are described as mammal-like reptiles in classical systematics, but are referred to as "stem-mammals" or "proto-mammals"...
s (classically called "mammal-like reptiles") are excluded from the traditional classifications as they were not true mammals nor were they reptiles. Thus some authors have argued for a new classification based purely on phylogeny, disregarding the anatomy and physiology (see below).
Phylogenetic classification
All early tetrapods and tetrapodomorphs that were not amphibians in the strict phylogenetic sense, nor amniotes, were once placed together in the paraphyletic group
LabyrinthodontiaLabyrinthodont is an obsolete term for any member of the extinct superorder of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times...
. Labyrinthodonts were distinguished mainly by their complex dentine infolding
toothTeeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to tear, scrape, and chew food. Some animals, particularly carnivores, also use teeth for hunting or defense. The roots of teeth are covered by gums...
structure, a feature shared with crossopterygian fish. The labyrinthodonts were divided into the Ichthyostegalia (another paraphyletic assemblage of primitive tetrapods and kin, such as
IchthyostegaIchthyostega is an early tetrapod genus that lived in the Upper Devonian period, 367-362.5 million years ago. It was a labyrinthodont that represents an intermediate form between fish and amphibians. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
), the
TemnospondyliTemnospondyli are an important and extremely diverse taxon of small to giant primitive amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous...
(possibly members of Amphibia), and the
AnthracosauriaAnthracosauria refers to a group 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....
(close relatives of
amnioteThe amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include mammals, birds and reptiles, 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. In humans,...
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
centrumCentrum 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
AnthracosauriaAnthracosauria refers to a group 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....
, which would eventually give rise to amniotes.
Tetrapod groups
A partial taxonomy of the tetrapods:
- Phylum Chordata
-
- Class Sarcopterygii
Sarcopterygii - Crossopterygii is traditionally the class of fleshy-finned, lobe-finned fishes, consisting of lungfish, coelacanths and all tetrapods.-Characteristics:Sarcopterygians - crossopterygians are bony fish with fleshy,...
- Subclass Tetrapodomorpha
Tetrapodomorpha is a clade of vertebrates, consisting of sarcopterygians with a number of features of tetrapods. Primitive forms, like Tiktaalik, have been referred to as "fishapods" by their discoverers, since they were half-fish half-tetrapods, at least in appearance.Tetrapodomorpha contains the...
- 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 pelagic animal...
- Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 380 million years ago, of Latvia. It has a large tetrapod-like head. Panderichthys exhibits transitional features between lobe-finned fishes and early tetrapods such as Acanthostega...
- Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian fish 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 was a Tiktaalik-like tetrapodomorph 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...
- Superclass Tetrapoda
-
-
- Family Elginerpeton
Elginerpeton is a 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 is an extinct tetrapod 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 is an early tetrapod genus that lived in the Upper Devonian period, 367-362.5 million years ago. It was a labyrinthodont that represents an intermediate form between fish and amphibians. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
- 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 class Amphibia...
- Family Tulerpeton
Tulerpeton is an extinct genus of Devonian reptiliomorph that was found in the Tula Region of Russia. It is considered one of the first true tetrapods to have arisen. This species is differentiated from the less derived "aquatic tetrapods" by a strengthened limb structure...
- Family Crassigyrinidae
Crassigyrinus is an extinct genus of carnivorous stem group tetrapod from the Early Carboniferous of Scotland and a possible specimen from Greer, West Virginia. The type specimen was originally described as Macromerium scoticum and lacked a complete skull...
- Family Loxommatidae
The Baphetids or Loxommatids were large tetrapod 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
The Colosteidae are a family of temnospondyl amphibians that lived in the Carboniferous period....
- Family 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 Diadectes
Diadectes was a genus of large, very reptile-like tetrapods that lived during the Early Permian. It is one of the very first herbivorous tetrapods, and also one of the first fully terrestrial animals to attain large size.-Description:...
- Batrachomorpha
Batrachomorpha is a name given to recent and extinct amphibians that are not related to reptiles.-Origin of the term: Are salamanders amphibians?:...
(directly above, below, or redundant to Amphibia)
- Class Amphibia
Amphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
— Amphibians
- Subclass Lepospondyli
Lepospondyli are a group of small but diverse Carboniferous to early Permian amphibians. Six different clades 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 are an important and extremely diverse taxon of small to giant primitive amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous...
- Subclass 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...
— frogs, salamanders
- Superorder Reptiliomorpha
Reptiliomorpha refers to reptile-like amphibians and the amniotes which evolved from them. The name was once was used to refer to reptile-like labyrinthodonts, but this usage of the term is obsolete.-Characteristics:...
contains among others:
- Series Amniota
The amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include mammals, birds and reptiles, 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. In humans,...
, which contains among others:
- Class Reptilia
Reptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
— Reptiles
- Class Aves
Birds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
— Birds
- Class Synapsida — Mammal-like reptiles
- Class Mammalia — Mammals
Phylogeny
Cladogram modified after Ruta, Jeffery, & Coates (2003).
{{clade| style=font-size:90%;line-height:100%
|label1=
Tetrapoda
|1={{clade
|1=
AcanthostegaAcanthostega is an extinct tetrapod 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...
|2={{clade
|1=
IchthyostegaIchthyostega is an early tetrapod genus that lived in the Upper Devonian period, 367-362.5 million years ago. It was a labyrinthodont that represents an intermediate form between fish and amphibians. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
|2={{clade
|1=
HynerpetonHynerpeton 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 class Amphibia...
|2={{clade
|1=
TulerpetonTulerpeton is an extinct genus of Devonian reptiliomorph that was found in the Tula Region of Russia. It is considered one of the first true tetrapods to have arisen. This species is differentiated from the less derived "aquatic tetrapods" by a strengthened limb structure...
|2={{clade
|1=
WhatcheeriidaeWhatcheeriidae 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...
|2={{clade
|1=
CrassigyrinusCrassigyrinus is an extinct genus of carnivorous stem group tetrapod from the Early Carboniferous of Scotland and a possible specimen from Greer, West Virginia. The type specimen was originally described as Macromerium scoticum and lacked a complete skull...
|2={{clade
|1=
Caerorhachis
|2={{clade
|1={{clade
|1={{clade
|1=Gephyrostegidae
|2={{clade
|1=
EoherpetontidaeEoherpeton is the only genus in the Eoherpetontidaea family of the extinct suborder Embolomeri....
|2=
EmbolomeriThe Embolomeri is a suborder of reptiliomorphs. The Embolomeri first evolved from an amphibian-like tetrapod in the Early Carboniferous . They were among the reptiliomorphs that survived the Permian-Triassic extinction event that wiped out around 90% of all life on Earth, excluding the amniotes...
}} }}
|2={{clade
|1=
SeymouriamorphaSeymouriamorpha were a small but widespread group of reptiliomorphs. Many seymouriamorphs were terrestrial or semi-aquatic. However, small aquatic larvae bearing external gills were found, making them unquestionably amphibians. The adults were terrestrial. They ranged from lizard-sized creatures ...
|label2= Crown group Tetrapoda
|2={{clade
|1={{clade
|1=Crown Amniota
|2={{clade
|1=
DiadectomorphaDiadectomorpha are a clade of large reptile-like tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota...
|2=Crown Amniota}} }}
|label2=
LepospondyliLepospondyli are a group of small but diverse Carboniferous to early Permian amphibians. Six different clades 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...
|2={{clade
|1={{clade
|1=
NectrideaNectridea is an extinct order of lepospondyl amphibians from the Carboniferous and Permian periods, which included animals such as Diplocaulus. In appearance, they would have resembled modern newts or aquatic salamanders. They had long flattened tails to aid in swimming, and well developed hind...
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AdelospondyliAdelospondyli are an order of elongate, presumably aquatic, Carboniferous amphibians. The skull is solidly roofed, and elongate, with the orbits located very far forward. The limbs are well developed. There is a single family, the Adelogyrinidae...
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AistopodaAïstopoda are an order of highly specialised snake-like amphibians known from the Carboniferous and Early Permian of Europe and North America, ranging from tiny forms only , to nearly in length. The first appear in the fossil record in the Mississippian period and continue through to the Early...
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MicrosauriaMicrosauria is an extinct Order of lepospondyl amphibians from the late Carboniferous and early Permian periods.The microsaurs all had short tails and small legs, but were otherwise quite varied in form...
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LissamphibiaThe 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...
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BaphetidaeThe Baphetids or Loxommatids were large tetrapod predators of the Late Carboniferous period of Europe. Fragmentary remains from the Early Carboniferous of Canada have been tentatively assigned to the group....
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TemnospondyliTemnospondyli are an important and extremely diverse taxon of small to giant primitive amphibians that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few stragglers continued into the Cretaceous...
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ColosteidaeThe Colosteidae are a family of temnospondyl amphibians that lived in the Carboniferous period....
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EryopoideaEryopoidea are a taxon of late Carboniferous and Permian temnospondyli amphibians, known from North America and Europe. Carroll includes no less than ten families, but Yates and Warren replace this with a cladistic approach and include only two Permian families, the Eryopidae and Zatrachydidae...
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DissorophoideaDissorophoideans are a clade of medium-sized, temnospondyl amphibians that appeared during the Late Pennsylvanian in Euramerica, and continued through to the Late Permian and even possibly the Early Triassic of Gondwana...
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ArchegosauroideaArchegosauroidea is an extinct superfamily of Permian temnospondyls. The superfamily is assigned to the clade Stereospondylomorpha and is the sister taxon to the suborder Stereospondyli. It includes the families Actinodontidae and Archegosauridae, and possibly the genus Intasuchus, which is placed...
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RhinesuchidaeRhinesuchidae is a family of Temnospondyli that lived in the Permian and Triassic period.-External links:*...
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RhytidosteidaeRhytidosteidae is a family of Temnospondyli that lived in the Permian and Triassic.-References:*Yates, AM , A new tiny rhytidosteid from the Early Triassic of Australia and the possibility of hidden temnospondyl diversity. J. Vert Paleontol. 20:484-489.-External links:*...
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ChigutisauridaeChigutisauridae is an extinct family of large Temnospondyl amphibians. The only genera recognized as belonging to Chigutisauridae at the current time are all from Gondwana and include Koolasuchus and Siderops.-References:*Sengupta, D.P. 1995. Palaeontology 38: 19-59.*Sengupta, D.P. 2003...
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PlagiosauridaePlagiosauridae is a family of Temnospondyli that lived in the Triassic period.-References:*Milner, A.R. 1994, Late Triassic and Jurassic amphibians: fossil record and phylogeny, pp. 5-22 in Fraser & Sues In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods, Cambridge University Press,...
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BrachyopidaeBrachyopidae is an extinct family of Temnospondyl amphibians.-References:*Warren, A. A., & Marsicano, C. Revision of the Brachyopidae from the Triassic of the Sydney, Carnarvon and Tasmania Basin, Australia: Alcheringa, v. 22, p. 329-342.-External links:*...
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MastodonsauroideaThe Mastodonsauroidea are an extinct superfamily of temnospondyl amphibians known from the Triassic and Jurassic. Fossils belonging to this superfamily have been found in North America, Greenland, Europe, Asia, and Australia...
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MetoposauroideaMetoposauroidea was an extinct superfamily of Temnospondyli, that lived from the Middle to Upper Triassic in North America, Europe and North Africa. They consisted of the family Metoposauridae....
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TrematosauroideaTrematosauroidea are an important group of Triassic Temnospondyl amphibians. They flourished briefly during the Early Triassic, occurring worldwide before declining at the start of the Middle Triassic, although the group continued until the Late Triassic. They were medium-sized temnospondyls with...
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Note: The origin of the subclass Lissamphibia, to which all extant amphibians belong, is disputed. This cladogram is the result of one analysis conducted by Ruta, Jeffery, & Coates (2003) that placed Lissamphibia within Lepospondyli, with the latter clade being within the
crown groupA crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all members, and all of that ancestor's descendants...
Tetrapoda. A second analysis by the authors placed Lissamphibia within Temnospondyli, thus placing Lepospondyli outside crown group Tetrapoda and Temnospondyli within. Another prevailing theory not represented by either of the cladograms is a diphyletic grouping of Lissamphibia with both Lepospondyli and Temnospondyli, with gymnophionans belonging to the former clade and anurans and caudatans belonging to the latter. }}
Anatomical features of early tetrapods
The tetrapod's ancestral fish must have possessed similar traits to those inherited by the early tetrapods, including internal nostrils (to separate the breathing and feeding passages) and a large fleshy
finA fin is a surface used 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
palatalThe palate is the roof of the mouth in humans and vertebrate animals. It separates the oral cavity from the nasal cavity. The palate is divided into two parts, the anterior bony hard palate, and the posterior fleshy soft palate or velum...
and jaw structures were identical to those of early tetrapods, and their
dentitionDentition is the development of teeth and their arrangement in the mouth.All mammals except the monotremes, the xenarthrans, the pangolins, and the cetaceans have up to four distinct types of teeth, with a maximum number for each. These are the incisor , the canine, the premolar, and the molar ...
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
humerusThe humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
or
femurThe femur, or thigh bone, is the most proximal bone of the leg in vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs...
), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the
carpusIn tetrapods, the carpals is the sole cluster of the 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...
/
tarsusIn tetrapods, the tarsus are the cluster of bones in the foot between the tibia and fibula and the metatarsus. The bones of the tarsus do not belong to individual toes, whereas those of the metatarsus do...
and
phalangesThe hands are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm of a primate. They are the chief organs for physically manipulating the environment, used for both gross motor skills and fine motor skills...
of a hand.
The major difference between crossopterygians and early tetrapods was in relative development of front and back
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
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
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
period. Therefore, their ancestor would have lived earlier, during the
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
period. Devonian Ichthyostegids 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
gillA gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide...
structure between the cheek and shoulder, and the
vertebral columnIn human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs...
. They had, however, lost several other fish features such as the fin rays in the
tailThe 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 and birds...
.
In order to propagate in the terrestrial
environmentThe natural environment, commonly referred to simply as the environment, is a term that encompasses all living and non-living things occurring naturally on Earth or some region thereof....
, certain challenges had to be overcome. The animal's body needed additional support, because
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
was no longer a factor. A new method of
respirationIn animal physiology, respiration is 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
atmosphericThe Earth's atmosphere 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...
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
, instead of oxygen dissolved in water. A means of
locomotionAnimal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, 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
matrixIn biology, matrix is the material between animal or plant cells, the material 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 vacuitiesIn anatomy, the orbital bone is the cavity or socket of the skull in which the eye and its appendages are situated.It can also mean the skin which surrounds the eye of a bird....
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 boneThe 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
parietalsThe parietal bones are bones in the human skull and form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles.-External:The external surface [Fig...
by a sutural junction between the postparietals and supratemporals. Also at the rear of the skull, all bones dorsal to the cleithrum were lost.
The lower jaw of, for example,
EryopsEryops 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 States...
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
splenialThe 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
maxillaThe 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 is sometimes called "upper maxilla", with the mandible being the "lower maxilla"...
and
premaxillaThe 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
enamelTooth enamel is the hardest and most highly mineralized substance of the body, and with dentin, cementum, and dental pulp is one of the four major tissues which make up the tooth in vertebrates. Tooth enamel is also found in the dermal denticles of sharks. It is the normally visible dental tissue...
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
pleurodontPleurodont 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
There is a
densityThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
difference between air and water that causes
smellsAn 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 called smells, 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
apparatusApparatus, is a mass noun used to describe equipment designed or assembled for a particular purpose...
was designed for aquatic detection.
Fish have a
lateral lineIn aquatic organisms , the lateral line is a sense organ 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
pressurePressure 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
habitatA habitat is an ecological or environmental area that is inhabited by a particular animal and plant species...
. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates. The
olfactoryOlfaction refers to 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...
epitheliumIn biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue...
would also have to be modified in order to detect airborne
odorAn 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 called smells, 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 indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
of light differs between air and water, so the
focal lengthThe focal length of an optical system is a measure of how strongly it 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
lensThe 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 sharp real...
was altered in order to properly function. The eye was now exposed to a relatively dry environment rather than being bathed by water, so
eyelidAn 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
vibrationsOscillation 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...
could not set up pulsations through the skull in order for it to function a proper auditory
organIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
. Typical of most labyrinthodonts, the spiracular gill pouch was retained as the otic notch, closed in by the
tympanumA Tympanal organ is a hearing organ in insects, consisting of a membrane stretched across a frame backed by an air sac. Sounds vibrate the membrane, and the vibrations are sensed by a chordotonal organ....
, a thin, tight
membraneA biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell. It is almost invariably a lipid bilayer, composed of a double layer of lipid molecules and proteins that may constitute close to 50% of membrane...
.
The
hyomandibulaThe hyomandibula 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 . In tetrapods, it has become the columella .-Sources:...
of fish migrated upwards from its jaw supporting position, and was reduced in size to form the
stapesThe stapes or stirrup is the stirrup-shaped small bone or ossicle in themiddle ear which is attached to the incus laterally and to the fenestra ovalis, the "oval window" medially. The oval window is adjacent to the vestibule of the inner ear...
. 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
lineagesAn 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
inertiaInertia is the resistance of any physical object, to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by...
and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz
frequenciesFrequency 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 girdleThe pectoral girdle is the set of bones which connect the upper limb to the axial skeleton on each side. It consists of the clavicle and scapula in humans and, in those species with three bones in the pectoral girdle, the coracoid. Some mammalian species The pectoral girdle is the set of bones...
of early tetrapods such as
Eryops was highly developed, with a larger size for both increased
muscleMuscle is the contractile tissue of the body 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 was retained as the
clavicleIn human anatomy, the clavicle or collar bone is classified as a long bone that makes up part of the shoulder girdle . It receives its name from the Latin clavicula because the bone rotates along its axis like a key when the shoulder is abducted. This movement is palpable...
, and the interclavicle was well-developed, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could have grown ventrally in such a way as to form a broad chest plate, although such was not the case in
Eryops. The upper portion of the girdle had a flat, scapular blade, with the
glenoid cavityOn the lateral angle of the scapula is a shallow pyriform, articular surface, the glenoid cavity , which is directed lateralward and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest.The cavity surface is covered with...
situated below performing as the
articulationArticulation may refer to:In linguistics:* Topic-focus articulation, a field of study concerned with marking old and new information in a clause* Manner of articulation, how speech organs involved in making a sound make contact...
surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.
The
pelvicIn human anatomy, the pelvis is the part of the trunk inferioposterior to the abdomen in the transition area between the trunk and the lower limbs...
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
ribIn vertebrate anatomy, ribs are the long curved bones which form the ribcage. In most vertebrates, ribs surround the chest they enable lungs to expand by expanding the chest, they also 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
iliumThe 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
pubisThe android 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.It is divisible into a body, a superior ramus and an inferior ramus....
in front and the ischium in behind. The three bones met at a single point in the center of the pelvic triangle called the
acetabulum, providing a surface of articulation for the femur.
The main strength of the ilio-sacral attachment of
Eryops was by
ligamentIn anatomy, the term ligament is used to denote three different types of structures:# Fibrous tissue that connects bones to other bones. They are sometimes called "articular ligaments", "fibrous ligaments", or "true ligaments"....
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
cartilageCartilage is a stiff yet 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
radiusThe radius is the bone of the forearm that extends from the lateral side of the elbow to the thumb side of the wrist. The radius is situated on the lateral side of the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
and
ulnaThe ulna is a long bone, prismatic in form. In anatomical position the ulna is placed at the medial side of the forearm closest to the body, parallel with the radius on both arms.-Articulations:The ulna articulates with:...
. 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
olecranonThe 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
carpusIn tetrapods, the carpals is the sole cluster of the 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
toeToes are the digits of the foot of an animal. 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
digitDigit may refer to:* Digit , one of several most distal parts of a limb* Phone number, slang as digit, as in "Let me get your digits so I can call you tonight."* Numerical digit, as used in mathematics or computer science...
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
analogueAnalogy is a cognitive process of transferring information 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
tibiaThe 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.The tibia is named for the greek aulos flute, also known as a tibia.-In humans:...
and
fibulaThe fibula or calf bone is a bone located on the lateral side of the tibia, with which it is connected above and below. It is the smaller of the two bones, and, in proportion to its length, the most slender of all the long bones...
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
alligatorAn Alligator is a crocodilian in the genus Alligator of the family Alligatoridae. The name alligator is an anglicized form of el lagarto the Spanish term for "lizard", the name by which early Spanish explorers and settlers in Florida called the alligator...
.
The
tongueThe tongue is a muscle on the floor of the mouth that manipulates food for chewing and swallowing . It is the primary organ of taste, as much of the upper surface of the tongue is covered in papillae and taste buds. A secondary function of the tongue is speech, in which the organ assists...
of modern adult amphibians is quite fleshy and attached to the front of the lower jaw, so it is reasonable to speculate that it was fastened in a similar fashion in primitive forms, although it was probably not specialized like it is in a frog.
It is taken that early tetrapods were not very active, thus a predatory
lifestyleLifestyle was originally coined by Austrian psychologist Alfred Adler in 1929. The current broader sense of the word dates from 1961.In sociology, a lifestyle is the way a person lives. A lifestyle is a characteristic bundle of behaviors that makes sense to both others and oneself in a given time...
was probably not the norm. It is more likely that it fed on fish either in the water or on those that became stranded at the margins of
lakeA lake is a terrain feature , a body of liquid on the surface of a world that is localized to the bottom of basin and moves slowly if it moves at all. Another definition is, a body of fresh or salt water of considerable size that is surrounded by land...
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
breatheIn animal physiology, respiration is 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
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
.
Eryops also inhaled, but its ribs were too closely spaced to suggest that it did this by expanding the rib cage. More likely, it breathed by
buccal pumpingBuccal 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
tissueTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
in the lung walls. Other special respiratory methods probably existed.
Circulation
Early tetrapods most likely had a three-chambered
heartThe heart is a muscular organ found in all vertebrates 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 angleIn geometry and trigonometry, a right angle is an angle of 90 degrees, corresponding to a quarter turn . It can be defined as the angle such that twice that angle amounts to a half turn, or 180°....
. 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
strengthPhysical strength is the ability of a person 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
rockIn geology, rock 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.
External links
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Devonian tetrapods
Carboniferous tetrapods