Tetrapods evolved from a group ofsemiaquatic animals within thetetrapodomorphs which, in turn, evolved from ancientlobe-finned fish (sarcopterygians) around390 million years ago in theMiddle Devonian period.[7] Early tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit the body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) aretrackways from the Middle Devonian, and body fossils became common near the end of theLate Devonian, around 370–360 million years ago. These Devonian species all belonged to the tetrapodstem group, meaning that they did not belong to any modern tetrapod group.
Limbs evolved prior toterrestrial locomotion, but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with asemiaquatic lifestyle to exploit food and shelter on land. The firstcrown-tetrapods (those descended from thelast common ancestors of extant tetrapods) appeared by theTournaisian age of theEarly Carboniferous.[1]
The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The transition from abody plan forgill-basedaquatic respiration andtail-propelledaquatic locomotion to one that enables the animal to survive out of water and move around on land is one of the most profound evolutionary changes known.[8][9] Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic fish ancestors. These include distinct head and neck structures for feeding and movements,appendicular skeletons (shoulder andpelvic girdles in particular) forweight bearing and locomotion, more versatileeyes for seeing,middle ears for hearing, and more efficientheart andlungs for oxygen circulation and exchange outside water.
Stem-tetrapods and "fish-a-pods" were primarilyaquatic.Modern amphibians are generallysemiaquatic; the first stages of their lives are as waterborneeggs and fish-likelarvae with gills known astadpoles, and later undergometamorphosis to grow limbs and lungs and become partly terrestrial and partly aquatic. However, most tetrapod species today areamniotes, a group of mostlyterrestrial tetrapods that evolved early in theLate Carboniferous. The key amniote innovation is theamnion, which enables the eggs to retain their aqueous contents on land. Some tetrapods, such assnakes andcaecilians, have lost some or all of their limbs through further speciation and evolution; some have only concealedvestigial bones as a remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during theCarboniferous period,[10] while others, such aswhales, as recently as theCenozoic.[11][12]
The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with the earliest members of the group.[13][14][15][16]
A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinctdigits (fingers and toes), as well as legless vertebrates with limbed ancestors.[14][15] Limbs and digits are majorapomorphies (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to the group. The first vertebrates with limbs and digits evolved in theDevonian, including theLate Devonian-ageIchthyostega andAcanthostega, as well as the trackmakers of theMiddle Devonian-ageZachelmie trackways.[7]
Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage throughconvergent evolution. To resolve this potential concern, the apomorphy-based definition is often supported by an equivalentcladistic definition. Cladistics is a modern branch oftaxonomy which classifies organisms through evolutionary relationships, as reconstructed byphylogenetic analyses. A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered amonophyleticclade, a group with all of its component taxa sharing a single common ancestor.[15] In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates.[16]
A simplified cladogram demonstrating differing definitions of Tetrapoda: * Under theapomorphy-based definition used by many paleontologists, tetrapods originate at the orange star ("First vertebrates with tetrapod limb") * When restricted to thecrown group, tetrapods originate at the "last common ancestor of recent tetrapods"
A portion of tetrapod workers, led by French paleontologistMichel Laurin, prefer to restrict the definition of tetrapod to thecrown group.[13][17] A crown group is a subset of a category of animal defined by the most recent common ancestor of living representatives. This cladistic approach defines "tetrapods" as the nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is anode-basedclade, a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).[15]
Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such asIchthyostega andAcanthostega, certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along thestem group, a subset of animals related to, but not within, the crown group. The stem and crown group together are combined into thetotal group, given the nameTetrapodomorpha, which refers to all animals closer to living tetrapods than to Dipnoi (lungfishes), the next closest group of living animals.[18] Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as the presence of limbs and digits.
Laurin's approach to the definition of tetrapods is rooted in the belief that the term has more relevance forneontologists (an informal term used for biologists specializing in living organizms) than paleontologists (who primarily use the apomorphy-based definition).[16] In 1998, he re-established the defunct historical termStegocephali to replace the apomorphy-based definition of tetrapod used by many authors.[19] Other paleontologists use the termstem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes.[20] The term "fishapod" was popularized after the discovery and 2006 publication ofTiktaalik, an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits.
The two subclades of crown tetrapods areBatrachomorpha andReptiliomorpha. Batrachomorphs are all animals sharing a more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians.[21] Gaffney (1979) provided the nameNeotetrapoda to the crown group of tetrapods, though few subsequent authors followed this proposal.[16]
The earliest fossils attributed to crown-group tetrapods are footprints from the earliest Carboniferous (Tournaisian) of Australia, which appear to belong to earlyamniotes or potentially evensauropsids. Prior to the discovery of these prints, the earliest evidence of crown-group tetrapods weretemnospondyl footprints from slightly later in the Tournaisian, with the earliest body fossils being of the temnospondylBalanerpeton from theViséan.[1]
Tetrapoda includes the four traditional livingclasses: amphibians, reptiles, birds and mammals. Overall, the biodiversity oflissamphibians,[22] as well as of tetrapods generally,[23] has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises, such as thePermian–Triassic extinction event, which at least affected amniotes.[24] The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of species and the number of niches that tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on a variety of diets.[23] The following table shows summary estimates for each tetrapod class from theIUCN Red List of Threatened Species, 2014.3, for the number ofextant species that have been described in the literature, as well as the number ofthreatened species.[25]
IUCN global summary estimates for extant tetrapod species as of 2023[25]
Carl Linnaeus's 1735 classification of animals, with tetrapods occupying the first three classes
The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on grossanatomical andphysiological traits.[26]Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have a full complement of limbs. Similar considerations apply tocaecilians andaquatic mammals. Newer taxonomy is frequently based oncladistics instead, giving a variable number of major "branches" (clades) of the tetrapodfamily tree.
As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example, the birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. Inphylogenetic nomenclature, in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-typeof dinosaurs.
The tetrapods, including all large- and medium-sized land animals, have been among the best understood animals since earliest times. ByAristotle's time, the basic division between mammals, birds and egg-laying tetrapods (the "herptiles") was well known, and the inclusion of the legless snakes into this group was likewise recognized.[27] With the birth of modernbiological classification in the 18th century,Linnaeus used the same division, with the tetrapods occupying the first three of his six classes of animals.[28] While reptiles and amphibians can be quite similar externally, the French zoologistPierre André Latreille recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.[29]
With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominantly done by experts working within classes. In the early 1930s, Americanvertebrate palaeontologistAlfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in hisVertebrate Paleontology.[30] This classical scheme with minor variations is still used in works where systematic overview is essential, e.g.Benton (1998) and Knobill and Neill (2006).[31][32] While mostly seen in general works, it is also still used in some specialist works like Fortuny et al. (2011).[33] The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):[34]
SubclassTheria – live-bearing mammals, including marsupials and placentals
This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique fromcladistics. The earliest tetrapods are grouped under class Amphibia, although several of the groups are more closely related toamniotes than tomodern day amphibians. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendants oftheropod dinosaurs.Basal non-mammaliansynapsids ("mammal-like reptiles") traditionally also sort under class Reptilia as a separate subclass,[26] but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely onphylogeny, disregarding the anatomy and physiology.
The first tetrapods probably evolved in theEmsian stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments.[35][36]The very earliest tetrapods would have been animals similar toAcanthostega, with legs and lungs as well as gills, but still primarily aquatic and unsuited to life on land.
The earliest tetrapods inhabited saltwater, brackish-water, and freshwater environments, as well as environments of highly variable salinity. These traits were shared with many early lobed-finned fishes. As early tetrapods are found on two Devonian continents, Laurussia (Euramerica) andGondwana, as well as the island ofNorth China, it is widely supposed that early tetrapods were capable of swimming across the shallow (and relatively narrow) continental-shelf seas that separated these landmasses.[37][38][39]
A notable feature ofTiktaalik is the absence of bones covering the gills. These bones would otherwise connect the shoulder girdle with skull, making the shoulder girdle part of the skull. With the loss of the gill-covering bones, the shoulder girdle is separated from the skull, connected to the torso by muscle and other soft-tissue connections. The result is the appearance of the neck. This feature appears only in tetrapods andTiktaalik, not other tetrapodomorph fishes.Tiktaalik also had a pattern of bones in the skull roof (upper half of the skull) that is similar to the end-Devonian tetrapodIchthyostega. The two also shared a semi-rigid ribcage of overlapping ribs, which may have substituted for a rigid spine. In conjunction with robust forelimbs and shoulder girdle, bothTiktaalik andIchthyostega may have had the ability to locomote on land in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging behind. Finally,Tiktaalik fin bones are somewhat similar to the limb bones of tetrapods.[43][44]
However, there are issues with positingTiktaalik as a tetrapod ancestor. For example, it had a long spine with far more vertebrae than any known tetrapod or other tetrapodomorph fish. Also the oldest tetrapod trace fossils (tracks and trackways) predate it by a considerable margin. Several hypotheses have been proposed to explain this date discrepancy: 1) The nearest common ancestor of tetrapods andTiktaalik dates to the Early Devonian. By this hypothesis, the lineage is the closest to tetrapods, butTiktaalik itself was a late-surviving relic.[45] 2)Tiktaalik represents a case of parallel evolution. 3) Tetrapods evolved more than once.[46][47]
The oldest evidence for the existence of tetrapods comes fromtrace fossils: tracks (footprints) andtrackways found inZachełmie, Poland, dated to theEifelian stage of the Middle Devonian,390 million years ago,[7] although these traces have also been interpreted as the ichnogenusPiscichnus (fish nests/feeding traces).[48] The adult tetrapods had an estimated length of 2.5 m (8 feet), and lived in a lagoon with an average depth of 1–2 m, although it is not known at what depth the underwater tracks were made. The lagoon was inhabited by a variety of marine organisms and was apparently salt water. The average water temperature was 30 degrees C (86 F).[49][50] The second oldest evidence for tetrapods, also tracks and trackways, date from ca. 385 Mya (Valentia Island, Ireland).[51][52]
The oldest partial fossils of tetrapods date from theFrasnian beginning ≈380 mya. These includeElginerpeton andObruchevichthys.[53] Some paleontologists dispute their status as true (digit-bearing) tetrapods.[54]
All known forms of Frasnian tetrapods became extinct in theLate Devonian extinction, also known as the end-Frasnian extinction.[55] This marked the beginning of a gap in the tetrapod fossil record known as theFamennian gap, occupying roughly the first half of the Famennian stage.[55]
The oldest near-complete tetrapod fossils,Acanthostega andIchthyostega, date from the second half of the Fammennian.[56][57] Although both were essentially four-footed fish,Ichthyostega is the earliest known tetrapod that may have had the ability to pull itself onto land and drag itself forward with its forelimbs. There is no evidence that it did so, only that it may have been anatomically capable of doing so.[58][59]
The publication in 2018 ofTutusius umlambo andUmzantsia amazana from high latitude Gondwana setting indicate that the tetrapods enjoyed a global distribution by the end of the Devonian and even extended into the high latitudes.[60]
Ichthyostega (a four-limbed stem-tetrapod, Late Devonian)
The end-Fammenian marked another extinction, known as the end-Fammenian extinction or theHangenberg event, which is followed by another gap in the tetrapod fossil record,Romer's gap, also known as theTournaisian gap.[61] This gap, which was initially 30 million years, but has been gradually reduced over time, currently occupies much of the 13.9-million year Tournaisian, the first stage of the Carboniferous period.[62]Tetrapod-like vertebrates first appeared in the Early Devonian period, and species with limbs and digits were around by the Late Devonian.[63] These early "stem-tetrapods" included animals such asIchthyostega,[49] with legs and lungs as well as gills, but still primarily aquatic and poorly adapted for life on land. The Devonian stem-tetrapods went through two majorpopulation bottlenecks during theLate Devonian extinctions, also known as theend-Frasnian andend-Fammenian extinctions. These extinction events led to the disappearance of stem-tetrapods with fish-like features.[64] When stem-tetrapods reappear in the fossil record in earlyCarboniferous deposits, some 10 million years later, the adult forms of some are somewhat adapted to a terrestrial existence.[62][65] Why they went to land in the first place is still debated.
Edops (an early temnospondyl, Late Carboniferous - Early Permian)
During the early Carboniferous, the number of digits onhands and feet of stem-tetrapods became standardized at no more than five, as lineages with more digits died out (exceptions within crown-group tetrapods arose among some secondarily aquatic members). By the very beginning of the Carboniferous,[1] the stem-tetrapods had radiated into two branches of true ("crown group") tetrapods, one ancestral to modern amphibians and the other ancestral to amniotes.Modern amphibians are most likely derived from thetemnospondyls, a particularly diverse and long-lasting group of tetrapods. A less popular proposal draws comparisons to the "lepospondyls", an eclectic mixture of various small tetrapods, including burrowing, limbless, and other bizarrely-shaped forms. Thereptiliomorphs (sometimes known as "anthracosaurs") were the relatives and ancestors of theamniotes (reptiles, mammals, and kin). The first amniotes are known from the early part of theLate Carboniferous. All basal amniotes had a small body size, like many of their contemporaries, though some Carboniferous tetrapods evolved into large crocodile-like predators, informally known as "labyrinthodonts".[66][67] Amphibians must return to water to lay eggs; in contrast, amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land.
Amphibians and amniotes were affected by theCarboniferous rainforest collapse (CRC), an extinction event that occurred around 307 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Amniotes and temnospondyls in particular were more suited to the new conditions. They invaded new ecological niches and began diversifying their diets to include plants and other tetrapods, previously having been limited to insects and fish.[68]
Diadectes (a terrestrial diadectomorph, Early Permian)
In thePermian period, amniotes became particularly well-established, and two important clades filled in most terrestrial niches: thesauropsids and thesynapsids. The latter were the most important and successful Permian land animals, establishing complex terrestrial ecosystems of predators and prey while acquiring various adaptations retained by their modern descendants, the mammals. Sauropsid diversity was more subdued during the Permian, but they did begin to fracture into several lineages ancestral to modern reptiles. Amniotes were not the only tetrapods to experiment with prolonged life on land. Some temnospondyls,seymouriamorphs, anddiadectomorphs also successfully filled terrestrial niches in the earlier part of the Permian. Non-amniote tetrapods declined in the later part of the Permian.
The end of the Permian saw a major turnover in fauna during thePermian–Triassic extinction event. There was a protracted loss of species, due to multiple extinction pulses.[69] Many of the once large and diverse groups died out or were greatly reduced.
Thediapsid reptiles (a subgroup of the sauropsids) strongly diversified during theTriassic, giving rise to theturtles,pseudosuchians (crocodilian ancestors),dinosaurs,pterosaurs, andlepidosaurs, along with many other reptile groups on land and sea. Some of the new Triassic reptiles would not survive into theJurassic, but others would flourish during the Jurassic.Lizards, turtles, dinosaurs, pterosaurs,crocodylomorphs, andplesiosaurs were particular beneficiaries of the Triassic-Jurassic transition.Birds, a particular subset oftheropod dinosaurs capable of flight via feathered wings, evolved in the Late Jurassic. In theCretaceous,snakes developed from lizards,rhynchocephalians (tuataras and kin) declined, and modern birds and crocodilians started to establish themselves.
Among the characteristic Paleozoic non-amniote tetrapods, few survived into the Mesozoic.Temnospondyls briefly recovered in the Triassic, spawning the large aquaticstereospondyls and the small terrestrial lissamphibians (the earliest frogs, salamanders, and caecilians). However, stereospondyl diversity would crash at the end of the Triassic. By the Late Cretaceous, the only surviving amphibians were lissamphibians. Many groups of synapsids, such asanomodonts andtherocephalians, that once comprised the dominant terrestrial fauna of the Permian, also became extinct during the Triassic. During the Jurassic, one synapsid group (Cynodontia) gave rise to the modernmammals, which survived through the rest of the Mesozoic to later diversify during the Cenozoic. TheCretaceous-Paleogene extinction event at the end of the Mesozoic killed off many organisms, including all the non-avian dinosaurs and nearly all marine reptiles. Birds survived and diversified during the Cenozoic, similar to mammals.
Following the great extinction event at the end of the Mesozoic, representatives of seven major groups of tetrapods persisted into theCenozoic era. One of them, a group of semiaquatic reptiles known as theChoristodera, became extinct 11 million years ago for unclear reasons.[70] The seven Cenozoic tetrapods groups are:
Stem tetrapods are all animals more closely related to tetrapods than to lungfish, but excluding the tetrapod crown group. The cladogram below illustrates the relationships of stem-tetrapods. All these lineages are extinct except for Dipnomorpha and Tetrapoda; from Swartz, 2012:[71]
Crown tetrapods are defined as the nearest common ancestor of all living tetrapods (amphibians, reptiles, birds, and mammals) along with all of the descendants of that ancestor.
The inclusion of certain extinct groups in the crown Tetrapoda depends on the relationships of modern amphibians, orlissamphibians. There are currently three major hypotheses on the origins of lissamphibians. In the temnospondyl hypothesis (TH), lissamphibians are most closely related to dissorophoidtemnospondyls, which would make temnospondyls tetrapods. In the lepospondyl hypothesis (LH), lissamphibians are the sister taxon of lysorophianlepospondyls, making lepospondyls tetrapods and temnospondyls stem-tetrapods. In the polyphyletic hypothesis (PH), frogs and salamanders evolved from dissorophoid temnospondyls while caecilians come out of microsaur lepospondyls, making both lepospondyls and temnospondyls true tetrapods.[72][73]
This hypothesis comes in a number of variants, most of which have lissamphibians coming out of the dissorophoid temnospondyls, usually with the focus on amphibamids and branchiosaurids.[74]
The temnospondyl hypothesis is the currently favored or majority view, supported by Rutaet al (2003a,b), Ruta and Coates (2007), Coateset al (2008), Sigurdsen and Green (2011), and Schoch (2013, 2014).[73][75]
Cladogram modified after Coates, Ruta and Friedman (2008).[76]
This hypothesis has batrachians (frogs and salamanders) coming out of dissorophoid temnospondyls, with caecilians out of microsaur lepospondyls. There are two variants, one developed byCarroll,[78] the other by Anderson.[79]
Cladogram modified after Schoch, Frobisch, (2009).[80]
The tetrapod's ancestral fish, tetrapodomorph, possessed similar traits to those inherited by the early tetrapods, including internal nostrils and large, fleshypectoral andpelvic fins built on bones that could give rise to the tetrapod limb. To propagate in the terrestrialenvironment, animals had to overcome certain challenges. Their bodies needed additional support, becausebuoyancy was no longer a factor. Water retention was now important, since it was no longer the livingmatrix, and could be lost easily to the environment. Finally, animals needed new sensory input systems to have any ability to function reasonably on land.
The brain only filled half of the skull in the early tetrapods. The rest was filled with fatty tissue or fluid, which gave the brain space for growth as they adapted to a life on land.[81] Thepalatal and jaw structures of tetramorphs were similar to those of early tetrapods, and theirdentition was similar too, with labyrinthine teeth fitting in a pit-and-tooth arrangement on the palate. A major difference between early tetrapodomorph fishes and early tetrapods was in the relative development of the front and backskull 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 notable characteristic 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; theorbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull.
In tetrapodomorph fishes such asEusthenopteron, the part of the body that would later become the neck was covered by a number of gill-covering bones known as theopercular series. These bones functioned as part of pump mechanism for forcing water through the mouth and past the gills. When the mouth opened to take in water, the gill flaps closed (including the gill-covering bones), thus ensuring that water entered only through the mouth. When the mouth closed, the gill flaps opened and water was forced through the gills.
InAcanthostega, a basal tetrapod, the gill-covering bones have disappeared, although the underlying gill arches are still present. Besides the opercular series,Acanthostega also lost the throat-covering bones (gular series). The opercular series and gular series combined are sometimes known as the operculo-gular or operculogular series. Other bones in the neck region lost inAcanthostega (and later tetrapods) include the extrascapular series and the supracleithral series. Both sets of bones connect the shoulder girdle to the skull. With the loss of these bones, tetrapods acquired a neck, allowing the head to rotate somewhat independently of the torso. This, in turn, required stronger soft-tissue connections between head and torso, including muscles and ligaments connecting the skull with the spine and shoulder girdle. Bones and groups of bones were also consolidated and strengthened.[82]
In Carboniferous tetrapods, the neck joint (occiput) provided a pivot point for the spine against the back of the skull. In tetrapodomorph fishes such asEusthenopteron, no such neck joint existed. Instead, thenotochord (a rod made of proto-cartilage) entered a hole in the back of the braincase and continued to the middle of the braincase.Acanthostega had the same arrangement asEusthenopteron, and thus no neck joint. The neck joint evolved independently in different lineages of early tetrapods.[83]
All tetrapods appear to hold their necks at the maximum possible vertical extension when in a normal, alert posture.[84]
Tetrapods had a tooth structure known as "plicidentine" characterized by infolding of the enamel as seen in cross-section. The more extreme version found in early tetrapods is known as "labyrinthodont" or "labyrinthodont plicidentine". This type of tooth structure has evolved independently in several types of bony fishes, both ray-finned and lobe finned, some modern lizards, and in a number of tetrapodomorph fishes. The infolding appears to evolve when a fang or large tooth grows in a small jaw, erupting when it is still weak and immature. The infolding provides added strength to the young tooth, but offers little advantage when the tooth is mature. Such teeth are associated with feeding on soft prey in juveniles.[85][86]
With the move from water to land, the spine 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.
The shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The early sarcopterygians'cleithrum was retained as theclavicle, and theinterclavicle 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. The upper portion of the girdle had a flat,scapular blade (shoulder bone), with theglenoid cavity situated below performing as thearticulation surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.
Thepelvic 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 sacralribs. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was theilium, while the broad ventral plate was composed of thepubis in front and theischium in behind. The three bones met at a single point in the center of the pelvic triangle called theacetabulum, providing a surface of articulation for the femur.
Fleshy lobe-fins supported on bones seem to have been an ancestral trait of all bony fishes (Osteichthyes). The ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a different direction. Thetetrapodomorph ancestors of the tetrapods further developed their lobe fins. The paired fins had bones distinctlyhomologous to thehumerus,ulna, andradius in the fore-fins and to thefemur,tibia, andfibula in the pelvic fins.[87]
The paired fins of the early sarcopterygians were smaller than tetrapod limbs, but the skeletal structure was very similar in that the early sarcopterygians had a single proximal bone (analogous to thehumerus orfemur), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of thecarpus/tarsus andphalanges of a hand.
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 nearright angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal'sstrength 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 Carboniferousrocks.
Early tetrapods had a wide gaping jaw with weak muscles to open and close it. In the jaw were moderate-sized palatal and vomerine (upper) and coronoid (lower) fangs, as well rows of smaller teeth. This was in contrast to the larger fangs and small marginal teeth of earlier tetrapodomorph fishes such asEusthenopteron. Although this indicates a change in feeding habits, the exact nature of the change in unknown. Some scholars have suggested a change to bottom-feeding or feeding in shallower waters (Ahlberg and Milner 1994). Others have suggesting a mode of feeding comparable to that of the Japanese giant salamander, which uses both suction feeding and direct biting to eat small crustaceans and fish. A study of these jaws shows that they were used for feeding underwater, not on land.[88]
In later terrestrial tetrapods, two methods of jaw closure emerge: static and kinetic inertial (also known as snapping). In the static system, the jaw muscles are arranged in such a way that the jaws have maximum force when shut or nearly shut. In the kinetic inertial system, maximum force is applied when the jaws are wide open, resulting in the jaws snapping shut with great velocity and momentum. Although the kinetic inertial system is occasionally found in fish, it requires special adaptations (such as very narrow jaws) to deal with the high viscosity and density of water, which would otherwise impede rapid jaw closure.
The tetrapodtongue is built from muscles that once controlled gill openings. The tongue is anchored to thehyoid bone, which was once the lower half of a pair of gill bars (the second pair after the ones that evolved into jaws).[89][90][91] The tongue did not evolve until the gills began to disappear.Acanthostega still had gills, so this would have been a later development. In an aquatically feeding animals, the food is supported by water and can literally float (or get sucked in) to the mouth. On land, the tongue becomes important.
The evolution of early tetrapod respiration was influenced by an event known as the "charcoal gap", a period of more than 20 million years, in the middle and late Devonian, when atmospheric oxygen levels were too low to sustain wildfires.[92] During this time, fish inhabitinganoxic waters (very low in oxygen) would have been under evolutionary pressure to develop their air-breathing ability.[93][94][95]
Early tetrapods probably relied on four methods ofrespiration: withlungs, withgills,cutaneous respiration (skin breathing), and breathing through the lining of the digestive tract, especially the mouth.
The early tetrapodAcanthostega had at least three and probably four pairs of gill bars, each containing deep grooves in the place where one would expect to find the afferent branchial artery. This strongly suggests that functional gills were present.[96] Some aquatic temnospondyls retained internal gills at least into the early Jurassic.[97] Evidence of clear fish-like internal gills is present inArchegosaurus.[98]
Lungs originated as an extra pair of pouches in the throat, behind the gill pouches.[99] They were probably present in the last common ancestor of bony fishes. In some fishes they evolved into swim bladders for maintainingbuoyancy.[100][101] Lungs and swim bladders are homologous (descended from a common ancestral form) as is the case for the pulmonary artery (which delivers de-oxygenated blood from the heart to the lungs) and the arteries that supply swim bladders.[102] Air was introduced into the lungs by a process known asbuccal pumping.[103][104]
In the earliest tetrapods, exhalation was probably accomplished with the aid of the muscles of the torso (the thoracoabdominal region). Inhaling with the ribs was either primitive for amniotes, or evolved independently in at least two different lineages of amniotes. It is not found in amphibians.[105][106] The muscularized diaphragm is unique to mammals.[107]
Although tetrapods are widely thought to have inhaled through buccal pumping (mouth pumping), according to an alternative hypothesis, aspiration (inhalation) occurred through passive recoil of theexoskeleton in a manner similar to the contemporary primitive ray-finned fishPolypterus. This fish inhales through itsspiracle (blowhole), an anatomical feature present in early tetrapods. Exhalation is powered by muscles in the torso. During exhalation, the bony scales in the upper chest region become indented. When the muscles are relaxed, the bony scales spring back into position, generating considerable negative pressure within the torso, resulting in a very rapid intake of air through the spiracle.[108][109][110]
Skin breathing, known ascutaneous respiration, is common in fish and amphibians, and occur both in and out of water. In some animals waterproof barriers impede the exchange of gases through the skin. For example, keratin in human skin, the scales of reptiles, and modern proteinaceous fish scales impede the exchange of gases. However, early tetrapods had scales made of highly vascularized bone covered with skin. For this reason, it is thought that early tetrapods could engage some significant amount of skin breathing.[111]
Although air-breathing fish can absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the ability of lungs to discharge CO2 came about gradually, and was not fully attained until the evolution of amniotes. The same limitation applies to gut air breathing (GUT), i.e., breathing with the lining of the digestive tract.[112] Tetrapod skin would have been effective for both absorbing oxygen and discharging CO2, but only up to a point. For this reason, early tetrapods may have experienced chronichypercapnia (high levels of blood CO2). This is not uncommon in fish that inhabit waters high in CO2.[113]According to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a mechanism for relievingrespiratory acidosis (acidic blood caused by excess CO2) through compensatorymetabolic alkalosis.[114]
Early tetrapods probably had a three-chamberedheart, as do modern amphibians and lepidosaurian and chelonian 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.[115]
The difference indensity between air and water causessmells (certain chemical compounds detectable bychemoreceptors) to behave differently. An animal first venturing out onto land would have difficulty in locating such chemical signals if its sensory apparatus had evolved in the context of aquatic detection. Thevomeronasal organ also evolved in the nasal cavity for the first time, for detecting pheromones from biological substrates on land, though it was subsequently lost or reduced to vestigial in some lineages, likearchosaurs andcatarrhines, but expanded in others likelepidosaurs.[116]
Fish have alateral line system that detectspressure 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 early tetrapods, suggesting either an aquatic or largely aquatichabitat. Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by thehigher vertebrates.
Changes in the eye came about because the behavior of light at the surface of the eye differs between an air and water environment due to the difference inrefractive index, so thefocal length of thelens altered to function in air. Theeye was now exposed to a relatively dry environment rather than being bathed by water, soeyelids developed andtear ducts evolved to produce a liquid to moisten the eyeball.
Airvibrations could not set uppulsations through the skull as in a proper auditoryorgan. Thespiracle was retained as theotic notch, eventually closed in by thetympanum, a thin, tightmembrane of connective tissue also called the eardrum (however this and the otic notch were lost in the ancestralamniotes, and later eardrums were obtained independently).
Thehyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form thecolumella. Situated between the tympanum and braincase in an air-filled cavity, the columella was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the columella became an important element in animpedance matching system, coupling airborne sound waves to the receptor system of the inner ear. This system had evolved independently within several different amphibianlineages.
The impedance matching ear had to meet certain conditions to work. The columella had to be perpendicular to the tympanum, small and light enough to reduce itsinertia, and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHzfrequencies, the footplate of the columella is 1/20th the area of the tympanum. However, in early amphibians the columella was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the columella more probably just supported the brain case against the cheek.
Only in the early Triassic, about a hundred million years after they conquered land, did the tympanicmiddle ear evolve (independently) in all the tetrapod lineages.[120] About fifty million years later (late Triassic), in mammals, the columella was reduced even further to become thestapes.
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