Which Of The Following Animals Is A Tetrapod, An Amniote, And An Ectotherm?

Superclass of the first iv-limbed vertebrates and their descendants

Tetrapods Temporal range: Tournaisian - Present ^[i] PreꞒ Ꞓ O S D C P T J Thou Pg N

Clockwise from top left: Mercurana myristicapaulstris, a shrub frog; Dermophis mexicanus, a legless amphibian; Equus quagga, a plains zebra; Sterna maxima, a tern (seabird); Pseudotrapelus sinaitus, a Sinai agama; Tachyglossus aculeatus, a short-beaked echidna
Scientific nomenclature
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Eotetrapodiformes
Clade:	Elpistostegalia
Clade:	Stegocephalia
Superclass:	Tetrapoda Hatschek & Cori, 1896 [Laurin]^[two] ^[three]
Subgroups
Batrachomorpha / Amphibia various extinct clades Lissamphibia Reptiliomorpha / Pan-Amniota various extinct clades Amniota (Crown group) Synapsida Sauropsida

Tetrapods (;^[four] from Ancient Greek τετρα- (tetra-) 'four', and πούς (poús) 'foot') are iv-limbed animals constituting the superclass Tetrapoda (). Information technology includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mammals). Tetrapods evolved from a grouping of animals known every bit the Tetrapodomorpha which, in turn, evolved from ancient sarcopterygian fish around 390 million years ago in the middle Devonian menses;^[five] their forms were transitional between lobe-finned fishes and the four-limbed tetrapods. The outset crown-tetrapods (from a traditional, apomorphy-based perspective) appeared by the very early Carboniferous, 350 one thousand thousand years ago.^[six] The specific aquatic ancestors of the tetrapods and the procedure by which they colonized Earth'south land afterwards emerging from water remains unclear. The change from a body plan for breathing and navigating in water to a torso program enabling the animal to movement on land is one of the most profound evolutionary changes known.^[vii] ^[8] The first tetrapods (stem) or "fishapods" were primarily aquatic. Modern amphibians, which evolved from earlier groups, are by and large semiaquatic; the offset stage of their lives is as fish-like tadpoles, and later stages are partly terrestrial and partly aquatic. Withal, most tetrapod species today are amniotes, virtually of which are terrestrial tetrapods whose co-operative evolved from earlier tetrapods nearly 340 million years agone (crown amniotes evolved 318 one thousand thousand years ago).^{[ citation needed ]} The key innovation in amniotes over amphibians is the amnion, which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water. (Some amniotes later evolved internal fertilization, although many aquatic species outside the tetrapod tree had evolved such before the tetrapods appeared, e.1000. Materpiscis.)

1 group of amniotes diverged into the reptiles, which includes lepidosaurs, dinosaurs (which includes birds), crocodilians, turtles, and extinct relatives; while another group of amniotes diverged into the mammals and their extinct relatives. Amniotes include the tetrapods that further evolved for flight—such as birds from amongst the dinosaurs, and bats from among the mammals.

Some tetrapods, such every bit snakes and caecilians, have lost some or all of their limbs through farther speciation and evolution; some accept only concealed vestigial bones as a remnant of the limbs of their distant ancestors. Others returned to existence amphibious or otherwise living partially or fully aquatic lives, the starting time during the Carboniferous menses,^[ix] others equally recently as the Cenozoic.^[10] ^[11]

Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic ancestors. These include the structure of the jaw and teeth for feeding on land, limb girdles and extremities for land locomotion, lungs for respiration in air, and optics and ears for seeing and hearing in air.

Definitions [edit]

Tetrapods can be defined in cladistics equally 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. This is a node-based definition (the node being the nearest common antecedent). The group so divers is the crown group, or crown tetrapods. The term tetrapodomorph is used for the stem-based definition: whatsoever fauna that is more than closely related to living amphibians, reptiles, birds, and mammals than to living dipnoi (lungfishes). The group and so divers is known as the tetrapod total group.^[12]

Stegocephalia is a larger group equivalent to some broader uses of the word tetrapod, used by scientists who adopt to reserve tetrapod for the crown group (based on the nearest common ancestor of living forms).^[thirteen] Such scientists employ the term stem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including the tetrapodomorph fishes.^[14]

The 2 subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha. Batrachomorphs are all animals sharing a more contempo common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common beginnings with living amniotes than with living amphibians.^[fifteen]

Biodiversity [edit]

Tetrapoda includes four living classes: amphibians, reptiles, mammals, and birds. Overall, the biodiversity of lissamphibians,^[xvi] as well as of tetrapods more often than not,^[17] has grown exponentially over fourth dimension; the more than 30,000 species living today are descended from a single amphibian group in the Early on to Center Devonian. However, that diversification procedure was interrupted at to the lowest degree a few times by major biological crises, such as the Permian–Triassic extinction event, which at to the lowest degree afflicted amniotes.^[18] The overall composition of biodiversity was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of niches that tetrapods take occupied. The showtime tetrapods were aquatic and fed primarily on fish. Today, the Globe supports a great diversity of tetrapods that alive in many habitats and subsist on a diverseness of diets.^[17] The following tabular array shows summary estimates for each tetrapod class from the IUCN Scarlet Listing of Threatened Species, 2014.iii, for the number of extant species that accept been described in the literature, besides every bit the number of threatened species.^[19]

IUCN global summary estimates for extant tetrapod species as of 2014^[xix]
Tetrapod group	Course	Estimated number of described species^[19]	Threatened species in Scarlet List^[19]	Share of species scientifically described^[nineteen]	Best judge of percent of threatened species^[19]
Anamniotes lay eggs in h2o	Amphibians	7,302	1,957	88%	41%
Amniotes adapted to lay eggs on land	Sauropsids (Reptiles and Birds)	20,463	2,300	75%	xiii%
Amniotes adapted to lay eggs on land	Synapsids (Mammals)	5,513	1,199	100%	26%
Overall		33,278	5,456	eighty%	?

Nomenclature [edit]

Carl Linnaeus's 1735 classification of animals, with tetrapods occupying the beginning three classes

The nomenclature of tetrapods has a long history. Traditionally, tetrapods are divided into iv classes based on gross anatomical and physiological traits.^[xx] Snakes and other legless reptiles are considered tetrapods considering they are sufficiently similar other reptiles that accept a full complement of limbs. Similar considerations apply to caecilians and aquatic mammals. Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" (clades) of the tetrapod family unit tree.

Every bit is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological nomenclature sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly unlike characteristics. For case, the birds, which evolved from the dinosaurs, are divers as a separate grouping from them, because they stand for a distinct new blazon of concrete form and functionality. In phylogenetic classification, in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in dissimilarity to each other, but rather birds are a sub-type of dinosaurs.

History of classification [edit]

The tetrapods, including all large- and medium-sized land animals, accept been amongst the best understood animals since earliest times. By Aristotle's time, the basic partition betwixt mammals, birds and egg-laying tetrapods (the "herptiles") was well known, and the inclusion of the legless snakes into this group was too recognized.^[21] With the birth of modern biological nomenclature in the 18th century, Linnaeus used the same division, with the tetrapods occupying the kickoff three of his six classes of animals.^[22] While reptiles and amphibians can exist quite similar externally, the French zoologist Pierre André Latreille recognized the large physiological differences at the outset of the 19th century and carve up the herptiles into ii classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals.^[23]

Modern classification [edit]

With the basic nomenclature of tetrapods settled, a half a century followed where the nomenclature of living and fossil groups was predominately washed by experts working within classes. In the early on 1930s, American vertebrate palaeontologist Alfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his Vertebrate Paleontology.^[24] This classical scheme with modest variations is notwithstanding used in works where systematic overview is essential, due east.g. Benton (1998) and Knobill and Neill (2006).^[25] ^[26] While generally seen in general works, it is also still used in some specialist works like Fortuny et al. (2011).^[27] The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):^[28]

Superclass Tetrapoda – four-limbed vertebrates
- Class Amphibia – amphibians
  - Bracket Ichthyostegalia – early fish-like amphibians-now outside tetrapoda
  - Subclass Anthracosauria – reptile-like amphibians (frequently idea to be the ancestors of the amniotes)
  - Bracket Temnospondyli – big-headed Paleozoic and Mesozoic amphibians
  - Subclass Lissamphibia – modern amphibians
- Grade Reptilia – reptiles
  - Bracket Diapsida – diapsids, including crocodiles, dinosaurs (includes birds), lizards, snakes and turtles
  - Subclass Euryapsida – euryapsids
  - Bracket Synapsida – synapsids, including mammal-similar reptiles-now a divide group (ofttimes thought to be the ancestors of mammals)
  - Subclass Anapsida – anapsids
- Class Mammalia – mammals
  - Subclass Prototheria – egg-laying mammals, including monotremes
  - Subclass Allotheria – multituberculates
  - Bracket Theria – live-bearing mammals, including marsupials and placentals

This classification is the 1 most commonly encountered in school textbooks and popular works. While orderly and easy to employ, it has come under critique from cladistics. The earliest tetrapods are grouped nether form Amphibia, although several of the groups are more closely related to amniotes than to modern day amphibians. Traditionally, birds are non considered a blazon 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 of theropod dinosaurs. Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under class Reptilia as a separate subclass,^[xx] 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 on phylogeny, disregarding the anatomy and physiology.

Development [edit]

Beginnings [edit]

Tetrapods evolved from early bony fishes (Osteichthyes), specifically from the tetrapodomorph branch of lobe-finned fishes (Sarcopterygii), living in the early to center Devonian menstruum.

The first tetrapods probably evolved in the Emsian stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments.^[29] ^[30] The very earliest tetrapods would have been animals similar to Acanthostega, with legs and lungs also as gills, but still primarily aquatic and unsuited to life on land.

The earliest tetrapods inhabited saltwater, stagnant-water, and freshwater environments, besides as environments of highly variable salinity. These traits were shared with many early lobed-finned fishes. Every bit early tetrapods are found on two Devonian continents, Laurussia (Euramerica) and Gondwana, too as the isle of North China, it is widely supposed that early on tetrapods were capable of swimming across the shallow (and relatively narrow) continental-shelf seas that separated these landmasses.^[31] ^[32] ^[33]

Since the early 20th century, several families of tetrapodomorph fishes take been proposed every bit the nearest relatives of tetrapods, among them the rhizodonts (notably Sauripterus),^[34] ^[35] the osteolepidids, the tristichopterids (notably Eusthenopteron), and more than recently the elpistostegalians (also known equally Panderichthyida) notably the genus Tiktaalik.^[36]

A notable characteristic of Tiktaalik is the absence of bones covering the gills. These basic would otherwise connect the shoulder girdle with skull, making the shoulder girdle role of the skull. With the loss of the gill-covering basic, the shoulder girdle is separated from the skull, continued to the body by muscle and other soft-tissue connections. The upshot is the appearance of the neck. This feature appears only in tetrapods and Tiktaalik, 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 tetrapod Ichthyostega. The two as well shared a semi-rigid ribcage of overlapping ribs, which may take substituted for a rigid spine. In conjunction with robust forelimbs and shoulder girdle, both Tiktaalik and Ichthyostega may take had the power to locomote on state in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging backside. Finally, Tiktaalik fin basic are somewhat similar to the limb bones of tetrapods.^[37] ^[38]

All the same, there are issues with positing Tiktaalik as a tetrapod ancestor. For example, it had a long spine with far more vertebrae than whatsoever known tetrapod or other tetrapodomorph fish. As well the oldest tetrapod trace fossils (tracks and trackways) predate it past a considerable margin. Several hypotheses have been proposed to explain this date discrepancy: ane) The nearest mutual ancestor of tetrapods and Tiktaalik dates to the Early Devonian. By this hypothesis, the lineage is the closest to tetrapods, only Tiktaalik itself was a late-surviving relic.^[39] two) Tiktaalik represents a case of parallel evolution. 3) Tetrapods evolved more than than once.^[40] ^[41]

History [edit]

The oldest bear witness for the existence of tetrapods comes from trace fossils, tracks (footprints) and trackways found in Zachełmie, Poland, dated to the Eifelian stage of the Middle Devonian, 390 million years ago,^[5] although these traces have also been interpreted every bit the ichnogenus Piscichnus (fish nests/feeding traces).^[42] The adult tetrapods had an estimated length of 2.5 k (8 feet), and lived in a lagoon with an boilerplate depth of 1–ii thousand, although it is non known at what depth the underwater tracks were made. The lagoon was inhabited by a variety of marine organisms and was plainly salt h2o. The boilerplate water temperature was 30 degrees C (86 F).^[43] ^[44] The 2d oldest prove for tetrapods, too tracks and trackways, engagement from ca. 385 Mya (Valentia Island, Ireland).^[45] ^[46]

The oldest partial fossils of tetrapods appointment from the Frasnian beginning ≈380 mya. These include Elginerpeton and Obruchevichthys.^[47] Some paleontologists dispute their status as true (digit-bearing) tetrapods.^[48]

All known forms of Frasnian tetrapods became extinct in the Late Devonian extinction, too known as the end-Frasnian extinction.^[49] This marked the start of a gap in the tetrapod fossil tape known as the Famennian gap, occupying roughly the offset half of the Famennian stage.^[49]

The oldest near-consummate tetrapod fossils, Acanthostega and Ichthyostega, appointment from the 2nd half of the Fammennian.^[l] ^[51] 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 frontward with its forelimbs. At that place is no show that information technology did then, only that information technology may have been anatomically capable of doing and so.^[52] ^[53]

The publication in 2018 of Tutusius umlambo and Umzantsia amazana from high latitude Gondwana setting signal that the tetrapods enjoyed a global distribution by the end of the Devonian and even extend into the loftier latitudes.^[54]

The end-Fammenian marked another extinction, known as the end-Fammenian extinction or the Hangenberg effect, which is followed by some other gap in the tetrapod fossil record, Romer'south gap, besides known as the Tournaisian gap.^[55] This gap, which was initially 30 million years, but has been gradually reduced over fourth dimension, currently occupies much of the 13.9-million year Tournaisian, the first phase of the Carboniferous period.^[56]

Palaeozoic [edit]

Devonian stem-tetrapods [edit]

Tetrapod-like vertebrates first appeared in the early Devonian period.^[57] These early "stem-tetrapods" would take been animals similar to Ichthyostega,^[43] with legs and lungs as well equally gills, just all the same primarily aquatic and unsuited to life on country. The Devonian stem-tetrapods went through ii major bottlenecks during the Late Devonian extinctions, also known as the end-Frasnian and stop-Fammenian extinctions. These extinction events led to the disappearance of stem-tetrapods with fish-like features.^[58] When stem-tetrapods reappear in the fossil record in early Carboniferous deposits, some 10 million years later, the adult forms of some are somewhat adapted to a terrestrial existence.^[56] ^[59] Why they went to land in the first place is still debated.

Carboniferous [edit]

During the early Carboniferous, the number of digits on hands and feet of stem-tetrapods became standardized at no more than v, as lineages with more than digits died out (exceptions within crown-group tetrapods arose amongst some secondarily aquatic members). By mid-Carboniferous times, the stem-tetrapods had radiated into two branches of truthful ("crown group") tetrapods. Modern amphibians are derived from either the temnospondyls or the lepospondyls (or possibly both), whereas the anthracosaurs were the relatives and ancestors of the amniotes (reptiles, mammals, and kin). The get-go amniotes are known from the early part of the Tardily Carboniferous. All basal amniotes, like basal batrachomorphs and reptiliomorphs, had a modest trunk size.^[60] ^[61] Amphibians must return to water to lay eggs; in contrast, amniote eggs take a membrane ensuring gas substitution out of h2o and can therefore be laid on land.

Amphibians and amniotes were affected by the Carboniferous Rainforest Plummet (CRC), an extinction event that occurred ≈300 1000000 years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Amniotes were more than suited to the new conditions. They invaded new ecological niches and began diversifying their diets to include plants and other tetrapods, previously having been express to insects and fish.^[62]

Permian [edit]

In the Permian period, in addition to temnospondyl and anthracosaur clades, at that place were two important clades of amniote tetrapods, the sauropsids and the synapsids. The latter were the most of import and successful Permian animals.

The cease of the Permian saw a major turnover in beast during the Permian–Triassic extinction upshot. There was a protracted loss of species, due to multiple extinction pulses.^[63] Many of the once big and diverse groups died out or were profoundly reduced.

Mesozoic [edit]

The diapsids (a subgroup of the sauropsids) began to diversify during the Triassic, giving rise to the turtles, crocodiles, and dinosaurs and lepidosaurs. In the Jurassic, lizards developed from some lepidosaurs. In the Cretaceous, snakes developed from lizards and mod birds branched from a group of theropod dinosaurs. Past the late Mesozoic, the groups of large, primitive tetrapod that first appeared during the Paleozoic such equally temnospondyls and amniote-like tetrapods had gone extinct. Many groups of synapsids, such equally anomodonts and therocephalians, that once comprised the dominant terrestrial fauna of the Permian, also became extinct during the Mesozoic; all the same, during the Jurassic, one synapsid group (Cynodontia) gave ascension to the mod mammals, which survived through the Mesozoic to later diversify during the Cenozoic. Also, the Cretaceous-Paleogene extinction event killed off many organisms, including all dinosaurs except neornithes, which later on diversified during the Cenozoic.

Cenozoic [edit]

Following the great faunal turnover at the end of the Mesozoic, representatives of seven major groups of tetrapods persisted into the Cenozoic era. One of them, the Choristodera, became extinct 11 1000000 years ago for unknown reasons.^[64] The surviving six are:

Lissamphibia: frogs, salamanders, and caecilians
Mammalia: monotremes, marsupials and placentals
Lepidosauria: tuataras and lizards (including amphisbaenians and snakes)
Testudines: turtles
Crocodilia: crocodiles, alligators, caimans and gharials
Dinosauria: birds

Cladistics [edit]

Stem grouping [edit]

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 stalk-tetrapods, from Swartz, 2012:^[65]

Crown grouping [edit]

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 mod amphibians, or lissamphibians. In that location are currently iii major hypotheses on the origins of lissamphibians. In the temnospondyl hypothesis (Thursday), lissamphibians are most closely related to dissorophoid temnospondyls, which would make temnospondyls tetrapods. In the lepospondyl hypothesis (LH), lissamphibians are the sis taxon of lysorophian lepospondyls, 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.^[66] ^[67]

Temnospondyl hypothesis (Thursday) [edit]

This hypothesis comes in a number of variants, nigh of which have lissamphibians coming out of the dissorophoid temnospondyls, unremarkably with the focus on amphibamids and branchiosaurids.^[68]

The Temnospondyl Hypothesis is the currently favored or majority view, supported by Ruta et al (2003a,b), Ruta and Coates (2007), Coates et al (2008), Sigurdsen and Greenish (2011), and Schoch (2013, 2014).^[67] ^[69]

Cladogram modified after Coates, Ruta and Friedman (2008).^[70]

Crown-group Tetrapoda

		full group Lissamphibia



		total group Amniota

Lepospondyl hypothesis (LH) [edit]

Cladogram modified after Laurin, How Vertebrates Left the H2o (2010).^[71]

Stegocephalia		† stem tetrapods total group Amniota total group Lissamphibia
("Tetrapoda")

Polyphyly hypothesis (PH) [edit]

This hypothesis has batrachians (frogs and salamander) coming out of dissorophoid temnospondyls, with caecilians out of microsaur lepospondyls. There are two variants, 1 developed past Carroll,^[72] the other by Anderson.^[73]

Cladogram modified after Schoch, Frobisch, (2009).^[74]

Anatomy and physiology [edit]

The tetrapod's ancestral fish, tetrapodomorph, possessed similar traits to those inherited by the early tetrapods, including internal nostrils and a big fleshy fin built on bones that could requite rise to the tetrapod limb. To propagate in the terrestrial surroundings, animals had to overcome certain challenges. Their bodies needed additional support, because buoyancy was no longer a factor. Water retention was now important, since it was no longer the living matrix, and could be lost easily to the surround. Finally, animals needed new sensory input systems to have whatsoever ability to function reasonably on country.

Skull [edit]

Their palatal and jaw structures of tetramorphs were similar to those of early tetrapods, and their dentition was similar too, with labyrinthine teeth plumbing equipment in a pit-and-tooth system on the palate. A major deviation betwixt early tetrapodomorph fishes and early on tetrapods was in the relative development of the front and back skull portions; the snout is much less developed than in nigh early on tetrapods and the post-orbital skull is exceptionally longer than an amphibian'south. A notable feature 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 end was curt; the orbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull.

Neck [edit]

In tetrapodomorph fishes such as Eusthenopteron, the part of the body that would later on become the neck was covered by a number of gill-covering bones known as the opercular series. These basic functioned equally part of pump machinery for forcing water through the oral cavity and past the gills. When the rima oris opened to take in h2o, the gill flaps closed (including the gill-covering basic), thus ensuring that water entered merely through the mouth. When the mouth closed, the gill flaps opened and water was forced through the gills.

In Acanthostega, a basal tetrapod, the gill-covering bones take disappeared, although the underlying gill arches are still present. Besides the opercular serial, Acanthostega too lost the throat-covering bones (gular series). The opercular serial and gular serial combined are sometimes known every bit the operculo-gular or operculogular series. Other bones in the cervix region lost in Acanthostega (and subsequently tetrapods) include the extrascapular series and the supracleithral serial. Both sets of bones connect the shoulder girdle to the skull. With the loss of these basic, tetrapods acquired a neck, assuasive the head to rotate somewhat independently of the torso. This, in turn, required stronger soft-tissue connections between head and trunk, including muscles and ligaments connecting the skull with the spine and shoulder girdle. Bones and groups of bones were also consolidated and strengthened.^[75]

In Carboniferous tetrapods, the neck joint (occiput) provided a pivot betoken for the spine confronting the dorsum of the skull. In tetrapodomorph fishes such as Eusthenopteron, no such neck joint existed. Instead, the notochord (a rod made of proto-cartilage) entered a pigsty in the back of the braincase and connected to the middle of the braincase. Acanthostega had the same arrangement every bit Eusthenopteron, and thus no neck joint. The cervix joint evolved independently in different lineages of early tetrapods.^[76]

All tetrapods announced to concur their necks at the maximum possible vertical extension when in a normal, alert posture.^[77]

Dentition [edit]

Cross-section of a labyrinthodont tooth

Tetrapods had a tooth construction known equally "plicidentine" characterized past infolding of the enamel equally seen in cross-section. The more extreme version establish in early tetrapods is known as "labyrinthodont" or "labyrinthodont plicidentine". This type of molar structure has evolved independently in several types of bony fishes, both ray-finned and lobe finned, some modernistic 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 still weak and immature. The infolding provides added strength to the young molar, simply offers little reward when the molar is mature. Such teeth are associated with feeding on soft prey in juveniles.^[78] ^[79]

Centric skeleton [edit]

With the motility from h2o to state, the spine had to resist the bending caused by torso 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, only the slowly strengthening limbs now transmitted their back up to the axis of the body.

Girdles [edit]

The shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The early sarcopterygians' cleithrum was retained as the clavicle, and the interclavicle was well-adult, lying on the underside of the chest. In primitive forms, the two clavicles and the interclavical could take grown ventrally in such a way equally to form a broad chest plate. The upper portion of the girdle had a flat, scapular blade (shoulder bone), with the glenoid cavity situated below performing as the articulation surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.

The pelvic girdle also was much larger than the simple plate found in fishes, accommodating more than muscles. It extended far dorsally and was joined to the backbone past one or more than specialized sacral ribs. The hind legs were somewhat specialized in that they not but supported weight, but also provided propulsion. The dorsal extension of the pelvis was the ilium, while the broad ventral plate was composed of the pubis in front and the ischium in behind. The three bones met at a single point in the centre of the pelvic triangle called the acetabulum, providing a surface of articulation for the femur.

Limbs [edit]

Fleshy lobe-fins supported on bones seem to have been an bequeathed trait of all bony fishes (Osteichthyes). The ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a dissimilar direction. The Tetrapodomorph ancestors of the Tetrapods further developed their lobe fins. The paired fins had bones distinctly homologous to the humerus, ulna, and radius in the fore-fins and to the femur, tibia, and fibula in the pelvic fins.^[eighty]

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 the humerus or femur), two bones in the next segment (forearm or lower leg), and an irregular subdivision of the fin, roughly comparable to the structure of the carpus / tarsus and phalanges of a hand.

Locomotion [edit]

In typical early tetrapod posture, the upper arm and upper leg extended well-nigh straight horizontal from its trunk, and the forearm and the lower leg extended downward from the upper segment at a near right angle. The trunk weight was not centered over the limbs, but was rather transferred 90 degrees outward and downwards through the lower limbs, which touched the basis. Nearly of the animal'due south strength was used to simply elevator its trunk off the basis for walking, which was probably slow and difficult. With this sort of posture, it could only brand brusk broad strides. This has been confirmed by fossilized footprints constitute in Carboniferous rocks.

Feeding [edit]

Early on tetrapods had a wide gaping jaw with weak muscles to open and close information technology. In the jaw were moderate-sized palatal and vomerine (upper) and coronoid (lower) fangs, as well rows of smaller teeth. This was in dissimilarity to the larger fangs and small marginal teeth of earlier tetrapodomorph fishes such equally Eusthenopteron. Although this indicates a change in feeding habits, the exact nature of the modify in unknown. Some scholars have suggested a change to bottom-feeding or feeding in shallower waters (Ahlberg and Milner 1994). Others have suggesting a manner of feeding comparable to that of the Japanese giant salamander, which uses both suction feeding and direct bitter to eat modest crustaceans and fish. A report of these jaws shows that they were used for feeding underwater, not on land.^[81]

In later terrestrial tetrapods, two methods of jaw closure emerge: static and kinetic inertial (also known equally snapping). In the static system, the jaw muscles are bundled in such a way that the jaws have maximum force when shut or almost shut. In the kinetic inertial system, maximum force is applied when the jaws are wide open up, 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 every bit very narrow jaws) to deal with the high viscosity and density of water, which would otherwise impede rapid jaw closure.

The tetrapod tongue is congenital from muscles that once controlled gill openings. The tongue is anchored to the hyoid bone, which was one time the lower half of a pair of gill bars (the 2d pair later on the ones that evolved into jaws).^[82] ^[83] ^[84] The natural language did not evolve until the gills began to disappear. Acanthostega still had gills, and so this would accept been a after development. In an aquatically feeding animals, the nutrient is supported by water and tin can literally float (or get sucked in) to the rima oris. On land, the tongue becomes important.

Respiration [edit]

The evolution of early tetrapod respiration was influenced by an upshot known as the "charcoal gap", a period of more than than 20 million years, in the middle and tardily Devonian, when atmospheric oxygen levels were too low to sustain wildfires.^[85] During this time, fish inhabiting anoxic waters (very low in oxygen) would have been under evolutionary pressure to develop their air-breathing power.^[86] ^[87] ^[88]

Early tetrapods probably relied on four methods of respiration: with lungs, with gills, cutaneous respiration (skin breathing), and breathing through the lining of the digestive tract, especially the mouth.

Gills [edit]

The early tetrapod Acanthostega had at least iii and probably four pairs of gill confined, each containing deep grooves in the place where one would wait to discover the afferent branchial artery. This strongly suggests that functional gills were present.^[89] Some aquatic temnospondyls retained internal gills at to the lowest degree into the early Jurassic.^[90] Prove of clear fish-like internal gills is present in Archegosaurus.^[91]

Lungs [edit]

Lungs originated as an extra pair of pouches in the throat, behind the gill pouches.^[92] They were probably present in the last common ancestor of bony fishes. In some fishes they evolved into swim bladders for maintaining buoyancy.^[93] ^[94] Lungs and swim bladders are homologous (descended from a mutual ancestral class) as is the example for the pulmonary artery (which delivers de-oxygenated blood from the heart to the lungs) and the arteries that supply swim bladders.^[95] Air was introduced into the lungs past a process known as buccal pumping.^[96] ^[97]

In the primeval 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 dissimilar lineages of amniotes. It is non institute in amphibians.^[98] ^[99] The muscularized diaphragm is unique to mammals.^[100]

Recoil aspiration [edit]

Although tetrapods are widely thought to accept inhaled through buccal pumping (mouth pumping), according to an alternative hypothesis, aspiration (inhalation) occurred through passive recoil of the exoskeleton in a manner similar to the gimmicky primitive ray-finned fish polypterus. This fish inhales through its spiracle (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 leap back into position, generating considerable negative pressure within the torso, resulting in a very rapid intake of air through the spiracle. ^[101] ^[102] ^[103]

Cutaneous respiration [edit]

Skin breathing, known as cutaneous respiration, is common in fish and amphibians, and occur both in and out of h2o. In some animals waterproof barriers impede the exchange of gases through the peel. For example, keratin in homo pare, the scales of reptiles, and mod proteinaceous fish scales impede the substitution of gases. However, early tetrapods had scales fabricated of highly vascularized bone covered with pare. For this reason, it is idea that early on tetrapods could appoint some pregnant amount of skin breathing.^[104]

Carbon dioxide metabolism [edit]

Although air-breathing fish tin absorb oxygen through their lungs, the lungs tend to be ineffective for discharging carbon dioxide. In tetrapods, the power of lungs to discharge CO_two 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.^[105] Tetrapod peel would accept been effective for both absorbing oxygen and discharging CO₂, just only up to a bespeak. For this reason, early tetrapods may take experienced chronic hypercapnia (high levels of blood CO_two). This is not uncommon in fish that inhabit waters loftier in CO₂.^[106] Co-ordinate to one hypothesis, the "sculpted" or "ornamented" dermal skull roof bones found in early tetrapods may have been related to a machinery for relieving respiratory acidosis (acidic blood caused by excess CO₂) through compensatory metabolic alkalosis.^[107]

Circulation [edit]

Early tetrapods probably had a three-chambered center, as exercise modern amphibians and lepidosaurian and chelonian reptiles, in which oxygenated claret from the lungs and de-oxygenated claret from the respiring tissues enters by split atria, and is directed via a spiral valve to the appropriate vessel — aorta for oxygenated blood and pulmonary vein for deoxygenated claret. The screw valve is essential to keeping the mixing of the 2 types of blood to a minimum, enabling the animal to have college metabolic rates, and be more active than otherwise.^[108]

Senses [edit]

Olfaction [edit]

The divergence in density between air and water causes smells (certain chemical compounds detectable by chemoreceptors) to behave differently. An brute first venturing out onto country would have difficulty in locating such chemic signals if its sensory apparatus had evolved in the context of aquatic detection. The vomeronasal organ besides evolved in the nasal cavity for the first time, for detecting pheromones from biological substrates on country, though information technology was subsequently lost or reduced to vestigial in some lineages, like archosaurs and catarrhines, only expanded in others like lepidosaurs.^[109]

Lateral line organisation [edit]

Fish have a lateral line system that detects pressure fluctuations in the h2o. Such pressure is non-detectable in air, but grooves for the lateral line sense organs were establish on the skull of early tetrapods, suggesting either an aquatic or largely aquatic habitat. Modern amphibians, which are semi-aquatic, exhibit this characteristic whereas it has been retired by the higher vertebrates.

Vision [edit]

Changes in the centre came well-nigh because the behavior of calorie-free at the surface of the eye differs between an air and water environment due to the difference in refractive alphabetize, and then the focal length of the lens altered to part in air. The eye was now exposed to a relatively dry environment rather than being bathed by water, so eyelids developed and tear ducts evolved to produce a liquid to moisten the eyeball.

Early tetrapods inherited a fix of v rod and cone opsins known as the vertebrate opsins.^[110] ^[111] ^[112]

Four cone opsins were present in the first vertebrate, inherited from invertebrate ancestors:

LWS/MWS (long—to—medium—wave sensitive) - dark-green, xanthous, or red
SWS1 (short—wave sensitive) - ultraviolet or violet - lost in monotremes (platypus, echidna)
SWS2 (brusque—wave sensitive) - violet or blue - lost in therians (placental mammals and marsupials)
RH2 (rhodopsin—similar cone opsin) - green - lost separately in amphibians and mammals, retained in reptiles and birds

A single rod opsin, rhodopsin, was present in the first jawed vertebrate, inherited from a jawless vertebrate antecedent:

RH1 (rhodopsin) - blue-green - used night vision and color correction in depression-light environments

Balance [edit]

Tetrapods retained the balancing function of the inner ear from fish ancestry.

Hearing [edit]

Air vibrations could not fix pulsations through the skull every bit in a proper auditory organ. The spiracle was retained as the otic notch, somewhen closed in by the tympanum, a thin, tight membrane of connective tissue as well called the eardrum (however this and the otic notch were lost in the ancestral amniotes, and after eardrums were obtained independently).

The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the columella. Situated betwixt 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 an impedance matching system, coupling airborne sound waves to the receptor organization of the inner ear. This organisation had evolved independently inside several different amphibian lineages.

The impedance matching ear had to see certain conditions to work. The columella had to be perpendicular to the tympanum, small-scale and light plenty to reduce its inertia, and suspended in an air-filled cavity. In modern species that are sensitive to over i kHz frequencies, the footplate of the columella is one/20th the area of the tympanum. However, in early amphibians the columella was likewise large, making the footplate surface area oversized, preventing the hearing of high frequencies. So information technology appears they could only hear high intensity, low frequency sounds—and the columella more than probably just supported the encephalon instance against the cheek.

Simply in the early Triassic, about a hundred million years subsequently they conquered country, did the tympanic centre ear evolve (independently) in all the tetrapod lineages.^[113] About fifty million years later (late Triassic), in mammals, the columella was reduced fifty-fifty further to become the stapes.

Come across likewise [edit]

Trunk form
Geologic timescale
Hexapoda
Marine tetrapods
Octopod
Prehistoric life
Quadrupedalism § Quadrupeds vs. tetrapods

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