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107-592: Amniotes are tetrapod vertebrate animals belonging to the clade Amniota , a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the Carboniferous period . Those of Amniota are defined as the smallest crown clade containing humans , the Greek tortoise , and the Nile crocodile . Amniotes are distinguished from

214-454: A crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy. Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy. The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows

321-481: A (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis , as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or

428-614: 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 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,

535-438: A larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4-inch) in diameter because of diffusion problems, like

642-576: A membrane ensuring gas exchange out of water and can therefore be laid on land. Amphibians and amniotes were affected by the Carboniferous 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. In

749-453: 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. Gaffney (1979) provided the name Neotetrapoda to the crown group of tetrapods, though few subsequent authors followed this proposal. Tetrapoda includes three living classes: amphibians, reptiles, and mammals. Overall,

856-543: A pair of vestigial spurs that are remnants of the hindlimbs . Tetrapods evolved from a group of primitive semiaquatic animals known as the Tetrapodomorpha which, in turn, evolved from ancient lobe-finned fish ( sarcopterygians ) around 390  million years ago in the Middle Devonian period . Tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit

963-483: 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 the Carboniferous period, others as recently as the Cenozoic . One fundamental subgroup of amniotes, the sauropsids , diverged into the reptiles : lepidosaurs (lizards, snakes, and the tuatara ), archosaurs ( crocodilians and dinosaurs , of which birds are

1070-400: A rigid spine. In conjunction with robust forelimbs and shoulder girdle, both Tiktaalik and Ichthyostega 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. However, there are issues with positing Tiktaalik as

1177-417: A robust, air-breathing, respiratory system , allow amniotes to live on land as true terrestrial animals . Amniotes have the ability to procreate without water bodies . Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs (reptiles, birds and monotremes ) or nurturing fertilized eggs within

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1284-401: A separate subclass, 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. Tetrapods evolved from early bony fishes (Osteichthyes), specifically from the tetrapodomorph branch of lobe-finned fishes ( Sarcopterygii ), living in

1391-431: A series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting

1498-1770: A simplified version of the relationships found by Laurin & Reisz (1995), with the exception of turtles, which more recent morphological and molecular phylogenetic studies placed firmly within diapsids . The cladogram covers the group as defined under Gauthier's definition. † Diadectomorpha [REDACTED] Synapsida (mammals and their extinct relatives) [REDACTED] † Mesosauridae [REDACTED] † Millerettidae [REDACTED] † Pareiasauria [REDACTED] † Procolophonoidea [REDACTED] † Captorhinidae [REDACTED] † Protorothyrididae [REDACTED] Diapsida (lizards, snakes, turtles , crocodiles , dinosaurs , birds, etc.) [REDACTED] Following studies in 2022 and 2023, with Drepanosauromorpha placed sister to Weigeltisauridae ( Coelurosauravus ) in Avicephala based on Senter (2004): † Seymouriamorpha [REDACTED] † Diadectomorpha [REDACTED] † Araeoscelida [REDACTED] † Captorhinidae [REDACTED] † Protorothyris [REDACTED] † Vaughnictis [REDACTED] † Eothyris [REDACTED] † Caseidae [REDACTED] † Oedaleops [REDACTED] † Varanopsidae [REDACTED] † Ophiacodontidae [REDACTED] † Edaphosauridae [REDACTED] † Haptodus [REDACTED] † Sphenacodontidae [REDACTED] Therapsida [REDACTED] [REDACTED] [REDACTED] † Acleistorhinidae [REDACTED] Tetrapod A tetrapod ( / ˈ t ɛ t r ə ˌ p ɒ d / ; from Ancient Greek τετρα- (tetra-) 'four' and πούς (poús) 'foot')

1605-497: A single common ancestor. In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates. A portion of tetrapod workers, led by French paleontologist Michel Laurin , prefer to restrict the definition of tetrapod to the crown group . 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

1712-413: A species to have living descendants in order for it to be included in the crown group. Extinct side branches on the family tree that are descended from the most recent common ancestor of living members will still be part of a crown group. For example, if we consider the crown-birds (i.e. all extant birds and the rest of the family tree back to their most recent common ancestor), extinct side branches like

1819-432: A stem group allows the order of these acquisitions to be established, and thus the ecological and functional setting of the evolution of the major features of the group in question. Stem groups thus offer a route to integrate unique palaeontological data into questions of the evolution of living organisms. Furthermore, they show that fossils that were considered to lie in their own separate group because they did not show all

1926-452: A subset of animals related to, but not within, the crown group. The stem and crown group together are combined into the total group , given the name Tetrapodomorpha , which refers to all animals closer to living tetrapods than to Dipnoi ( lungfishes ), the next closest group of living animals. Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as

2033-449: A subset), turtles , and various other extinct forms. The remaining group of amniotes, the synapsids , include mammals and their extinct relatives. Amniotes include the only tetrapods that further evolved for flight—such as birds from among the dinosaurs, the extinct pterosaurs from earlier archosaurs, and bats from among the mammals. The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with

2140-468: 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 and Tiktaalik dates to the Early Devonian. By this hypothesis,

2247-614: A variety of diets. The following table shows summary estimates for each tetrapod class from the IUCN Red List of Threatened Species , 2014.3, for the number of extant species that have been described in the literature, as well as the number of threatened species . The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have

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2354-570: A variety of marine organisms and was apparently salt water. The average water temperature was 30 degrees C (86 F). The second oldest evidence for tetrapods, also tracks and trackways, date from ca. 385 Mya ( Valentia Island , Ireland). The oldest partial fossils of tetrapods date from the Frasnian beginning ≈380 mya. These include Elginerpeton and Obruchevichthys . Some paleontologists dispute their status as true (digit-bearing) tetrapods. All known forms of Frasnian tetrapods became extinct in

2461-410: Is also still used in some specialist works like Fortuny et al. (2011). The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001): 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 from cladistics . The earliest tetrapods are grouped under class Amphibia, although several of

2568-708: Is any four- limbed vertebrate animal of the superclass Tetrapoda ( / t ɛ ˈ t r æ p ə d ə / ). Tetrapods include all extant and extinct amphibians and amniotes , with the latter in turn evolving into two major clades , the sauropsids ( reptiles , including dinosaurs and therefore birds ) and synapsids (extinct pelycosaurs , therapsids and all extant mammals , including humans ). Hox gene mutations have resulted in some tetrapods becoming limbless ( snakes , legless lizards , and caecilians ) or two-limbed ( cetaceans , moas , and some lizards ). Nevertheless, these limbless groups still qualify as tetrapods through their ancestry, and some retain

2675-504: Is facilitated by their astragalus. Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs during the Carboniferous period. After the Carboniferous rainforest collapse , amniotes spread around Earth's land and became the dominant land vertebrates. They almost immediately diverged into two groups, namely the sauropsids (including all reptiles and birds ) and synapsids (including mammals and extinct ancestors like " pelycosaurs " and therapsids ). Among

2782-423: Is given the designation "crown-", to separate it from the group as commonly defined. Both birds and mammals are traditionally defined by their traits, and contain fossil members that lived before the last common ancestors of the living groups or, like the mammal Haldanodon , were not descended from that ancestor although they lived later. Crown-Aves and Crown-Mammalia therefore differ slightly in content from

2889-618: Is no consensus phylogeny. Stem arthropods constitute a group that has seen attention in connection with the Burgess Shale fauna. Several of the finds , including the enigmatic Opabinia and Anomalocaris have some, though not all, features associated with arthropods , and are thus considered stem arthropods. The sorting of the Burgess Shale fauna into various stem groups finally enabled phylogenetic sorting of this enigmatic assemblage and also allowed for identifying velvet worms as

2996-448: Is presented in simplified form below. With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny , but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants". As Gauthier makes use of

3103-465: 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 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 also shared a semi-rigid ribcage of overlapping ribs, which may have substituted for

3210-477: 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.g. Materpiscis .) Some tetrapods, such as snakes and caecilians , have lost some or all of their limbs through further speciation and evolution; some have only concealed vestigial bones as

3317-461: Is the crown group and all organisms more closely related to it than to any other extant organisms. In a tree analogy, it is the crown group and all branches back to (but not including) the split with the closest branch to have living members. The Pan-Aves thus contain the living birds and all (fossil) organisms more closely related to birds than to crocodilians (their closest living relatives). The phylogenetic lineage leading back from Neornithes to

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3424-402: 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. The publication in 2018 of Tutusius umlambo and Umzantsia amazana from high latitude Gondwana setting indicate that the tetrapods enjoyed a global distribution by

3531-453: Is thought by some to make the Cambrian explosion easier to understand without invoking unusual evolutionary mechanisms; however, application of the stem group concept does nothing to ameliorate the difficulties that phylogenetic telescoping poses to evolutionary theorists attempting to understand both macroevolutionary change and the abrupt character of the Cambrian explosion . Overemphasis on

3638-514: The Eifelian stage of the Middle Devonian, 390  million years ago , although these traces have also been interpreted as the ichnogenus Piscichnus (fish nests/feeding traces). 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

3745-629: The Late Devonian extinction , also known as the end-Frasnian extinction. This marked the beginning of a gap in the tetrapod fossil record known as the Famennian gap, occupying roughly the first half of the Famennian stage. The oldest near-complete tetrapod fossils, Acanthostega and Ichthyostega , date from the second half of the Fammennian. Although both were essentially four-footed fish, Ichthyostega

3852-405: The Late Devonian extinctions , also known as the end-Frasnian and end-Fammenian extinctions. These extinction events led to the disappearance of stem-tetrapods with fish-like features. 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. Why they went to land in

3959-460: The Permian period, amniotes became particularly well-established, and two important clades filled in most terrestrial niches: the sauropsids and the synapsids . 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

4066-1153: The Visean age of the Early Carboniferous . 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 a body plan for gill -based aquatic respiration and tail -propelled aquatic 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. 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 and pelvic girdles in particular) for weight bearing and locomotion, more versatile eyes for seeing, middle ears for hearing, and more efficient heart and lungs for oxygen circulation and exchange outside water. Stem-tetrapods and "fish-a-pods" were primarily aquatic . Modern amphibians , which evolved from earlier groups , are generally semiaquatic ;

4173-455: The amnion , which derives from Greek ἀμνίον ( amnion ), which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός ( amnos ), meaning "lamb". Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion , chorion , and allantois . Amniotes develop directly into

4280-405: The dinosaurs and the pterosaurs . The last common ancestor of birds and crocodilians—the first crown group archosaur—was neither bird nor crocodilian and possessed none of the features unique to either. As the bird stem group evolved, distinctive bird features such as feathers and hollow bones appeared. Finally, at the base of the crown group, all traits common to extant birds were present. Under

4387-531: The dodo or great auk are still descended from the most recent common ancestor of all living birds , so fall within the bird crown group. One very simplified cladogram for birds is shown below: † Archaeopteryx other extinct groups Neornithes (modern birds, some extinct like the dodo) In this diagram, the clade labelled "Neornithes" is the crown group of birds: it includes the most recent common ancestor of all living birds and its descendants, living or not. Although considered to be birds (i.e. members of

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4494-466: The last common ancestor of the crown group and their closest living relatives. It follows from the definition that all members of a stem group are extinct. The "stem group" is the most used and most important of the concepts linked to crown groups, as it offers a means to reify and name paraphyletic assemblages of fossils that otherwise do not fit into systematics based on living organisms. While often attributed to Jefferies (1979), Willmann (2003) traced

4601-426: The lungfish , our nearest relatives among the fishes. In addition to a series of lobe-finned fishes , they also include some of the early labyrinthodonts . Exactly what labyrinthodonts are in the stem group tetrapods rather than the corresponding crown group is uncertain, as the phylogeny of early tetrapods is not well understood. This example shows that crown and stem group definitions are of limited value when there

4708-473: The placenta . The ancestors of true amniotes, such as Casineria kiddi , which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from

4815-419: The rib cage ), the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys , more complex kidneys , the presence of an astragalus for better extremity range of motion , the diminished role of skin breathing , and the complete loss of metamorphosis , gills , and lateral lines . The presence of an amniotic buffer, of a water-impermeable skin , and of

4922-415: The tristichopterids (notably Eusthenopteron ), and more recently the elpistostegalians (also known as Panderichthyida) notably the genus Tiktaalik . A notable feature of Tiktaalik 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

5029-591: The 13.9-million year Tournaisian, the first stage of the Carboniferous period. Tetrapod-like vertebrates first appeared in the Early Devonian period, and species with limbs and digits were around by the Late Devonian. These early "stem-tetrapods" included animals such as Ichthyostega , 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 major population bottlenecks during

5136-616: 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 the Cenozoic era. One of them, a group of semiaquatic reptiles known as the Choristodera , became extinct 11 million years ago for unclear reasons. The seven Cenozoic tetrapods groups are: Stem tetrapods are all animals more closely related to tetrapods than to lungfish, but excluding

5243-521: The Crocodilia branch. Basal branch names such as Avemetatarsalia are usually more obscure. However, not so advantageous are the facts that "Pan-Aves" and "Aves" are not the same group, the circumscription of the concept of "Pan-Aves" (synonymous with Avemetatarsalia) is only evident by examination of the above tree, and calling both groups "birds" is ambiguous. Stem mammals are those in the lineage leading to living mammals, together with side branches, from

5350-503: The French zoologist Pierre 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. 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

5457-679: The Permian saw a major turnover in fauna during the Permian–Triassic extinction event . There was a protracted loss of species, due to multiple extinction pulses. Many of the once large and diverse groups died out or were greatly reduced. The diapsid reptiles (a subgroup of the sauropsids) strongly diversified during the Triassic , giving rise to the turtles , pseudosuchians (crocodilian ancestors), dinosaurs , pterosaurs , and lepidosaurs , along with many other reptile groups on land and sea. Some of

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5564-441: The apomorphy-based definition is often supported by an equivalent cladistic definition. Cladistics is a modern branch of taxonomy which classifies organisms through evolutionary relationships, as reconstructed by phylogenetic analyses . A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a monophyletic clade , a group with all of its component taxa sharing

5671-506: The best understood animals since earliest times. By Aristotle '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. With the birth of modern biological classification in the 18th century, Linnaeus used the same division, with the tetrapods occupying the first three of his six classes of animals. While reptiles and amphibians can be quite similar externally,

5778-503: The biodiversity of lissamphibians , as well as of tetrapods generally, 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 the Permian–Triassic extinction event , which at least affected amniotes. The overall composition of biodiversity

5885-482: 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. In phylogenetic 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-type of dinosaurs. The tetrapods, including all large- and medium-sized land animals, have been among

5992-517: The body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) are trackways from the Middle Devonian , and body fossils became common near the end of the Late Devonian , around 370–360 million years ago. These Devonian species all belonged to the tetrapod stem group , meaning that they were not directly related to any modern tetrapod group. Broad anatomical descriptors like "tetrapod" and "amphibian" can approximate some members of

6099-617: The characteristic Paleozoic non-amniote tetrapods, few survived into the Mesozoic. Temnospondyls briefly recovered in the Triassic, spawning the large aquatic stereospondyls 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 as anomodonts and therocephalians , that once comprised

6206-457: The clade Aves), Archaeopteryx and other extinct groups are not included in the crown group, as they fall outside the Neornithes clade, being descended from an earlier ancestor. An alternative definition does not require any members of a crown group to be extant, only to have resulted from a "major cladogenesis event". The first definition forms the basis of this article. Often, the crown group

6313-549: The class Reptilia is paraphyletic —it has given rise to two other classes not included in Reptilia. Most species described as microsaurs , formerly grouped in the extinct and prehistoric amphibian group lepospondyls , has been placed in the newer clade Recumbirostra , and shares many anatomical features with amniotes which indicates they were amniotes themselves. A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton ,

6420-514: The closest living relatives of arthropods. Stem priapulids are other early Cambrian to middle Cambrian faunas, appearing in Chengjiang to Burgess Shale. The genus Ottoia has more or less the same build as modern priapulids , but phylogenetic analysis indicates that it falls outside the crown group, making it a stem priapulid. The name plesion has a long history in biological systematics, and plesion group has acquired several meanings over

6527-457: The common definition of Aves and Mammalia. This has caused some confusion in the literature. The cladistic idea of strictly using the topology of the phylogenetic tree to define groups necessitates other definitions than crown groups to adequately define commonly discussed fossil groups. Thus, a host of prefixes have been defined to describe various branches of the phylogenetic tree relative to extant organisms. A pan-group or total group

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6634-483: The diagnostic features of a living clade, can nevertheless be related to it by lying in its stem group. Such fossils have been of particular importance in considering the origins of the tetrapods , mammals , and animals . The application of the stem group concept also influenced the interpretation of the organisms of the Burgess shale . Their classification in stem groups to extant phyla, rather than in phyla of their own,

6741-401: The diapsid line of descent. Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs , a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle. Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion , hence

6848-509: The divergence of the lineage from the Sauropsida to the last common ancestor of the living mammals. This group includes the synapsids as well as mammaliaforms like the morganucodonts and the docodonts ; the latter groups have traditionally and anatomically been considered mammals even though they fall outside the crown group mammals. Stem tetrapods are the animals belonging to the lineage leading to tetrapods from their divergence from

6955-522: The dominant terrestrial fauna of the Permian, also became extinct during the Triassic. During the Jurassic, one synapsid group ( Cynodontia ) gave rise to the modern mammals , which survived through the rest of the Mesozoic to later diversify during the Cenozoic. The Cretaceous-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

7062-590: The earliest known crown group amniotes, the oldest known sauropsid is Hylonomus and the oldest known synapsid is Asaphestera , both of which are from Nova Scotia during the Bashkirian age of the Late Carboniferous around 318  million years ago . This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma, or 312–330 Ma, and by a fossilized birth–death process study at 322–340 Ma. The term amniote comes from

7169-405: The earliest members of the group. A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct digits (fingers and toes), as well as legless vertebrates with limbed ancestors. Limbs and digits are major apomorphies (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to

7276-428: The early 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 . 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). While mostly seen in general works, it

7383-399: The early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof . In their ancestors, this notch held a spiracle , an unnecessary structure in an animal without an aquatic larval stage. There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular

7490-726: The early to middle Devonian period . The first tetrapods probably evolved in the Emsian stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments. The very earliest tetrapods would have been animals similar to Acanthostega , 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 ) and Gondwana , as well as

7597-520: The eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood. A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water. Fish and amphibian eggs have only one inner membrane,

7704-415: The embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for

7811-567: The end of the Devonian and even extend into the high latitudes. The end-Fammenian marked another extinction, known as the end-Fammenian extinction or the Hangenberg event , which is followed by another gap in the tetrapod fossil record, Romer's gap , also known as the Tournaisian gap. This gap, which was initially 30 million years, but has been gradually reduced over time, currently occupies much of

7918-505: The extinct moa ) The crown group here is Neornithes , all modern bird lineages back to their last common ancestor. The closest living relatives of birds are crocodilians . If we follow the phylogenetic lineage leading to Neornithes to the left, the line itself and all side branches belong to the stem birds until the lineage merges with that of the crocodilians. In addition to non-crown group primitive birds like Archaeopteryx , Hesperornis and Confuciusornis , stem group birds include

8025-446: The first place is still debated. During the early Carboniferous, the number of digits on hands 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 mid-Carboniferous times, the stem-tetrapods had radiated into two branches of true ("crown group") tetrapods, one ancestral to modern amphibians and

8132-421: The first stages of their lives are as waterborne eggs and fish-like larvae known as tadpoles , and later undergo metamorphosis to grow limbs and become partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes , most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous . The key innovation in amniotes over amphibians

8239-418: The formulator of phylogenetic systematics , as a way of classifying living organisms relative to their extinct relatives in his "Die Stammesgeschichte der Insekten", and the "crown" and "stem" group terminology was coined by R. P. S. Jefferies in 1979. Though formulated in the 1970s, the term was not commonly used until its reintroduction in 2000 by Graham Budd and Sören Jensen . It is not necessary for

8346-432: The full bifurcating phylogeny. Stem birds perhaps constitute the most cited example of a stem group, as the phylogeny of this group is fairly well known. The following cladogram, based on Benton (2005), illustrates the concept: Crocodilia Pterosauria Hadrosauridae Stegosauria Sauropoda Tyrannosauridae Archaeopteryx Neognathae (including the extinct dodo ) Paleognathae (including

8453-486: The group. The first vertebrates with limbs and digits evolved in the Devonian , including the Late Devonian -age Ichthyostega and Acanthostega , as well as the trackmakers of the Middle Devonian -age Zachelmie trackways . Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through convergent evolution . To resolve this potential concern,

8560-419: The groups are more closely related to amniotes than to modern 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 of theropod dinosaurs . Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under class Reptilia as

8667-437: The inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for

8774-515: The increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size. Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys (metanephros) and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside

8881-408: The island of North 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. Since the early 20th century, several families of tetrapodomorph fishes have been proposed as the nearest relatives of tetrapods, among them the rhizodonts (notably Sauripterus ), the osteolepidids ,

8988-409: The latter, however, was limited by their position in the terrestrial food-chain , which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators. While

9095-579: The lineage is the closest to tetrapods, but Tiktaalik itself was a late-surviving relic. 2) Tiktaalik represents a case of parallel evolution. 3) Tetrapods evolved more than once. [REDACTED] Coelacanthiformes (coelacanths) [REDACTED] Dipnoi (lungfish) [REDACTED] †Tetrapodomorph fishes [REDACTED] Tetrapoda [REDACTED] The oldest evidence for the existence of tetrapods comes from trace fossils : tracks (footprints) and trackways found in Zachełmie , Poland, dated to

9202-432: The mother ( marsupial and placental mammals ). This distinguishes amniotes from anamniotes ( fish and amphibians) that have to spawn in aquatic environments . Most amniotes still require regular access to drinking water for rehydration, like the semiaquatic amphibians do. They have better homeostasis in drier environments, and more efficient non-aquatic gas exchange to power terrestrial locomotion , which

9309-444: The name. A problem with this definition is that the trait ( apomorphy ) in question does not fossilize , and the status of fossil forms has to be inferred from other traits. Older classifications of the amniotes traditionally recognised three classes based on major traits and physiology : This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics , as

9416-464: The narrower one. Often, an (extinct) grouping is identified as belonging together. Later, it may be realized other (extant) groupings actually emerged within such grouping, rendering them a stem grouping. Cladistically , the new groups should then be added to the group, as paraphyletic groupings are not natural. In any case, stem groupings with living descendants should not be viewed as a cohesive group, but their tree should be further resolved to reveal

9523-412: 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 a node-based clade , a group with a common ancestry descended from a single "node" (the node being

9630-408: The nearest common ancestor of living species). Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such as Ichthyostega and Acanthostega , certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along the stem group ,

9737-776: The new Triassic reptiles would not survive into the Jurassic , but others would flourish during the Jurassic. Lizards , turtles, dinosaurs, pterosaurs, crocodylomorphs , and plesiosaurs were particular beneficiaries of the Triassic-Jurassic transition. Birds , a particular subset of theropod dinosaurs capable of flight via feathered wings, evolved in the Late Jurassic. In the Cretaceous , snakes developed from lizards, rhynchocephalians (tuataras and kin) declined, and modern birds and crocodilians started to establish themselves. Among

9844-408: The number of holes behind each eye. In anapsids , the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians , squamates , and tuataras ), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this. Molecular testing firmly places them in

9951-474: The origin of the stem group concept to Austrian systematist Othenio Abel (1914), and it was discussed and diagrammed in English as early as 1933 by A. S. Romer . Alternatively, the term "stem group" is sometimes used in a wider sense to cover any members of the traditional taxon falling outside the crown group. Permian synapsids like Dimetrodon or Anteosaurus are stem mammals in the wider sense but not in

10058-407: The other ancestral to amniotes. Modern amphibians are most likely derived from the temnospondyls , 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. The reptiliomorphs (sometimes known as " anthracosaurs ") were

10165-404: The other living tetrapod clade — the non-amniote lissamphibians ( frogs / toads , salamanders , newts and caecilians ) — by the development of three extraembryonic membranes ( amnion for embryonic protection, chorion for gas exchange , and allantois for metabolic waste disposal or storage), thicker and keratinized skin , costal respiration (breathing by expanding/constricting

10272-639: The other tetrapods somewhere during Romer's gap , when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic). The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating . Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through

10379-493: The phylogenetic split from the remaining amniotes (the Sauropsida ). Pan-Mammalia is thus an alternative name for Synapsida . A stem group is a paraphyletic assemblage composed of the members of a pan-group or total group, above, minus the crown group itself (and therefore minus all living members of the pan-group). This leaves primitive relatives of the crown groups , back along the phylogenetic line to (but not including)

10486-438: The point where it merges with the crocodilian lineage, along with all side branches, constitutes pan-birds. In addition to non-crown group primitive birds like Archaeopteryx , Hesperornis and Confuciusornis , therefore, pan-group birds would include all dinosaurs and pterosaurs as well as an assortment of non-crocodilian animals like Marasuchus . Pan-Mammalia consists of all mammals and their fossil ancestors back to

10593-454: 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 for neontologists (zoologists specializing in living animals) than paleontologists (who primarily use the apomorphy-based definition). In 1998, he re-established the defunct historical term Stegocephali to replace the apomorphy-based definition of tetrapod used by many authors. Other paleontologists use

10700-430: The relatives and ancestors of the amniotes (reptiles, mammals, and kin). The first amniotes are known from the early part of the Late 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 ". Amphibians must return to water to lay eggs; in contrast, amniote eggs have

10807-493: The stem group, but a few paleontologists opt for more specific terms such as Stegocephali . Limbs evolved prior to terrestrial locomotion , but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with a semiaquatic lifestyle to exploit food and shelter on land. The first crown -tetrapods (those descended from the last common ancestors of extant tetrapods) appeared by

10914-521: The term stem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes. The term "fishapod" was popularized after the discovery and 2006 publication of Tiktaalik , an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits. The two subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha . Batrachomorphs are all animals sharing

11021-421: 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: Dipnomorpha (lungfishes and relatives) [REDACTED] Kenichthys Rhizodontidae [REDACTED] Marsdenichthys Canowindra Koharalepis Crown group The concept was developed by Willi Hennig ,

11128-597: The widely used total-group perspective, the Crocodylomorpha would become synonymous with the Crocodilia, and the Avemetatarsalia would become synonymous with the birds, and the above tree could be summarized as Crocodilia Birds An advantage of this approach is that declaring Theropoda to be birds (or Pan-aves ) is more specific than declaring it to be a member of the Archosauria, which would not exclude it from

11235-440: The years. One use is as "nearby group" (plesion means close to in Greek ), i.e. sister group to a given taxon , whether that group is a crown group or not. The term may also mean a group, possibly paraphyletic , defined by primitive traits (i.e. symplesiomorphies ). It is generally taken to mean a side branch splitting off earlier on the phylogenetic tree than the group in question. Placing fossils in their right order in

11342-537: 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

11449-417: 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 , and diadectomorphs 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

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