44-452: An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples . Traditionally, the Anapsida are considered the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolved, making anapsids paraphyletic . It is, however, doubtful that all anapsids lack temporal fenestra as a primitive trait, and that all
88-458: 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
132-482: 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
176-466: A 2018 study that captorhinids were the first amniotes to develop caudal autotomy as a defensive function. In studied specimens a split line is present in certain caudal vertebrae that is similar to those found in modern reptiles that perform caudal autonomy. This behaviour represented significant evolutionary benefit for the animals, allowing for escape and distracting predators, as well as minimizing blood loss at an injury site. Euconcordia cunninghami
220-493: 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 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
264-417: 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
308-1713: 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] Captorhinidae See text Romeriidae Price , 1937 Captorhinidae
352-594: A valid group, but is not favoured by current workers. Anapsids in the traditional meaning of the word are not a clade, but rather a paraphyletic group composed of all the early reptiles retaining the primitive skull morphology, grouped together by the absence of temporal openings. Gauthier, Kluge and Rowe (1988) attempted to redefine Anapsida so it would be monophyletic, defining it as the clade containing "extant turtles and all other extinct taxa that are more closely related to them than they are to other reptiles". This definition explicitly includes turtles in Anapsida; because
396-676: Is an extinct family of tetrapods , typically considered primitive reptiles , known from the late Carboniferous to the Late Permian . They had a cosmopolitan distribution across Pangea . Captorhinids are a clade of small to very large lizard-like animals that date from the Late Carboniferous through the Permian . Their skulls were much stronger than those of their relatives, the protorothyridids , and had teeth that were better able to deal with tough plant material. The postcranial skeleton
440-561: 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
484-583: Is no known case of a diapsid reverting to the excretion of urea (ureotelism), even when they return to semi-aquatic lifestyles. Crocodilians, for example, are still uricotelic, although they are also partly ammonotelic, meaning they excrete some of their waste as ammonia. Ureotelism appears to be the ancestral condition among primitive amniotes, and it is retained by mammals, which likely inherited ureotelism from their synapsid and therapsid ancestors. Ureotelism therefore would suggest that turtles were more likely anapsids than diapsids. The only known uricotelic chelonian
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#1732883892445528-450: 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
572-473: Is similar to those of seymouriamorphs and diadectomorphs ; these animals were grouped together with the captorhinids in the order Cotylosauria as the first reptiles in the early 20th century, but are now usually regarded as stem - amniotes no closer to reptiles than to mammals . Captorhinids have broad, robust skulls that are generally triangular in shape when seen in dorsal view. The premaxillae are characteristically downturned. The largest captorhinid,
616-417: Is the desert tortoise , which likely evolved it recently as adaptation to desert habitats. Some desert mammals are also uricotelic, so since practically all known mammals are ureotelic, uricotelic adaptation is a likely result of convergence among desert species. Therefore, turtles would have to be the only known case of a uricotelic reptile reverting to ureotelism. Anapsida is still sporadically recognized as
660-580: Is thought to be the basalmost known member of Captorhinidae. A phylogenic study of primitive reptile relationships by Muller & Reisz in 2006 recovered Thuringothyris as a sister taxon of the Captorhinidae. The same results were obtained in later phylogenic analyses. Captorhinidae contains a single subfamily, the Moradisaurinae . Moradisaurinae was named and assigned to the family Captorhinidae by A. D. Ricqlès and P. Taquet in 1982. Moradisaurinae
704-690: 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 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
748-456: 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
792-473: 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
836-430: 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
880-449: 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 a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and
924-447: The archosauromorph hypothesis is supported. Reanalysis of prior phylogenies suggests that they classified turtles as anapsids both because they assumed this classification (most of them were studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa broadly enough for constructing the cladogram . Testudines is suggested to have diverged from other diapsids between 200 and 279 million years ago, though
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#1732883892445968-550: 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 ,
1012-647: The debate is far from settled. Although procolophonids managed to survive into the Triassic , most of the other reptiles with anapsid skulls, including the millerettids , nycteroleterids , and pareiasaurs , became extinct in the Late Permian period by the Permian-Triassic extinction event . Despite the molecular studies, there is evidence that contradicts their classification as diapsids. All known diapsids excrete uric acid as nitrogenous waste (uricotelic), and there
1056-402: 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
1100-535: 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
1144-801: The early 20th century. However, more recent fossil finds have shown that Eunotosaurus was either a parareptile or a diapsid , and therefore unrelated to captorhinids. The following taxonomy follows Reisz et al. , 2011 and Sumida et al. , 2010 unless otherwise noted. The cladogram below follows the topology from a 2011 analysis by paleontologists Robert R. Reisz , Jun Liu, Jin-Ling Li and Johannes Müller. Paleothyris Thuringothyris Concordia Rhiodenticulatus Romeria Protocaptorhinus Saurorictus C. laticeps C. aguti C. magnus Captorhinikos Labidosaurus Labidosaurikos Moradisaurus Rothianiscus Gansurhinus Simões et al. (2022) recovered captorhinids as stem-amniotes instead, as
1188-400: 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
1232-415: 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
1276-419: The groups traditionally seen as anapsids truly lacked fenestra. While "anapsid reptiles" or "Anapsida" were traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls might be only distantly related. Scientists still debate the exact relationship between the basal (original) reptiles that first appeared in the late Carboniferous ,
1320-568: The herbivorous Moradisaurus , could reach an estimated snout-vent length of 2 meters (6.5 feet). Early, smaller forms possessed single rows of teeth, and were likely carnivorous or omnivorous, while the larger, more derived captorhinids belonging to the subfamily Moradisaurinae were herbivorous and developed multiple (up to 11) rows of teeth in the jaws alongside propalinal (back and forth) jaw motion, which created an effective apparatus for grinding and shredding plant matter. Histological and SEM analysis of captorhinid tail vertebrae concluded in
1364-515: 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
Anapsid - Misplaced Pages Continue
1408-410: 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
1452-432: 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
1496-448: 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
1540-408: 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
1584-455: 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
1628-423: The phylogenetic placement of turtles within Amniota is very uncertain, it is unclear what taxa, other than turtles themselves, would be included in such defined Anapsida, and whether its content would be similar to the Anapsida of tradition. Indeed, Gauthier, Kluge and Rowe (1988) themselves included only turtles and Captorhinidae in their Anapsida, while excluding the majority of anapsids in the traditional sense of
1672-468: The placement of turtles within diapsids; some place turtles within Archosauria , or, more commonly, as a sister group to extant archosaurs. One molecular study, published in 2012, suggests that turtles are lepidosauromorph diapsids, most closely related to the lepidosaurs ( lizards , snakes , and tuataras ). However, in a later paper from the same authors, published in 2014, based on more extensive data,
1716-729: 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 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,
1760-453: The skulls of these taxa makes it uncertain whether the ancestral reptiles had an anapsid-like skull as traditionally assumed or a synapsid-like skull instead. [REDACTED] Amniote 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
1804-401: 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 a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst
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1848-737: The various Permian reptiles that had anapsid skulls, and the Testudines ( turtles , tortoises , and terrapins ). However, it was later suggested that the anapsid-like turtle skull is due to reversion rather than to anapsid descent. The majority of modern paleontologists believe that the Testudines are descended from diapsid reptiles that lost their temporal fenestrae. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids, or, more commonly, within Archelosauria . All molecular studies have strongly upheld
1892-650: The word from it. Tsuji and Müller (2009) noted that the name Anapsida implies a morphology (lack of temporal openings) that is in fact absent in the skeletons of a number of taxa traditionally included in the group. A temporal opening in the skull roof behind each eye, similar to that present in the skulls of synapsids , has been discovered in the skulls of a number of members of Parareptilia (the group containing most of reptiles traditionally referred to as anapsids), including lanthanosuchoids , millerettids, bolosaurids , some nycteroleterids, some procolophonoids and at least some mesosaurs . The presence of temporal openings in
1936-424: Was defined as "all captorhinids more closely related to Moradisaurus than to Captorhinus ". The moradisaurines inhabited what is now China , Morocco , Niger , Russia , Texas and Oklahoma . Captorhinids were once thought to be the ancestors of turtles . The Middle Permian reptile Eunotosaurus from South Africa was seen as the " missing link " between cotylosaurs and chelonians throughout much of
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