| Sauropsids | |
|---|---|
 | |
| Clockwise from top left:Pareiasaurus (an extinctpareiasaurianparareptile),Mesosaurus (an extinctmesosaurian parareptile),Smaug breyeri (alizard),Dinemellia dinemelli (thewhite-faced buffalo-weaver),Crocodylus niloticus (theNile crocodile), andLabidosaurikos (an extinctcaptorhinid eureptile) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Tetrapoda |
| Clade: | Reptiliomorpha |
| Clade: | Amniota |
| Clade: | Sauropsida Huxley, 1864 |
| Subclades | |
Revisionist view: Traditional view:
Incertae sedis possible sauropsids: | |
Sauropsida (Greek for "lizard faces") is aclade ofamniotes, broadly equivalent to theclassReptilia, though typically used in a broader sense to also include extinctstem-group relatives of modern reptiles andbirds (which, astheropod dinosaurs, are nested within reptiles as more closely related to crocodilians than to lizards or turtles).[3] The most popular definition states that Sauropsida is thesibling taxon toSynapsida, the other clade of amniotes which includesmammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are a group oftheropoddinosaurs despite originally being named as a separate class inLinnaean taxonomy.
The base of Sauropsida is traditionally divided into main groups of "reptiles":Eureptilia ("true reptiles") andParareptilia ("next to reptiles"). Eureptilia encompasses all living reptiles (including birds), as well as various extinct groups.Parareptilia is typically considered to be an entirely extinct group, though a few hypotheses for the origin of turtles have suggested that they belong to the parareptiles. The cladesRecumbirostra andVaranopidae, traditionally thought to belepospondyls and synapsids respectively, may also bebasal sauropsids. The term "Sauropsida" originated in 1864 withThomas Henry Huxley,[4] who grouped birds with reptiles based on fossil evidence. The divisions of "Eureptilia" and "Parareptilia" have been challenged in a number of recent studies, who find that they do not representmonophyletic groups.
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The termSauropsida ("lizard faces") has a long history, and hails back toThomas Henry Huxley, who first used the term in 1863, originally using the term "sauroids"[5] and his opinion that birds had risen from thedinosaurs. He based this chiefly on the fossils ofHesperornis andArchaeopteryx, that were starting to become known at the time.[6] In theHunterian lectures delivered at theRoyal College of Surgeons in 1863, Huxley grouped the vertebrateclasses informally intomammals, sauroids, and ichthyoids (the latter containing theanamniotes), based on the gaps in physiological traits and lack oftransitional fossils that seemed to exist between the three groups. Early in the following year he proposed the names Sauropsida andIchthyopsida for the two latter.[4] Huxley did however include groups on the mammalian line (synapsids) likeDicynodon among the sauropsids. Thus, under the original definition, Sauropsida contained not only the groups usually associated with it today, but also several groups that today are known to be in the mammalian side of the tree.[7] Huxley stated in an 1867 lecture that "The members of the class Aves so nearly approach the Reptilia in all the essential and fundamental points of their structure, that the phrase 'Birds and greatly modified Reptiles' would hardly be an exaggerated expression of the closeness of that resemblance."[5]
By the early 20th century, the fossils ofPermian synapsids fromSouth Africa had become well known, allowing palaeontologists to trace synapsid evolution in much greater detail. The term Sauropsida was taken up byE. S. Goodrich in 1916 much like Huxley's, to include lizards, birds and their relatives. He distinguished them frommammals and their extinct relatives, which he included in the sister group Theropsida (now usually replaced with the nameSynapsida). Goodrich's classification thus differs somewhat from Huxley's, in which the non-mammalian synapsids (or at least thedicynodontians) fell under the sauropsids. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ("first lizards"), which included somePaleozoicamphibians as well as earlyreptiles predating the sauropsid/synapsid split (and thus not true sauropsids). His concept differed from modern classifications in that he considered a modified fifthmetatarsal to be anapomorphy of the group, leading him to placeSauropterygia,Mesosauria and possiblyIchthyosauria andAraeoscelida in the Theropsida.[7]
In 1956,D. M. S. Watson observed that sauropsids and synapsids diverged very early in the reptilian evolutionary history, and so he divided Goodrich's Protosauria between the two groups. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals respectively, making themparaphyletic, unlike Goodrich's definition. Thus his Sauropsida includedProcolophonia,Eosuchia,Protorosauria,Millerosauria,Chelonia (turtles),Squamata (lizards and snakes),Rhynchocephalia,Rhynchosauria,Choristodera,Thalattosauria,Crocodilia, "thecodonts" (paraphyleticbasalArchosauria), non-aviandinosaurs,pterosaurs andsauropyterygians. However, his concept differed from the modern one in that he placed reptiles without anotic notch, such asaraeoscelids andcaptorhinids, with thetheropsids.[8]
This classification supplemented, but was never as popular as, the classification of the reptiles (according toRomer's classicVertebrate Paleontology[9]) into four subclasses according to the positioning oftemporal fenestrae, openings in the sides of the skull behind the eyes. Since the advent ofphylogenetic nomenclature, the termReptilia has fallen out of favor with many taxonomists, who have used Sauropsida in its place to include amonophyletic group containing the traditional reptiles and the birds.
The class Reptilia has been known to be anevolutionary grade rather than a clade for as long asevolution has been recognised. Reclassifying reptiles has been among the key aims ofphylogenetic nomenclature.[10] The term Sauropsida had from the mid 20th century been used to denote abranch-basedclade containing all amniote species which are not on the synapsid side of the split between reptiles and mammals. This group encompasses all now-living reptiles as well as birds, and as such is comparable to Goodrich's classification. The main difference is that better resolution of the early amniote tree has split up most of Goodrich's "Protosauria", though definitions of Sauropsida essentially identical to Huxley's (i.e. including the mammal-like reptiles) are also forwarded.[11][12] Some later cladistic work has used Sauropsida more restrictively, to signify thecrown group, i.e. all descendants of the last common ancestor ofextant reptiles and birds. A number of phylogenetic stem, node and crown definitions have been published, anchored in a variety of fossil and extant organisms, thus there is currently no consensus of the actual definition (and thus content) of Sauropsida as a phylogenetic unit.[13]
Some taxonomists, such as Benton (2004), have co-opted the term to fit into traditional rank-based classifications, making Sauropsida and Synapsida class-level taxa to replace the traditional Class Reptilia, while Modesto and Anderson (2004), using thePhyloCode standard, have suggested replacing the name Sauropsida with their redefinition of Reptilia, arguing that the latter is by far better known and should have priority.[13]
Cladistic definitions of Sauropsida include:
Eureptilia ("true reptiles") is one of the two traditional major subgroups of theclade Sauropsida, the other one beingParareptilia. Eureptilia includesDiapsida (the clade containing all modernreptiles andbirds), as well as a number of primitivePermo-Carboniferous forms previously classified underAnapsida, in the old (no longer recognised) order "Cotylosauria".[15]
Eureptilia is characterized by theskull having greatly reducedsupraoccipital,tabular, andsupratemporal bones that are no longer in contact with thepostorbital. Aside from Diapsida, the group notably containsCaptorhinidae, a diverse and long lived (Late Carboniferous-Late Permian) clade of initially small carnivores that later evolved into large herbivores.[16] Other primitive eureptiles such as the "protorothyrids" were all small, superficially lizard-like forms, that were probablyinsectivorous.[17] One primitive eureptile, the Late Carboniferous "protorothyrid"Anthracodromeus, is the oldest knownclimbing tetrapod.[18] Diapsids were the only eureptilian clade to continue beyond the end of the Permian.
The traditional classification of sauropsids and eureptiles has been challenged in recent studies, with several studies in the early 2020s finding that "Parareptilia" isparaphyletic, and the supposed "eureptilian" captorhinids andProtorothyris are not even sauropsids, butstem-amniotes, and thataraeoscelidians are not closely related to true diapsids, if they are even sauropsids at all,[19][20][21] and that the famous "earliest reptile"Hylonomus may also not be a true sauropsid.[21] In 2019 the new cladeNeoreptilia was coined as the clade uniting Parareptilia and Neodiapsida, under the phylogenetic hypothesis that parareptiles were monophyletic and relatively derived, placed as closer to neodiapsids than araeoscelidians,Hylonomus, "protorothyrids" andvaranopids (the last of which are conventionally viewed assynapsids).[22] This clade was later reused by other scholars in a different sense to include parts of former Parareptilia that were considered close to Neodiapsida, which in one paper included onlyProcolophonia, and Neodiapsida,[19] while another paper includedMesosauria andAcleistorhinidae within Neoreptilia in addition to the aforementioned taxa.[21]
A 2025 paper named the new cladeParapleurota within Sauropsida, comprising the former parareptile familyMillerettidae and Neodiapsida, which the paper found to be sister groups. The group is formally defined as the clade containing the most recent common ancestor ofMilleretta rubidgei andYoungina capensis, but notPetrolacosaurus kansensis,Orovenator mayorum,Procolophon trigoniceps, orMesosaurus tenuidens.[21] Members of the Parapleurota are distinguished by the presence of atympanic membrane inside the ear.[21] This allows airborne sounds to be efficiently transmitted through the ear, and is typically associated with heightened hearing ability.[23][24] Developmental biology and the fossil record both indicate that the presence of a tympanic ear is ancestral to extant reptiles.[25] Parapleurota displays stepwise evolution of the tympanic fossa, an opening in the back of the skull that holds the membrane. In basal members of the clade, the membrane is supported by thesquamosal andquadratojugal, while inNeodiapsida it is mostly or entirely supported by thequadrate.[26] Tympanic membranes also evolved independently inProcolophonia andstem-mammals.[26][25]
Sauropsids evolved from basal amniotes approximately 320 million years ago, in theCarboniferous Period of thePaleozoic Era. In theMesozoic Era (from about 250 million years ago to about 66 million years ago), sauropsids were the largest animals on land, in the water, and in the air. The Mesozoic is sometimes called the Age of Reptiles. In theCretaceous–Paleogene extinction event, the large-bodied sauropsids died out in theglobal extinction event at the end of the Mesozoic era. With the exception of a few species of birds, the entire dinosaur lineage became extinct; in the following era, theCenozoic, the remaining birds diversified so extensively that, today, nearly one out of every three species of land vertebrate is a bird species.
Thecladogram presented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[27] Allgenetic studies have supported the hypothesis that turtles (formerly categorized together with ancientanapsids) are diapsid reptiles, despite lacking any skull openings behind their eye sockets; some studies have even placed turtles among thearchosaurs,[27][28][29][30][31][32] though a few have recovered turtles as lepidosauromorphs instead.[33] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[27]
| Amniota |
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Laurin & Piñeiro (2017) and Modesto (2019) proposed an alternate phylogeny of basal sauropsids. In this tree, parareptiles include turtles and are closely related to non-araeoscelidian diapsids. The familyVaranopidae, otherwise included inSynapsida, is considered by Modesto a sauropsid group.[34][35]
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In a number of recent studies, the "microsaur" cladeRecumbirostra, historically consideredlepospondyl reptiliomorphs, have been recovered as early sauropsids,[36][37] though this assertion has been disputed by a number of authors, who contend that microsaurs are still reptillomorph stem-amniotes.[38]
Simoes et al (2022) foundCaptorhinidae,Protorothyris andAraeoscelidia to form a clade that was the sister group to crown Amniota (containing true sauropsids and synapsids). The same study also considered parareptiles to be polyphyletic, with some groups being closer to the crown group of reptiles than others.[39]
Cladogram after Simoes et al (2022):[39]
| Reptiliomorpha |
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This cladogram follows Jenkins et al. (2025). Traditional "parareptiles" are highlighted in orange:[21]
| Sauropsida |
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The last common ancestor of synapsids and Sauropsida lived at around 320mya during Carboniferous, known asReptiliomorpha.
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The earlysynapsids inherited abundant glands on their skins from their amphibian ancestors. Those glands evolved into sweat glands in synapsids, which granted them the ability to maintain constant body temperature but made them unable to save water from evaporation. Moreover, the way synapsids discharge nitrogenous waste is throughurea, which is toxic and must be dissolved in water to be secreted. Unfortunately, the upcomingPermian andTriassic periods were arid periods. As a result, only a small percent of early synapsids survived in the land from South Africa to Antarctica in today's geography. Unlike synapsids, sauropsids do not have those glands on the skin; their way of nitrogenous waste emission is throughuric acid which does not require water and can be excreted with feces. As a result, sauropsids were able to expand to all environments and reach their pinnacle. Even today, most vertebrates that live in arid environments are sauropsids, snakes and desert lizards for example.
Different from howsynapsids have their cortex in six different layers of neurons which is calledneocortex, the cerebrum of Sauropsida has a completely different structure. For the corresponding structure of the cerebrum in the classic view, the neocortex of synapsids is homology with only thearchicortex of the avian brain. However, in the modern view appeared since the 1960s, behavioral studies suggested that avianneostriatum andhyperstriatum can receive signals of vision, hearing, and body sensations, which means they act just like the neocortex. Comparing an avian brain to that to a mammal,nuclear-to-layered hypothesis proposed by Karten (1969), suggested that the cells which form layers in synapsids' neocortex, gather individually by type and form several nuclei. For synapsids, when one new function is adapted in evolution it will be assigned to a separate area of cortex, so for each function, synapsids will have to develop a separate area of cortex, and damage to that specific cortex may cause disability.[40] However, for Sauropsida functions are disassembled and assigned to all nuclei. In this case, brain function is highly flexible for Sauropsida, even with a small brain, many Sauropsida can still have a relatively high intelligence compared to mammals, for example, birds in the family Corvidae. So, it is possible that some non-avian dinosaurs, likeTyrannosaurus, which had tiny brains compared to their enormous body size, were more intelligent than previously thought.[41]
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Jenkins20252 was invoked but never defined (see thehelp page).
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