Sauropsida (Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern reptiles and birds (which, as theropod dinosaurs, are nested within reptiles as more closely related to crocodilians than to lizards or turtles). The most popular definition states that Sauropsida is the sibling taxon to Synapsida, the other clade of amniotes which includes mammals 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 of theropod dinosaurs despite originally being named as a separate class in Linnaean taxonomy.
The base of Sauropsida is traditionally divided into main groups of "reptiles": Eureptilia ("true reptiles") and Parareptilia ("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 clades Recumbirostra and Varanopidae, traditionally thought to be lepospondyls and synapsids respectively, may also be basal sauropsids. The term "Sauropsida" originated in 1864 with Thomas Henry Huxley, and his opinion that birds had risen from the dinosaurs. He based this chiefly on the fossils of Hesperornis and Archaeopteryx, that were starting to become known at the time. In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrate classes informally into mammals, sauroids, and ichthyoids (the latter containing the anamniotes), based on the gaps in physiological traits and lack of transitional fossils that seemed to exist between the three groups. Early in the following year he proposed the names Sauropsida and Ichthyopsida for the two latter. Huxley did however include groups on the mammalian line (synapsids) like Dicynodon 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. 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."
This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Since the advent of phylogenetic nomenclature, the term Reptilia has fallen out of favor with many taxonomists, who have used Sauropsida in its place to include a monophyletic group containing the traditional reptiles and the birds.
Cladistic definitions
upright=1.8|thumb|Sauropsida and the 19th-/20th-century conception of the class [[Reptilia. Both are superimposed on a cladogram of tetrapods, showing the difference in coverage.]]
The class Reptilia has been known to be an evolutionary grade rather than a clade for as long as evolution has been recognised. Reclassifying reptiles has been among the key aims of phylogenetic nomenclature. The term Sauropsida had from the mid 20th century been used to denote a branch-based clade 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. Some later cladistic work has used Sauropsida more restrictively, to signify the crown group, i.e. all descendants of the last common ancestor of extant 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.
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 the PhyloCode 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. Though formulated differently, this grouping was similar in scope and intention to the definition provided by Gauthier (1994).
Subdivisions
Eureptilia ("true reptiles") is one of the two traditional major subgroups of the clade Sauropsida, the other one being Parareptilia. Eureptilia includes Diapsida (the clade containing all modern reptiles and birds), as well as a number of primitive Permo-Carboniferous forms previously classified under Anapsida, in the old (no longer recognised) order "Cotylosauria", including Captorhinidae as well as Hylonomus and the "protorothyrids". Other primitive eureptiles such as the "protorothyrids" were all small, superficially lizard-like forms, that were probably insectivorous. One primitive eureptile, the Late Carboniferous "protorothyrid" Anthracodromeus, is the oldest known climbing tetrapod. 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" is paraphyletic, and the supposed "eureptilian" captorhinids and Protorothyris are not even sauropsids, but stem-amniotes, and that araeoscelidians are not closely related to true diapsids, if they are even sauropsids at all, 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 only Procolophonia, and Neodiapsida, Developmental biology and the fossil record both indicate that the presence of a tympanic ear is ancestral to extant reptiles. 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 the squamosal and quadratojugal, while in Neodiapsida it is mostly or entirely supported by the quadrate. All genetic studies have supported the hypothesis that turtles (formerly categorized together with ancient anapsids) are diapsid reptiles, despite lacking any skull openings behind their eye sockets; some studies have even placed turtles among the archosaurs, though a few have recovered turtles as lepidosauromorphs instead. The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.
In a number of recent studies, the "microsaur" clade Recumbirostra, historically considered lepospondyl reptiliomorphs, have been recovered as early sauropsids, though this assertion has been disputed by a number of authors, who contend that microsaurs are still reptillomorph stem-amniotes.
Simoes et al (2022) found Captorhinidae, Protorothyris and Araeoscelidia 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.
Cladogram after Simoes et al (2022):
Structure difference with synapsids
The last common ancestor of synapsids and Sauropsida lived at around 320mya during Carboniferous, known as Reptiliomorpha.
Thermal and secretion
The early synapsids 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 through urea, which is toxic and must be dissolved in water to be secreted. Unfortunately, the upcoming Permian and Triassic 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 through uric 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.
Brain structure
Different from how synapsids have their cortex in six different layers of neurons which is called neocortex, 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 the archicortex of the avian brain. However, in the modern view appeared since the 1960s, behavioral studies suggested that avian neostriatum and hyperstriatum 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. 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, like Tyrannosaurus, which had tiny brains compared to their enormous body size, were more intelligent than previously thought.