Synapsida is one of the two major clades of vertebrate animals in the group Amniota, the other being the Sauropsida. The synapsids were the dominant land animals in the late Paleozoic and early Mesozoic, but the only group that survived into the Cenozoic are mammals. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.
Pelycosaur is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile had been used, and pelycosaur was considered an order, but this is now thought to be incorrect, and seen as outdated.
Diadectomorpha is a clade of large tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian), They have typically been classified as advanced reptiliomorphs positioned close to, but outside of the clade Amniota, though some recent research has recovered them as the sister group to the traditional Synapsida within Amniota, based on inner ear anatomy and cladistic analyses. They include both large carnivorous and even larger herbivorous forms, some semi-aquatic and others fully terrestrial. The diadectomorphs seem to have originated during late Mississippian times, although they only became common after the Carboniferous rainforest collapse and flourished during the Late Pennsylvanian and Early Permian periods.
Eupelycosauria is a large clade of animals characterized by the unique shape of their skull, encompassing all mammals and their closest extinct relatives. They first appeared 308 million years ago during the Early Pennsylvanian epoch, with the fossils of Echinerpeton and perhaps an even earlier genus, Protoclepsydrops, representing just one of the many stages in the evolution of mammals, in contrast to their earlier amniote ancestors.
Varanopidae is an extinct family of amniotes that resembled monitor lizards and may have filled a similar niche, hence the name. Typically, they are considered synapsids that evolved from an Archaeothyris-like synapsid in the Late Carboniferous. However, some recent studies have recovered them being taxonomically closer to diapsid reptiles. A varanopid from the latest Middle Permian Pristerognathus Assemblage Zone is the youngest known varanopid and the last member of the "pelycosaur" group of synapsids.
Eothyrididae is an extinct family of very primitive, insectivorous synapsids. Only three genera are known, Eothyris, Vaughnictis and Oedaleops, all from the early Permian of North America. Their main distinguishing feature is the large caniniform tooth in front of the maxilla.
Caseidae are an extinct family of basal synapsids that lived from the Late Carboniferous to Middle Permian between about 300 and 265 million years ago. Fossils of these animals come from the south-central part of the United States, from various parts of Europe, and possibly from South Africa if the genus Eunotosaurus is indeed a caseid as some authors proposed in 2021. Caseids show great taxonomic and morphological diversity. The most basal taxa were small insectivorous and omnivorous forms that lived mainly in the Upper Carboniferous and Lower Permian, such as Eocasea, Callibrachion, and Martensius. This type of caseid persists until the middle Permian with Phreatophasma and may be Eunotosaurus. During the early Permian, the clade is mainly represented by many species that adopted a herbivorous diet. Some have evolved into gigantic forms that can reach 6–7 metres (20–23 ft) in length, such as Cotylorhynchus hancocki and Alierasaurus ronchii, making them the largest Permian synapsids. Caseids are considered important components of early terrestrial ecosystems in vertebrate history because the numerous herbivorous species in this family are among the first terrestrial tetrapods to occupy the role of primary consumer. The caseids experienced a significant evolutionary radiation at the end of the early Permian, becoming, with the captorhinid eureptiles, the dominant herbivores of terrestrial ecosystems in place of the edaphosaurids and diadectids.
Sphenacodontoidea is a node-based clade that is defined to include the most recent common ancestor of Sphenacodontidae and Therapsida and its descendants. Sphenacodontoids are characterised by a number of synapomorphies concerning proportions of the bones of the skull and the teeth.
Casea is a genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) in what is now Texas, United States. The genus is only represented by its type species, Casea broilii, named by Samuel Wendell Williston in 1910. The species is represented by a skull associated with a skeleton, a second skull, a partial skull with a better preserved dentition than that of the preceding skulls, and several incomplete postcranial skeletons. Three other Casea species were later erected, but these are considered today to be invalid or belonging to different genera. Casea was a small animal with a length of about 1.20 m and a weight of around 20 kg.
Eothyris is a genus of extinct synapsid in the family Eothyrididae from the early Permian. It was a carnivorous insectivorous animal, closely related to Oedaleops. Only the skull of Eothyris, first described in 1937, is known. It had a 6-centimetre-long (2.4-inch) skull, and its total estimated length was 30 centimetres. Eothyris is one of the most primitive synapsids known and is probably very similar to the common ancestor of all synapsids in many respects. The only known specimen of Eothyris was collected from the Artinskian-lower.
Oedaleops is an extinct genus of caseasaur synapsids from the Early Permian of the southwestern United States. Fossils have been found in the Cutler Formation in New Mexico, which dates back to the Wolfcampian stage of the Early Permian. All remains belong to the single known species Oedaleops campi. Oedaleops was closely related to Eothyris, and both are part of the family Eothyrididae. Like Eothyris, it was probably an insectivore.
Cotylorhynchus is an extinct genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) and possibly the early Middle Permian (Roadian) in what is now Texas and Oklahoma in the United States. The large number of specimens found make it the best-known caseid. Like all large herbivorous caseids, Cotylorhynchus had a short snout sloping forward and very large external nares. The head was very small compared to the size of the body. The latter was massive, barrel-shaped, and ended with a long tail. The limbs were short and robust. The hands and feet had short, broad fingers with powerful claws. The barrel-shaped body must have housed large intestines, suggesting that the animal had to feed on a large quantity of plants of low nutritional value. Caseids are generally considered to be terrestrial, though a semi-aquatic lifestyle has been proposed by some authors. The genus Cotylorhynchus is represented by three species, the largest of which could reach more than 6 m in length. However, a study published in 2022 suggests that the genus may be paraphyletic, with two of the three species possibly belonging to separate genera.
Ennatosaurus is an extinct genus of caseid synapsid that lived during the Middle Permian in northern European Russia. The genus is only represented by its type species, Ennatosaurus tecton, which was named in 1956 by Ivan Antonovich Efremov. The species is known from at least six skulls associated with their lower jaws, as well as from the postcranial bones of several juvenile individuals. Ennatosaurus has the typical caseid skull with a short snout tilted forward and very large external nares. However, it differs from other derived caseids by its postcranial skeleton with smaller proportions compared to the size of the skull. As with other advanced caseids, the teeth of Ennatosaurus were well suited for slicing and cutting vegetation. The presence of a highly developed hyoid apparatus indicates the presence of a massive and mobile tongue, which had to work in collaboration with the palatal teeth during swallowing. With a late Roadian - early Wordian age, Ennatosaurus is one of the last known caseids.
Phreatophasma is an extinct genus of synapsids from the Middle Permian of European Russia. It includes only one species, Phreatophasma aenigmatum, which is itself known from a single femur found in a mine near the town of Belebei in Bashkortostan. Phreatophasma comes from a fossil assemblage that is latest Ufimian to earliest Kazanian in age under the Russian stratigraphic scheme, correlating with the Roadian Age under the international stratigraphic timescale. Because the species is based on a single specimen with few diagnostic anatomical features, uncertainty remains as to where it belongs in tetrapod phylogeny; originally interpreted in 1954 as an enigmatic "theromorph" synapsid by Soviet paleontologist Ivan Yefremov, Phreatophasma was later described as a therapsid incertae sedis by American paleontologist Alfred Romer in 1956 and then as a member of a basal synapsid family called Caseidae starting with Everett C. Olson in 1962. Olson's classification was later supported by Canadian paleontologist Robert Reisz in 1986 and American paleontologist Robert L. Carroll in 1988. Ivakhneneko et al. (1997) and Maddin et al. (2008) both considered Phreatophasma an indeterminate synapsid.
Ianthodon is an extinct genus of basal haptodontiform synapsids from the Late Carboniferous about 304 million years ago. The taxon was discovered and named by Kissel & Reisz in 2004. The only species in the taxon, Ianthodon schultzei, was found by separating it from a block that also contained the remains of Petrolacosaurus and Haptodus. The evolutionary significance of the taxon wasn't realized until a publication in 2015. The fossil of this organism was discovered in Garnett, Kansas.
Datheosaurus is an extinct genus of caseasaur. It was at least 1.5 metres (5 ft) in length. It lived during the Latest Carboniferous to Early Permian in Poland.
Callibrachion is an extinct genus of caseid synapsids that lived in east-central France during the Lower Permian (Asselian). The holotype and only known specimen (MNHN.F.AUT490) is represented by an almost complete postcranial skeleton associated with skull fragments discovered at the end of the 19th century in the Permian Autun basin in Saône-et-Loire department, in the Bourgogne-Franche-Comté region. It belongs to an immature individual measuring less than 1.50 m in length. Callibrachion was long considered a junior synonym of the genus Haptodus and classified among the sphenacodontid pelycosaurs. In 2015, a new study found that Callibrachion was a different animal from Haptodus and that it was a caseasaur rather than a sphenacodontid. This was confirmed in 2016 by a cladistic analysis which recovered Callibrachion as a basal caseid. Callibrachion's sharp teeth and unenlarged ribcage indicate that this animal was likely faunivorous.
Olson's Extinction was a mass extinction that occurred 273 million years ago in the late Cisuralian or early Guadalupian epoch of the Permian period, predating the much larger Permian–Triassic extinction event. The event is named after American paleontologist Everett C. Olson, who first identified the gap in fossil record indicating a sudden change between the early Permian and middle/late Permian faunas. Some authors also place a hiatus in the continental fossil record around that time, but others disagree. This event has been argued by some authors to have affected many taxa, including embryophytes, marine metazoans, and tetrapods.
Euromycter is an extinct genus of caseid synapsids that lived in what is now southern France during the Early Permian about 285 million years ago. The holotype and only known specimen of Euromycter (MNHN.F.MCL-2) includes the complete skull with lower jaws and hyoid apparatus, six cervical vertebrae with proatlas, anterior part of interclavicle, partial right clavicle, right posterior coracoid, distal head of right humerus, left and right radius, left and right ulna, and complete left manus. It was collected by D. Sigogneau-Russell and D. Russell in 1970 at the top of the M1 Member, Grès Rouge Group, near the village of Valady, Rodez Basin. It was first assigned to the species "Casea" rutena by Sigogneau-Russell and Russell in 1974. More recently, it was reassigned to its own genus, Euromycter, by Robert R. Reisz, Hillary C. Maddin, Jörg Fröbisch and Jocelyn Falconnet in 2011. The preserved part of the skeleton suggests a size between 1,70 m (5,5 ft) and 1,80 m (5,9 ft) in length for this individual.
