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.
Dimetrodon is an extinct genus of non-mammalian synapsid belonging to the family Sphenacodontidae that lived during the Cisuralian age of the Early Permian period, around 295–272 million years ago. With most species measuring 1.7–4.6 m (5.6–15.1 ft) long and weighing 28–250 kg (62–551 lb), the most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It was an obligate quadruped and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the Southwestern United States, the majority of these coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, its fossils have also been found in Germany and over a dozen species have been named since the genus was first erected in 1878.
Diadectidae is an extinct family of early tetrapods that lived in what is now North America and Europe during the Late Carboniferous and Early Permian, and in Asia during the Late Permian. They were the first herbivorous tetrapods, and also the first fully terrestrial animals to attain large sizes. Footprints indicate that diadectids walked with an erect posture. They were the first to exploit plant material in terrestrial food chains, making their appearance an important stage in both vertebrate evolution and the development of terrestrial ecosystems.
Sphenacodontidae is an extinct family of sphenacodontoid synapsids. Small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian "pelycosaurs". The most recent one, Dimetrodon angelensis, is from the latest Kungurian or, more likely, early Roadian San Angelo Formation. However, given the notorious incompleteness of the fossil record, a recent study concluded that the Sphenacodontidae may have become extinct as recently as the early Capitanian. Primitive forms were generally small, but during the later part of the early Permian these animals grew progressively larger, to become the top predators of terrestrial environments. Sphenacodontid fossils are so far known only from North America and Europe.
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes Haptodus baylei, Haptodus garnettensis and Sphenacodon ferox, but not Edaphosaurus pogonias". They first appear during the Late Pennsylvanian epoch. From the end of the Carboniferous to the end of the Permian, most of them remained large, with only some secondarily becoming small in size.
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.
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.
Ctenospondylus is an extinct genus of sphenacodontid synapsid
Sphenacodon is an extinct genus of synapsid that lived from about 300 to about 280 million years ago (Ma) during the Late Carboniferous and Early Permian periods. Like the closely related Dimetrodon, Sphenacodon was a carnivorous member of the Eupelycosauria family Sphenacodontidae. However, Sphenacodon had a low crest along its back, formed from blade-like bones on its vertebrae instead of the tall dorsal sail found in Dimetrodon. Fossils of Sphenacodon are known from New Mexico and the Utah–Arizona border region in North America.
Secodontosaurus is an extinct genus of "pelycosaur" synapsids that lived from between about 285 to 272 million years ago during the Early Permian. Like the well known Dimetrodon, Secodontosaurus is a carnivorous member of the Eupelycosauria family Sphenacodontidae and has a similar tall dorsal sail. However, its skull is long, low, and narrow, with slender jaws that have teeth that are very similar in size and shape—unlike the shorter, deep skull of Dimetrodon, which has large, prominent canine-like teeth in front and smaller slicing teeth further back in its jaws. Its unusual long, narrow jaws suggest that Secodontosaurus may have been specialized for catching fish or for hunting prey that lived or hid in burrows or crevices. Although no complete skeletons are currently known, Secodontosaurus likely ranged from about 2 to 2.7 metres (7–9 ft) in length, weighing up to 110 kilograms (250 lb).
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.
Limnoscelis was a genus of large diadectomorph tetrapods from the Late Carboniferous to early Permian of western North America. It includes two species: the type species Limnoscelis paludis from New Mexico, and Limnoscelis dynatis from Colorado, both of which are thought to have lived concurrently. No specimens of Limnoscelis are known from outside of North America. Limnoscelis was carnivorous, and likely semiaquatic, though it may have spent a significant portion of its life on land. Limnoscelis had a combination of derived amphibian and primitive reptilian features, and its placement relative to Amniota has significant implications regarding the origins of the first amniotes.
Boii is an extinct genus of microsaur within the family Tuditanidae. It was found in Carboniferous coal from mines near the community of Kounov in the Czech Republic. The only remains of the genus consist of a crushed skull, shoulder girdle bones, and scales, which were similar to microsaurian elements originally referred to Asaphestera. Boii can be characterized by its heavily sculptured skull, thin ventral plate of the clavicles, and a larger number of fangs on the roof of the mouth. For many years the type and only known species, Boii crassidens, was considered to be a species of Sparodus, until 1966 when Robert Carroll assigned it to its own genus.
Aerosaurus is an extinct genus within Varanopidae, a family of non-mammalian synapsids. It lived between 252-299 million years ago during the Early Permian in North America. The name comes from Latin aes (aeris) “copper” and Greek sauros “lizard,” for El Cobre Canyon in northern New Mexico, where the type fossil was found and the site of former copper mines. Aerosaurus was a small to medium-bodied carnivorous synapsid characterized by its recurved teeth, triangular lateral temporal fenestra, and extended teeth row. Two species are recognized: A. greenleeorum (1937) and A. wellesi (1981).
Echinerpeton is an extinct genus of synapsid, including the single species Echinerpeton intermedium from the Late Carboniferous of Nova Scotia, Canada. The name means 'spiny lizard' (Greek). Along with its contemporary Archaeothyris, Echinerpeton is the oldest known synapsid, having lived around 308 million years ago. It is known from six small, fragmentary fossils, which were found in an outcrop of the Morien Group near the town of Florence. The most complete specimen preserves articulated vertebrae with high neural spines, indicating that Echinerpeton was a sail-backed synapsid like the better known Dimetrodon, Sphenacodon, and Edaphosaurus. However, the relationship of Echinerpeton to these other forms is unclear, and its phylogenetic placement among basal synapsids remains uncertain.
Milosaurus is an extinct genus of non-mammalian synapsids native to Illinois that was alive during the latest Carboniferous and earliest Permian. It was named in 1970 on the basis of FMNH 701, a partial skeleton, as well as referred material.
Macromerion is an extinct genus of non-mammalian synapsids, specifically Pelycosaurs, in the family Sphenacodontidae from Late Carboniferous deposits in the Czech Republic. It was named as a species of Labyrinthodon in 1875 and as its own genus in 1879.
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.
Gorgodon is an extinct genus of basal synapsids. The genus is monotypic, known only from the type species Gorgodon minutus from the Early Permian of the southwestern United States. The only known remains of Gorgodon are two fossils consisting of fragments of the skull. Gorgodon was described and named by paleontologist Everett C. Olson in 1962 from the San Angelo Formation in Knox County, Texas. Based on what is known of Gorgodon—the squamosal, quadrate, and pterygoid bones of the back of the skull, the maxilla and premaxilla bones that make up the front of the skull, and several teeth—Gorgodon had a relatively large temporal fenestra and a pair large, conical caniniform teeth at the front of the jaw. Other distinguishing features of Gorgodon include the fused connection between the quadrate and squamosal bones and a long transverse process of the pterygoid.
Gordodon is an extinct genus of non-mammalian synapsid that lived during the Early Permian of what is now Otero County, New Mexico. It was a member of the herbivorous sail-backed family Edaphosauridae and contains only a single species, the type species G. kraineri. Gordodon is unusual among early synapsids for its teeth, which were arranged similarly to those of modern mammals and unlike the simple, uniform lizard-like teeth of other early herbivorous synapsids. Gordodon had large incisor-like teeth at the front, followed by a prominent gap between them and a short row of peg-like teeth at the back. Gordodon was also relatively long-necked for an early synapsid, with elongated and gracile vertebrae in its neck and back. Like other edaphosaurids, Gordodon had a tall sail on its back made from the bony neural spines of its vertebrae. The spines also had bony knobs on them, a common trait of edaphosaurids, but the knobs of Gordodon are also unique for being more slender, thorn-like and randomly arranged along the spines. It is estimated to have been rather small at 1 m in length excluding the tail and 34 kg (75 lb) in weight.
