A tetrapod is any four-limbed vertebrate animal of the superclass Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Some tetrapods such as snakes, legless lizards, and caecilians had evolved to become limbless via mutations of the Hox gene, although some do still have a pair of vestigial spurs that are remnants of the hindlimbs.
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.
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reptile had been used, and pelycosaur was considered an order, but this is now thought to be incorrect, and seen as outdated.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.
Seymouria is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians, Seymouria were well-adapted to life on land, with many reptilian features—so many, in fact, that Seymouria was first thought to be a primitive reptile. It is primarily known from two species, Seymouria baylorensis and Seymouria sanjuanensis. The type species, S. baylorensis, is more robust and specialized, though its fossils have only been found in Texas. On the other hand, Seymouria sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in the Cutler Formation of New Mexico, and a pair of fully grown skeletons from the Tambach Formation of Germany, which were fossilized lying next to each other.
Caseasauria is one of the two main clades of early synapsids, the other being the Eupelycosauria. Caseasaurs are currently known only from the Late Carboniferous and the Permian, and include two superficially different families, the small insectivorous or carnivorous Eothyrididae, and the large, herbivorous Caseidae. These two groups share a number of specialised features associated with the morphology of the snout and external naris.
Ophiacodontidae is an extinct family of early synapsids from the Carboniferous and Permian. Archaeothyris, and Clepsydrops were among the earliest ophiacodontids, appearing in the Late Carboniferous. Ophiacodontids are among the most basal synapsids, an offshoot of the lineage which includes therapsids and their descendants, the mammals. The group became extinct by the Kungurian or the Roadian, replaced by anomodonts, theriodonts, and the diapsid reptiles.
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.
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).
Varanops is an extinct genus of Early Permian varanopid known from Texas and Oklahoma of the United States. It was first named by Samuel Wendell Williston in 1911 as a second species of Varanosaurus, Varanosaurus brevirostris. In 1914, Samuel W. Williston reassigned it to its own genus and the type species is Varanops brevirostris.
Clepsydrops is an extinct genus of primitive synapsids from the early Late Carboniferous that was related to Archaeothyris. The name means 'hour-glass appearance'.
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.
Trimerorhachis is an extinct genus of dvinosaurian temnospondyl within the family Trimerorhachidae. It is known from the Early Permian of the southwestern United States, with most fossil specimens having been found in the Texas Red Beds. The type species of Trimerorhachis, T. insignis, was named by American paleontologist Edward Drinker Cope in 1878. Cope named a second species from Texas, T. mesops, in 1896. The species T. rogersi and T. greggi are also from Texas, and the species T. sandovalensis is from New Mexico.
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).
Mesenosaurus is an extinct genus of amniote. It belongs to the family Varanopidae. This genus includes two species: the type species Mesenosaurus romeri from the middle Permian Mezen River Basin of northern Russia, and Mesenosaurus efremovi from the early Permian (Artinskian) Richards Spur locality. M. romeri’s stratigraphic range is the middle to late Guadalupian while M. efremovi’s stratigraphic range is the Cisuralian.
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.
The Organ Rock Formation or Organ Rock Shale is a formation within the late Pennsylvanian to early Permian Cutler Group and is deposited across southeastern Utah, northwestern New Mexico, and northeastern Arizona. This formation notably outcrops around Canyonlands National Park, Natural Bridges National Monument, and Monument Valley of northeast Arizona, southern Utah. The age of the Organ Rock is constrained to the latter half of the Cisuralian epoch by age dates from overlying and underlying formations. Important early terrestrial vertebrate fossils have been recovered from this formation in northern Arizona, southern Utah, and northern New Mexico. These include the iconic Permian terrestrial fauna: Seymouria, Diadectes, Ophiacodon, and Dimetrodon. The fossil assemblage present suggests arid environmental conditions. This is corroborated with paleoclimate data indicative of global drying throughout the early Permian.
Stenokranio is a genus of eryopid temnospondyl from the Permo-Carboniferous Remigiusberg Formation of Germany. It is represented by the type species, Stenokranio boldi, which was named for two specimens collected from the Remigiusberg quarry near Kusel, Saar–Nahe Basin, southwest Germany.
