Eureptilia ("true reptiles") is one of the two major clades of the Sauropsida, the other being Anapsida (or Parareptilia). Eureptilia includes not only all Diapsids, but also a number of primitive Permo-Carboniferous forms previously classified under the Anapsida, in the old (no longer recognised) order "Cotylosauria". Primitive eureptilians were all small, superficially lizard-like forms, that probably scurried through the Paleozoic undergrowth in search of insects. The diapsids are the only eureptilian clade to continue beyond the Permian Period. Eureptilia is defined by the skull having greatly reduced supraoccipital, tabular, and supratemporal bones that are no longer in contact with the postorbital. In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.
Brouffia orientalis is an extinct basal eureptile species known from a few bones found in Late Carboniferous (late Westphalian stage) coal seams in what is now the Czech Republic. It is known from a single partial skeleton, the holotype ČGH III B.21.C.587 and MP 451 (part and counterpart). It was collected in the Nýřany site from the Nýřany Member of the Kladno Formation. It was first named by was first named by Robert L. Carroll and Donald Baird in 1972. Experts disagree about its phylogenetic relationships to other contemporary and later reptiles and to ancestral amphibians.
Coelostegus prothales (Carroll and Baird 1972) is an extinct species of Late Carboniferous (late Westphalian stage) basal reptile known from Pilsen of Czech Republic, was derived from a sister to Paleothyris, was a sister to Hylonomus and both phylogenetically preceded Protorothyris. Distinct from Paleothyris, the skull of Coelostegus was little changed overall. The frontals had posterior processes. Here the supratemporals were angled down. The maxilla was deeper. The posterior rim of the jugal was a vertical suture. Overall the vertebrae and ribs were much more robust and the cervicals were slightly larger than the dorsals. The vertebrae were taller than long and the neural spines were taller than each centrum. The rest is little different or unknown.
Paleothyris acadiana was a small, agile, anapsid reptile which lived in the Middle Pennsylvanian epoch in Nova Scotia (approximately 312 to 304 million years ago). Paleothyris had sharp teeth and large eyes, meaning that it was a nocturnal hunter. It was about a foot long. It probably fed on insects and other smaller animals found on the floor of its forest home. Paleothyris was an early sauropsid, yet it still had some features that were more primitive, more labyrinthodont-like than reptile-like, especially its skull, which lacked fenestrae, holes found in the skulls of most modern reptiles and mammals.
Hylonomus (pronounced /haɪˈlɒnəməs/, hylo- "forest" + nomos "dweller") was a very early reptile. It lived 312 million years ago during the Late Carboniferous period. It is the earliest unquestionable reptile (Westlothiana is older, but may in fact be an amphibian, and Casineria is rather fragmentary). Hylonomus lyelli was 20 centimetres (8 in) long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and likely ate small invertebrates such as millipedes or early insects. Fossils of Hylonomus have been found in the remains of fossilized club moss stumps in Joggins, Nova Scotia, Canada. It is supposed that, after harsh weather, the club mosses would crash down, with the stumps eventually rotting and hollowing out. Small animals such as Hylonomus, seeking shelter, would enter and become trapped, starving to death. Fossils of the basal pelycosaur Archaeothyris and the basal diapsid Petrolacosaurus are also found in the same region of Nova Scotia, although from a higher stratum, dated approximately 6 million years later. Fossilized footprints found in New Brunswick have been attributed to Hylonomus, at an estimated age of 315 million years. his animal was discovered by John William Dawson in the mid-19th century. The species name comes from Dawson's teacher, the geologist Sir Charles Lyell. While it has traditionally been included in the group Protothyrididae, later studies have shown that it is probably more closely related to diapsids. It was discovered in 1851.