Diapsids are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ago during the late Carboniferous period. All diapsids other than the most primitive ones in the clade Araeoscelidia are sometimes placed into the clade Neodiapsida. The diapsids are extremely diverse, and include birds and all modern reptile groups, including turtles, which were historically thought to lie outside the group. Although some diapsids have lost either one hole (lizards), or both holes, or have a heavily restructured skull, they are still classified as diapsids based on their ancestry. At least 17,084 species of diapsid animals are extant: 9,159 birds, and 7,925 snakes, lizards, tuatara, turtles, and crocodiles.
Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens, have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.
Neornithischia is a clade of the dinosaur order Ornithischia. It is the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs. Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Parksosauridae; in addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.
Avicephala is a potentially polyphyletic grouping of extinct diapsid reptiles that lived during the Late Permian and Triassic periods characterised by superficially bird-like skulls and arboreal lifestyles. As a clade, Avicephala is defined as including the gliding weigeltisaurids and the arboreal drepanosaurs to the exclusion of other major diapsid groups. This relationship is not recovered in the majority of phylogenetic analyses of early diapsids and so Avicephala is typically regarded as an artificial or unnatural grouping. However, the clade was recovered again in 2021 following a redescription of Weigeltisaurus, raising the possibility that the clade may be valid after all, although subsequent analyses have not supported this result.
Megalaimidae, the Asian barbets, are a family of birds, comprising two genera with 35 species native to the forests of the Indomalayan realm from Tibet to Indonesia. They were once clubbed with all barbets in the family Capitonidae but the Old World species have been found to be distinctive and are considered, along with the Lybiidae and Ramphastidae, as sister groups.
Drepanosaurs are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. The various species of drepanosaurs were characterized by a bird-like skull, a barrell shaped body, and a horizontally narrow tail. A number of drepanosaurs had specialized grasping limbs and often prehensile tails similar to those of chameleons. Drepanosaurs are generally thought to have been arboreal (tree-dwelling), and probably insectivores. Some studies have alternately suggested fossorial (digging) and aquatic lifestyles for some members. Fossils of drepanosaurs have been found in North America and Europe. The name is taken from the family's namesake genus Drepanosaurus, which means "sickle lizard," a reference to their strongly curved claws.
Neoaves is a clade that consists of all modern birds with the exception of Palaeognathae and Galloanserae. Almost 95% of the roughly 10,000 known species of extant birds belong to the Neoaves.
Aequornithes, or core water birds, are defined as "the least inclusive clade containing Gaviidae and Phalacrocoracidae".
The order Falconiformes is represented by the extant family Falconidae and a handful of enigmatic Paleogene species. Traditionally, the other bird of prey families Cathartidae, Sagittariidae (secretarybird), Pandionidae (ospreys), Accipitridae (hawks) were classified in Falconiformes. A variety of comparative genome analyses published since 2008, however, found that falcons are part of a clade of birds called Australaves, which also includes seriemas, parrots and passerines. Within Australaves falcons are more closely related to the parrot-passerine clade than they are to the seriemas. The hawks, vultures and owls are placed in the clade Afroaves.
Strisores, sometimes called nightbirds, is a clade of birds that includes the living families and orders Caprimulgidae, Nyctibiidae (potoos), Steatornithidae (oilbirds), Podargidae (frogmouths), Apodiformes, as well as the Aegotheliformes (owlet-nightjars) whose distinctness was only recently realized. The Apodiformes and the Aegotheliformes form the Daedalornithes.
Afroaves is a clade of birds, consisting of the kingfishers and kin (Coraciiformes), woodpeckers and kin (Piciformes), hornbills and kin (Bucerotiformes), trogons (Trogoniformes), cuckoo roller (Leptosomiformes), mousebirds (Coliiformes), owls (Strigiformes), raptors (Accipitriformes) and New World vultures (Cathartiformes). The most basal clades are predatory, suggesting the last common ancestor of Afroaves was also a predatory bird.
Telluraves is a recently defined clade of birds defined by their arboreality. Based on most recent genetic studies, the clade unites a variety of bird groups, including the australavians as well as the afroavians. They appear to be the sister group of the Phaethoquornithes.
Passerea is a clade of neoavian birds that was proposed by Jarvis et al. (2014). Their genomic analysis recovered two major clades within Neoaves, Passerea and Columbea, and concluded that both clades appear to have many ecologically driven convergent traits.
Phaethoquornithes is a clade of birds that contains Eurypygimorphae and Aequornithes, which was first recovered by genome analysis in 2014. Members of Eurypygimorphae were originally classified in the obsolete group Metaves, and Aequornithes were classified as the sister taxon to Musophagiformes or Gruiformes.
Archelosauria is a clade grouping turtles and archosaurs and their fossil relatives, to the exclusion of lepidosaurs. The majority of phylogenetic analyses based on molecular data have supported a sister-group relationship between turtles and archosaurs. On the other hand, Archelosauria had not been historically supported by most morphological analyses, which have instead found turtles to either be descendants of parareptiles, early-diverging diapsids outside of Sauria, or close relatives of lepidosaurs within the clade Ankylopoda. Some recent morphological analyses have also found support for Archelosauria.
Coraciimorphae is a clade of birds that contains the order Coliiformes (mousebirds) and the clade Cavitaves. The name however was coined in the 1990s by Sibley and Ahlquist based on their DNA-DNA hybridization studies conducted in the late 1970s and throughout the 1980s. However their Coraciimorphae only contains Trogoniformes and Coraciiformes.
Eucavitaves is a clade that contains the order Trogoniformes (trogons) and the clade Picocoraciae. The name refers to the fact that the majority of them nest in cavities.
Aequorlitornithes is a clade of waterbirds recovered in a comprehensive genomic systematic study using nearly 200 species in 2015. It contains the clades Charadriiformes, Mirandornithes and Phaethoquornithes. Previous studies have found different placement for the clades in the tree.
The Odontoanserae is a proposed clade that includes the family Pelagornithidae and the clade Anserimorphae. The placement of the pseudo-toothed birds in the evolutionary tree of birds has been problematic, with some supporting the placement of them near the orders Procellariformes and Pelecaniformes based on features in the sternum.
Vegaviidae is an extinct family of ornithurine dinosaurs of uncertain phylogenetic placement, which existed during the Late Cretaceous and possibly the Paleocene. Definitive fossils attributed to the family have only been found in Antarctica, though other fossils from the Southern Hemisphere including Chile and New Zealand may represent this group. The putative Campanian vegaviid from Canada known as Maaqwi, more likely belongs to the Procellariformes.
