Archosauriformes (Greek for "ruling reptiles", + "form") are a clade of diapsid reptiles that developed from Archosauromorph ancestors some time in the Late Permian (roughly 250 million years ago). These reptiles, who are classified under the family Proterosuchidae, were superficially crocodile-like predatory semi-aquatic animals about 1.5 meters long, with a sprawling elbows-out stance and long snouts. Unlike the bulk of their therapsid contemporaries, the Proterosuchids happily survived the catastrophe at the end of the Permian, perhaps because they were opportunistic scavengers, perhaps because they could retreat into water to find respite from an overheated climate. Any such scenarios are hypothetical; what is clearer is that these animals were highly successful in their new environment, and evolved quickly. Within a few millions years at the opening of the Triassic, the Proterosuchids had given rise to the Erythrosuchidae (the first sauropsids to totally dominate their environment), who in turn were the ancestors of the small agile Euparkeriidae, from whom a number of successfully more advanced families - the Archosaurs proper - evolved rapidly to fill empty ecological niches in the devastated global system. Classes Ichthyosauria Sauropterygia Lepidosauria Archosauria Diapsids (two arches) are a group of tetrapod animals that developed two holes (temporal fenestra) in each side their skulls, about 300 million years ago during the late Carboniferous period. ... Orders  Crocodylia - Crocodilians  Rhynchocephalia - Tuataras  Squamata   Suborder Sauria - Lizards   Suborder Serpentes - Snakes Testudines - Turtles Superorder Dinosauria  Saurischia  Ornithischia The reptiles are a group of vertebrate animals. ... Groups Order Rhynchosauria Order Trilophosauria Order Prolacertiformes Plesion     Archosauriformes Division        Archosauria Archosauromorpha (Greek for ruling reptiles form) are a diverse group or Infraclass of diapsid reptiles that first appeared during the late Permian, but only became common during the Triassic. ... The Permian is a geologic period that extends from about 280 to 248 million years before the present (mya). ... Mega-annum, usually abbreviated as Ma, is a unit of time equal to one million years. ... Orders  Crocodylia - Crocodilians  Rhynchocephalia - Tuataras  Squamata   Suborder Sauria - Lizards   Suborder Serpentes - Snakes Testudines - Turtles Superorder Dinosauria  Saurischia  Ornithischia The reptiles are a group of vertebrate animals. ... Scientific classification or biological classification refers to how biologists group and categorize extinct and living species of organisms. ... Groups Biarmosuchia Dinocephalia Anomodontia Theriodontia    Cynodontia       (...mammals) Therapsids, previously known as the mammal-like reptiles, are a group of synapsids. ... The Permian-Triassic extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 252 million years ago (mya), forming the boundary of the Permian and Triassic geologic periods. ... The Triassic is a Geologic period that extends from about 248 to 202 million years (My or megayears) before the present. ... Superclasses Anapsida Eurapsida Diapsida Reptilia was an old kingdom or phylum classification that has since been divided into 4 classes. ... Groups Crurotarsi     Crocodylia (crocodiles) Ornithodira     Pterosauria     Dinosauria        Aves (birds) Archosaurs (Greek for ruling reptiles) are a group of diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian (Lower Triassic). ...

Pre-Euparkeria Archosauriformes were in the past included in the suborder Proterosuchia of the order Thecodontia. Under the cladistic methodology, the Proterosuchia are rejected as a paraphyletic assemblage, and the pre-Archosaurian taxa are simply considered as basal Archosauriformes. Thecodont (socket-toothed reptile), is a catch-all (paraphyletic) group, now considered an obsolete term, that was formerly used to describe a group of the earliest archosaurs that lived during the Permian and Triassic periods. ... Greek clados = branch) or phylogenetic systematics is a branch of biology that determines the evolutionary relationships of living things based on derived similarities. ... In phylogenetics, a grouping of organisms is said to be paraphyletic (Greek para = near and phyle = race) if all the members of the group have a common ancestor, but the group does not include all the descendants of the most recent common ancestor of all group members. ...

External links

• Paleos (http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.400.html)

• Mikko's Phylogeny Archive (http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Reptilia/Archosauromorpha/Archosauriformes.htm)