Archosaur

Archosaurs Temporal range: Early Triassic–Present, 248–0 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Birds and crocodilians (in this case, a great egret and a mugger crocodile) are the only living archosaur groups.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Clade:	Eucrocopoda
Clade:	Archosauria Cope, 1869
Subgroups
Avemetatarsalia (birds and their extinct relatives) Pseudosuchia (crocodilians and their extinct relatives) Archosaurs of uncertain affinity †Albisaurus †Avalonianus †Avipes †Incertovenator? †Jushatyria †Palaeosaurus †Picrodon †Protecovasaurus? †Sikannisuchus †Smok †Zanclodon †Phytosauria?(paraphyletic?)[2]
Synonyms
Avesuchia Benton, 1999

Archosauria (lit. 'ruling reptiles') or archosaurs (/ˈɑːrkəˌsɔːr/^[3]) is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistic sense of the term includes all living and extinct relatives of birds and crocodilians such as non-avian dinosaurs, pterosaurs, phytosaurs, aetosaurs and rauisuchians as well as many Mesozoic marine reptiles. Modern paleontologists define Archosauria as a crown group that includes the most recent common ancestor of living birds and crocodilians, and all of its descendants.

The base of Archosauria splits into two clades: Pseudosuchia, which includes crocodilians and their extinct relatives; and Avemetatarsalia, which includes birds and their extinct relatives (such as non-avian dinosaurs and pterosaurs).^[4] Older definitions of the group Archosauria rely on shared morphological characteristics, such as an antorbital fenestra in the skull, serrated teeth, and an upright stance. Some extinct reptiles, such as proterosuchids and euparkeriids, also possessed these features yet originated prior to the split between the crocodilian and bird lineages. The older morphological definition of Archosauria nowadays roughly corresponds to Archosauriformes, a group named to encompass crown-group archosaurs and their close relatives.^[4]

The oldest true archosaur fossils are known from the Early Triassic period, though the first archosauriforms and archosauromorphs (reptilians closer to archosaurs than to lizards or other lepidosaurs) appeared in the Permian. Archosaurs quickly diversified in the aftermath of the Permian-Triassic mass extinction (~252 Ma), which wiped out most of the then-dominant therapsid competitors such as the gorgonopsians and anomodonts, and the subsequent arid Triassic climate allowed the more drought-resilient archosaurs (largely due to their uric acid-based urinary system) to eventually become the largest and most ecologically dominant terrestrial vertebrates from the Middle Triassic period up until the Cretaceous–Paleogene extinction event (~66 Ma).^[5] Birds and several crocodyliform lineages were the only archosaurs to survive the K-Pg extinction, rediversifying in the subsequent Cenozoic era. Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day.