Archosauromorpha

Archosauromorpha, meaning "ruling lizard forms," is a group of diapsid reptiles that includes all reptiles more closely related to archosaurs—like crocodilians, dinosaurs, and birds—than to lepidosaurs, such as tuataras, lizards, and snakes. The first archosauromorphs appeared in the late Middle Permian or Late Permian, but they became far more common and diverse during the Triassic.

Though the name Archosauromorpha was coined in 1946, it wasn't until the 1980s that scientists settled on which reptiles belonged to it. Today, the clade is understood to contain four main groups: the stocky, plant-eating allokotosaurs and rhynchosaurs; the hugely varied Archosauriformes; and a polyphyletic collection of long-necked reptiles including *Protorosaurus*, tanystropheids, and *Prolacerta*. Some researchers also place pantestudines (turtles and their extinct relatives) and the semiaquatic choristoderes within Archosauromorpha.

Archosauromorpha is one of the most diverse reptile groups, but its members share several skeletal features: laminae on the vertebrae, a posterodorsal process on the premaxilla, no notochordal canals, and the loss of the entepicondylar foramen in the humerus.

Friedrich von Huene first used the term Archosauromorpha in 1946 for reptiles closer to archosaurs than to lepidosaurs, but there was little agreement on ancient reptile relationships until the late 20th century, so the name saw little use for decades. The rise of cladistics helped clarify that a split between the archosaur and lepidosaur lineages occurred sometime in the Permian, with certain reptiles clearly aligning with archosaurs. Jacques Gauthier revived the term Archosauromorpha for the archosaur lineage at a 1982 meeting of the American Society of Zoologists and later in his 1984 Ph.D. thesis. His version included four groups: Rhynchosauria, "Prolacertiformes," "Trilophosauria," and Archosauria (now equivalent to Archosauriformes). Cladistic analyses in the 1980s by Gauthier, Michael J. Benton, and Susan E. Evans applied this scheme to large reptile studies.

Michel Laurin (1991) defined Archosauromorpha as a node-based clade containing the most recent common ancestor of *Prolacerta*, *Trilophosaurus*, *Hyperodapedon*, and all its descendants. David Dilkes (1998) offered a broader, branch-based definition: *Protorosaurus* and all other saurians more closely related to it than to Lepidosauria. Gauthier, in *Phylonyms* (2020), redefined it as a node-based clade containing *Gallus*, *Alligator*, *Mesosuchus*, *Trilophosaurus*, *Prolacerta*, and *Protorosaurus*. The name Pan-Archosauria was created for the wider total group, similar to Dilkes’ definition.

In 2016, Martin Ezcurra named Crocopoda ("crocodile feet"), a subgroup of Archosauromorpha that includes all archosauromorphs more closely related to allokotosaurs (like *Azendohsaurus* and *Trilophosaurus*), rhynchosaurs (like *Rhynchosaurus*), or archosauriforms (like *Proterosuchus*) than to *Protorosaurus* or tanystropheids (like *Tanystropheus*). This group roughly matches Laurin’s definition.

Since the 1980s, Archosauromorpha has consistently included four reptile groups, though their definitions and validity have been questioned. The least controversial is Rhynchosauria ("beak reptiles"), a monophyletic clade of stocky herbivores, many with beak-like premaxillae and wide heads. Another group was traditionally represented by *Trilophosaurus*, an iguana-like herbivore. Gauthier called this group "Trilophosauria," but a 2015 study proposed Allokotosauria ("strange reptiles") instead, after finding that Azendohsauridae—Triassic reptiles once mistaken for prosauropod dinosaurs—were close relatives of trilophosaurids. Rhynchosaurs and allokotosaurs did not survive the end of the Triassic.

The most famous group, Archosauriformes, not only survived but continued to diversify after the Triassic-Jurassic extinction. It includes dinosaurs and pterosaurs, with two subclades still alive today: semiaquatic crocodilians and birds (the last feathered dinosaurs). Gauthier originally used Archosauria for what is now called Archosauriformes; today, Archosauria is restricted to the ancestors of crocodilians (Pseudosuchia) and birds (Avemetatarsalia).

The final unambiguous members are the most controversial: the first archosauromorphs to appear, including *Protorosaurus*, tanystropheids, and *Prolacerta*.