Archosauromorpha

Archosauromorpha, meaning "ruling lizard forms" in Greek, is a group of diapsid reptiles. It includes all reptiles that are 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.

The name Archosauromorpha was first used in 1946, but its exact membership wasn't settled until the 1980s. 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.

Though it is one of the most diverse reptile groups, its members share several skeletal features: laminae on the vertebrae, a posterodorsal process on the premaxilla, a lack of notochordal canals, and the loss of the entepicondylar foramen in the humerus.

**History and definition**

Friedrich von Huene coined the term Archosauromorpha in 1946 for reptiles closer to archosaurs than to lepidosaurs. However, because there was little agreement on ancient reptile relationships before the late 20th century, the name was rarely used for decades.

The rise of cladistics helped clarify some reptile relationships, revealing a split between the archosaur and lepidosaur lineages sometime in the Permian. Jacques Gauthier reintroduced 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 of the clade included four groups: Rhynchosauria, "Prolacertiformes", "Trilophosauria", and Archosauria (now roughly equivalent to Archosauriformes). Cladistic studies in the 1980s by Gauthier, Michael J. Benton, and Susan E. Evans used this classification.

In 1991, Michel Laurin defined Archosauromorpha as a node-based clade containing the most recent common ancestor of *Prolacerta*, *Trilophosaurus*, *Hyperodapedon*, and all its descendants. David Dilkes (1998) gave a broader, branch-based definition: *Protorosaurus* and all other saurians closer to it than to Lepidosauria. In 2020, Gauthier and co-authors 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, matching Dilkes's earlier definition.

In 2016, Martin Ezcurra named a subgroup called Crocopoda ("crocodile feet"). It 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 of Archosauromorpha.

**Members**

Since the 1980s, Archosauromorpha has consistently included four reptile groups, though their definitions have been questioned. The least controversial is Rhynchosauria ("beak reptiles"), a monophyletic group of stocky herbivores. Many rhynchosaurs had highly modified skulls 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 an alternative. It found that Azendohsauridae—Triassic reptiles once mistaken for "prosauropod" dinosaurs—were close relatives of *Trilophosaurus* and other trilophosaurids. These are now united as Allokotosauria ("strange reptiles").

Rhynchosaurs and allokotosaurs did not survive the end of the Triassic, but the most famous archosauromorph group—Archosauriformes—did. This diverse group includes dinosaurs and pterosaurs. Two subclades survive 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. These were the first archosauromorphs to appear, represented by *Protorosaurus*, tanystropheids, and *Prolacerta*.