Archosauromorph

Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, lizards, and snakes). Archosauromorphs first appeared during the middle Permian, though they became much more common and diverse during the Triassic period. Although Archosauromorpha was first named in 1946, its membership did not become well-established until the 1980s. Currently Archosauromorpha encompasses four main groups of reptiles: the stocky, herbivorous allokotosaurs and rhynchosaurs, the hugely diverse Archosauriformes, and a polyphyletic grouping of various long-necked reptiles including Protorosaurus, tanystropheids, and Prolacerta. Other groups including pantestudines (turtles and their extinct relatives) and the semiaquatic choristoderes have also been placed in Archosauromorpha by some authors. Archosauromorpha is one of the most diverse groups of reptiles, but its members can be united by several shared skeletal characteristics. These include laminae on the vertebrae, a posterodorsal process of the premaxilla, a lack of notochordal canals, and the loss of the entepicondylar foramen of the humerus.

Unambiguous members Since the seminal studies of the 1980s, Archosauromorpha has consistently been found to contain four specific reptile groups, although the definitions and validity of the groups themselves have been questioned. The least controversial group is Rhynchosauria ("beak reptiles"), a monophyletic clade of stocky herbivores. Many rhynchosaurs had highly modified skulls, with beak-like premaxillary bones and wide heads. Another group of archosauromorphs has traditionally been represented by Trilophosaurus, an unusual iguana-like herbivorous reptile quite different from the rhynchosaurs. Gauthier used the name "Trilophosauria" for this group, but a 2015 study offered an alternative name. This study found that Azendohsauridae, Triassic reptiles previously mistaken for "prosauropod" dinosaurs, were in fact close relatives of Trilophosaurus and the rest of Trilophosauridae. Trilophosaurids and azendohsaurids are now united under the group Allokotosauria ("strange reptiles"). These two groups did not survive the end of the Triassic period, but the most famous group of archosauromorphs not only survived, but have continued to diversify and dominate beyond the Triassic-Jurassic extinction. These were the Archosauriformes, a diverse assortment of animals including the famous dinosaurs and pterosaurs. Two subclades of Archosauriformes survive to the present day: the semiaquatic crocodilians and the last of the feathered dinosaurs: birds. Gauthier used the name Archosauria to refer to what is now called the Archosauriformes; in modern studies, the name Archosauria has a more restricted definition that only includes the ancestors of crocodilians (i.e. Pseudosuchia) and birds (i.e. Avemetatarsalia). The final unambiguous members of Archosauromorpha represent the most controversial group. These …

The term Archosauromorpha was first used by Friedrich von Huene in 1946 to refer to reptiles more closely related to archosaurs than to lepidosaurs. However, there was little consensus on ancient reptile relationships prior to the late 20th century, so the term Archosauromorpha was seldom used until many years after its creation. The advent of cladistics helped to sort out at least some of the relationships within Reptilia, and it became clear that there was a split between the archosaur lineage and the lepidosaur lineage somewhere within the Permian, with certain reptiles clearly closer to archosaurs and others allied with lepidosaurs. Jacques Gauthier reused the term Archosauromorpha for the archosaur lineage at the 1982 annual meeting of the American Society of Zoologists, and later used it within his 1984 Ph.D. thesis. Archosauromorpha, as formulated by Gauthier, included four main groups of reptiles: Rhynchosauria, "Prolacertiformes", "Trilophosauria", and Archosauria (now equivalent to the group Archosauriformes). Cladistic analyses created during the 1980s by Gauthier, Michael J. Benton, and Susan E. Evans implemented Gauthier's classification scheme within large studies of reptile relations. Michel Laurin (1991) defined Archosauromorpha as the clade containing the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon and all of its descendants. David Dilkes (1998) formulated a more inclusive (and currently more popular) definition of Archosauromorpha, defining it as the clade containing Protorosaurus and all other saurians that are more closely related to Protorosaurus than to Lepidosauria. In 2016, Martin Ezcurra named a subgroup of Archosauromorpha, Crocopoda ("crocodile feet"). Crocopoda is defined as all archosauromorphs more closely related to allokotosaurs (specifically Azendohsaurus.

Although the most diverse clade of living archosauromorphs are birds, early members of the group were evidently reptilian, superficially similar to modern lizards. When archosauromorphs first appeared in the fossil record in the Permian, they were represented by long-necked, lightly-built sprawling reptiles with moderately long, tapering snouts.This body plan, similar to that of modern monitor lizards, is also shared by Triassic archosauromorphs such as tanystropheids and Prolacerta. Other early groups such as trilohpsaurids, azendohsaurids, and rhynchosaurs deviate from this body plan by evolving into stockier forms with semi-erect postures and higher metabolisms. The archosauriforms went to further extremes of diversity, encompassing giant sauropod dinosaurs, flying pterosaurs and birds, semiaquatic crocodilians, phytosaurs, and proterochampsians, and apex predators such as erythrosuchids, pseudosuchians, and theropod dinosaurs. Despite the staggering diversity of archosauromorphs, they can still be united as a clade thanks to several subtle skeletal features. Skull Most archosauromorphs more "advanced" than Protorosaurus possessed an adaptation of the premaxilla (tooth-bearing bone at the tip of the snout) known as a posterodorsal or postnarial process. This was a rear-facing branch of bone that stretched up below and behind the external nares (nostril holes) to contact the nasal bones on the upper edge of the snout. A few advanced archosauriforms reacquired the plesiomorphic ("primitive") state present in other reptiles, that being a short or absent posterodorsal process of the premaxilla, with the rear edge of the nares formed primarily by the maxilla bones instead. As for the nares themselves, they were generally large and oval-shaped, positioned high and close to the midline of the skull.