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) than lepidosaurs (such as tuataras, lizards, and snakes). 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. 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 a node-based clade containing the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon and all of its descendants. David Dilkes (1998) formulated a more inclusive (and more common) definition of Archosauromorpha, defining it as a branch-based total group clade containing Protorosaurus and all other saurians that are more closely related to Protorosaurus than to Lepidosauria. Gauthier, as an author for Phylonyms (2020), redefined Archosauromorpha as a node-based clade containing Gallus, Alligator, Mesosuchus, Trilophosaurus, Prolacerta, and Protorosaurus. The new name Pan-Archosauria was established for the broader total group of Archosauromorpha, similar to the definition of Dilkes (1998).
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 and Trilophosaurus), rhynchosaurs (specifically Rhynchosaurus), or archosauriforms (specifically Proterosuchus) rather than Protorosaurus or tanystropheids (specifically Tanystropheus). This group roughly corresponds to Laurin's definition of Archosauromorpha.
Members
Unambiguous members
left|thumb|238x238px|An interaction between two archosauromorphs: [[Ornithosuchus ( a member of Archosauriformes) scavenging on Hyperodapedon (a rhynchosaur)]]
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.
thumb|[[Shringasaurus, a horned allokotosaurian from the family Azendohsauridae.]]
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).
left|thumb|274x274px|The skeleton of [[Protorosaurus, one of the oldest archosauromorphs and namesake of the problematic group "Protorosauria"]]
The final unambiguous members of Archosauromorpha represent the most controversial group. These were the first archosauromorphs to appear, and can be characterized by their long necks, sprawling posture, and carnivorous habits. One name for the group, Protorosauria, is named after Protorosaurus, the oldest archosauromorph known from good remains. Another name, Prolacertiformes, is in reference to a different member, Prolacerta. Protorosauria/Prolacertiformes has had a complicated history, and many taxa have entered and left the group as paleontologists discover and re-evaluate reptiles of the Triassic. By far the most famous of these are tanystropheids such as Tanystropheus, known for having necks longer than their entire body. Other notable genera include Boreopricea, Pamelaria, and Macrocnemus, as well as strange gliding reptiles such as Sharovipteryx and Mecistotrachelos. A landmark 1998 study by David Dilkes completely deconstructed the concept of Prolacertiformes as a traditional monophyletic group (i.e. one whose members have a single common ancestor). He argued that Prolacerta was much closer to Archosauriformes than to other "prolacertiforms", invalidating the name. The aquatic thalattosaurs If this evidence is accurate, then turtles are part of basal Archosauromorpha. Likewise, extinct turtle relatives known as Pantestudines would also fall within Archosauromorpha. Some geneticists have proposed a name to refer to reptiles within the group formed by relatives of turtles and archosaurs. This name is the clade Archelosauria. Since Pantestudines may encompass the entire aquatic reptile order Sauropterygia, this means that Archosauromorpha (as Archelosauria) may be a much wider group than commonly believed. However, anatomical data disagrees with this genetic evidence, instead placing Pantestudines within Lepidosauromorpha but many modern studies have supported Archelosauria. Several recent studies place sauropterygians within Archosauromorpha group, forming a large clade including Ichthyosauromorpha and Thalattosauria as opposed to the Pantestudine relations.
Anatomy
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.
Thin, plate-like ridges known as laminae develop to connect the vertebral components, sloping down from the elongated transverse processes to the centra. Laminae are practically unique to archosauromorphs, being present even in the earliest Permian genera such as Aenigmastropheus and Eorasaurus. However, they are also known to occur in the bizarre semiaquatic reptile Helveticosaurus, In all adult archosauromorphs with the exception of Aenigmastropheus, the vertebrae lack notochordal canals, holes which perforate the centra. This also sets the archosauromorphs apart from most other Permian and Triassic reptiles.
The following cladogram is based on a large analysis of archosauriforms published by M.D. Ezcurra in 2016.