Archosaur in the context of Bipeds

Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped /ˈbaɪpɛd/, meaning 'two feet' (from Latin bis 'double' and pes 'foot'). Types of bipedal movement include walking or running (a bipedal gait) and hopping.

Several groups of modern species are habitual bipeds whose normal method of locomotion is two-legged. In the Triassic period some groups of archosaurs, a group that includes crocodiles and dinosaurs, developed bipedalism; among the dinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds are members of a clade of exclusively bipedal dinosaurs, the theropods. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare, hopping mice, pangolins and hominin apes such as australopithecines, including humans, as well as many other extinct groups evolving the trait independently.

The Triassic (/traɪˈæsɪk/; sometimes symbolized as 🝈) is a geologic period and a stratigraphic system that spans 50.5 million years from the end of the Permian Period 251.902 Ma (million years ago) to the beginning of the Jurassic Period 201.4 Ma. The Triassic Period is the first and shortest geologic period of the Mesozoic Era, and the seventh period of the Phanerozoic Eon. The start and the end of the Triassic Period featured major extinction events.

Chronologically, the Triassic Period is divided into three epochs: (i) the Early Triassic, (ii) the Middle Triassic, and (iii) the Late Triassic. The Triassic Period began after the Permian–Triassic extinction event that much reduced the biosphere of planet Earth. The fossil record of the Triassic Period presents three categories of organisms: (i) animals that survived the Permian–Triassic extinction event, (ii) new animals that briefly flourished in the Triassic biosphere, and (iii) new animals that evolved and dominated the Mesozoic Era. Reptiles, especially archosaurs, were the chief terrestrial vertebrates during this time. A specialized group of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period. Archosaurs that became dominant in this period were primarily pseudosuchians, relatives and ancestors of modern crocodilians, while some archosaurs specialized in flight, the first time among vertebrates, becoming the pterosaurs. Therapsids, the dominant vertebrates of the preceding Permian period, saw a brief surge in diversification in the Triassic, with dicynodonts and cynodonts quickly becoming dominant, but they declined throughout the period with the majority becoming extinct by the end. However, the first stem-group mammals (mammaliamorphs), themselves a specialized subgroup of cynodonts, appeared during the Triassic and would survive the extinction event, allowing them to radiate during the Jurassic. Amphibians were primarily represented by the temnospondyls, giant aquatic predators that had survived the end-Permian extinction and saw a new burst of diversification in the Triassic, before going extinct by the end; however, early crown-group lissamphibians (including stem-group frogs, salamanders and caecilians) also became more common during the Triassic and survived the extinction event. The earliest known neopterygian fish, including early holosteans and teleosts, appeared near the beginning of the Triassic, and quickly diversified to become among the dominant groups of fish in both freshwater and marine habitats.

Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and an example of a complex evolutionary novelty. They are among the characteristics that distinguish the extant birds from other living groups.

Although feathers cover most of the bird's body, they arise only from certain well-defined tracts on the skin. They aid in flight, thermal insulation, and waterproofing. In addition, coloration helps in communication and protection. The study of feathers is called plumology (or plumage science).

The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves).

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between non-avian dinosaurs, Archaeopteryx, and modern birds is still under debate.

Crocodilia (/krɒkəˈdɪliə/) is an order of semiaquatic, predatory reptiles that are known as crocodilians. They appeared 83.5 million years ago in the Late Cretaceous period (Campanian stage) and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order includes the true crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae), and the gharial and false gharial (family Gavialidae). Although the term "crocodiles" is sometimes used to refer to all of these families, the term "crocodilians" is less ambiguous.

Extant crocodilians have flat heads with long snouts and tails that are compressed on the sides, with their eyes, ears, and nostrils at the top of the head. Alligators and caimans tend to have broader U-shaped jaws that, when closed, show only the upper teeth, whereas crocodiles usually have narrower V-shaped jaws with both rows of teeth visible when closed. Gharials have extremely slender, elongated jaws. The teeth are conical and peg-like, and the bite is powerful. All crocodilians are good swimmers and can move on land in a "high walk" position, traveling with their legs erect rather than sprawling. Crocodilians have thick skin covered in non-overlapping scales and, like birds, have a four-chambered heart and lungs with unidirectional airflow.

The Mesozoic Era is the era of Earth's geological history, lasting from about 252 to 66 million years ago, comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms such as cycads, ginkgoaceae and araucarian conifers; a hot greenhouse climate; and the tectonic break-up of Pangaea. The Mesozoic is the middle of the three eras since complex life evolved: the Paleozoic, the Mesozoic, and the Cenozoic.

The Mesozoic is commonly known as the Age of the Dinosaurs because the terrestrial animals that dominated both hemispheres for the majority of it were Dinosaurs. This era began in the wake of the Permian–Triassic extinction event, the largest mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs, pterosaurs, mosasaurs, and plesiosaurs. The Mesozoic was a time of significant tectonic, climatic, and evolutionary activity. The supercontinent Pangaea began to break apart into separate landmasses. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today.

Pseudosuchia (from Ancient Greek ψεύδος (pseúdos), meaning "false", and Σοῦχος (Soûkhos), meaning "Sobek") is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs", in contrast to the "bird-line archosaurs" or Avemetatarsalia. Despite Pseudosuchia meaning "false crocodiles", the name is a misnomer as true crocodilians are now defined as a subset of the group.

The clade Pseudosuchia is potentially equivalent to another term, Crurotarsi, even though the latter has a different, node-based definition: "all taxa the least inclusive clade containing Rutiodon carolinensis (Emmons, 1856), and Crocodylus niloticus (Laurenti, 1768)." Many paleontologists of the late 20th century took this proposal for granted, using Crurotarsi as the term for crocodilian ancestors. In 2011, a major revision of Triassic archosaur relations proposed that Rutiodon's group, Phytosauria, was not as closely related to other traditional "crurotarsans", at least compared to avemetatarsalians such as pterosaurs and dinosaurs. Under that interpretation, Crurotarsi would be a much broader clade than Pseudosuchia. Other recent studies continue to support a more traditional phylogeny with phytosaurs as an early branch of Pseudosuchia. If the traditional interpretation is maintained, Pseudosuchia and Crurotarsi are roughly equivalent categories.

Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes referred to as parasuchians. Phytosauria, Parasuchia, Parasuchidae, and Phytosauridae have often been considered equivalent groupings containing the same species. Some recent studies have offered a more nuanced approach, defining Parasuchidae and Phytosauridae as nested clades within Phytosauria as a whole. The clade Phytosauria was defined by Paul Sereno in 2005 as Rutiodon carolinensis and all taxa more closely related to it than to Aetosaurus ferratus, Rauisuchus tiradentes, Prestosuchus chiniquensis, Ornithosuchus woodwardi, or Crocodylus niloticus (the Nile crocodile). Phytosaurs were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodilians in size, appearance, and lifestyle, as an example of convergence or parallel evolution.

The name phytosaur means 'plant lizard', as the first fossils of phytosaurs were mistakenly thought to belong to plant-eaters.

Aetosaurs (/eɪˌɛtoʊˈsɔːr/) are heavily armored reptiles belonging to the extinct order Aetosauria (/eɪˌɛtoʊˈsɔːriə/; from Greek, ἀετός (aetos, "eagle") and σαυρος (sauros, "lizard")). They were medium- to large-sized omnivorous or herbivorous pseudosuchians, part of the branch of archosaurs more closely related to crocodilians than to birds and non-avian dinosaurs. All known aetosaurs are restricted to the Late Triassic, and in some strata from this time they are among the most abundant fossil vertebrates. They have small heads, upturned snouts, erect limbs, and a body ornamented with four rows of plate-like osteoderms (bony scutes). Aetosaur fossil remains are known from Europe, North and South America, parts of Africa, and India. Since their armoured plates are often preserved and are abundant in certain localities, aetosaurs serve as important Late Triassic tetrapod index fossils. Many aetosaurs had wide geographic ranges, but their stratigraphic ranges were relatively short. Therefore, the presence of particular aetosaurs can accurately date a site in which they are found.

Nearly all aetosaurs (except for the genus Aetosauroides) belong to the family Stagonolepididae. Over 20 genera of aetosaurs have been described, and recently there has been controversy regarding the description of some of these genera. Two distinct subdivisions of aetosaurs are currently recognized, Desmatosuchia and Aetosaurinae, based primarily on broad differences in skull morphology. Osteoderms structure is generally one of the most useful traits for inferring aetosaur relations more precisely. Among other archosaurs, aetosaurs are most closely related to Revueltosaurus, a small reptile originally known from teeth mistakenly referred to herbivorous dinosaurs.

"Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians than to birds and other dinosaurs. First named in the 1940s, Rauisuchia was a name exclusive to Triassic archosaurs which were generally large (often 4 to 6 metres (13 to 20 ft)), carnivorous, and quadrupedal with a pillar-erect hip posture, though exceptions exist for all of these traits. Rauisuchians, as a traditional taxonomic group, were considered distinct from other Triassic archosaur groups such as early dinosaurs, phytosaurs (crocodile-like carnivores), aetosaurs (armored herbivores), and crocodylomorphs (lightly-built crocodilian ancestors).

However, more recent studies on archosaur evolution have upended this idea based on phylogenetic analyses and cladistics, a modern approach to taxonomy based on clades (nested monophyletic groups of common ancestry). Since the early 2010s, archosaur classification schemes have stabilized on a system where Rauisuchia is rendered an evolutionary grade, or even a wastebin taxon. Crocodylomorphs most likely originated from a rauisuchian ancestor based on a myriad of shared traits, and some "rauisuchians" (such as Postosuchus and Rauisuchus) appear to be more closely related to crocodylomorphs than to other "rauisuchians" (such as Prestosuchus and Saurosuchus).

Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinosaurs were the largest terrestrial animals for much of the Mesozoic Era, and one group of small feathered dinosaurs (Aves, i.e. birds) has survived up to the present day. Pterosaurs were the first flying vertebrates and persisted through the Mesozoic before dying out at the Cretaceous-Paleogene (K-Pg) extinction event. Both dinosaurs and pterosaurs appeared in the Triassic Period, shortly after avemetatarsalians as a whole. The name Avemetatarsalia was first established by British palaeontologist Michael Benton in 1999. An alternate name is Pan-Aves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodilians.

Although dinosaurs and pterosaurs were the only avemetatarsalians to survive past the end of the Triassic, other groups flourished during the Triassic. The most basal (earliest-branching) and plesiomorphic ("primitive") known avemetatarsalians were the aphanosaurs. Aphanosaurs were rare, four-legged carnivores which were only properly distinguished as a group in 2017. The split between dinosaurs and pterosaurs occurred just after aphanosaurs branched off the archosaur family tree. This split corresponds to the subgroup Ornithodira (Ancient Greek ὄρνις (órnis, "bird") + δειρή (deirḗ, "throat"), defined as the last common ancestor of dinosaurs and pterosaurs, and all of its descendants. Until the discovery of aphanosaurs, Ornithodira and Avemetatarsalia were considered roughly equivalent concepts.

Euparkeriidae is an extinct family of small carnivorous archosauriforms which lived from the Early Triassic to the Middle Triassic (Anisian). While most other early archosauriforms walked on four limbs, euparkeriids were probably facultative bipeds that had the ability to walk on their hind limbs at times. The most well known member of Euparkeriidae is the species Euparkeria capensis, which was named by paleontologist Robert Broom from the Karoo Basin of South Africa in 1913 and is known from several nearly complete skeletons. The family name was first proposed by German paleontologist Friedrich von Huene in 1920; Huene classified euparkeriids as members of Pseudosuchia, a traditional name for crocodilian-line archosaurs from the Triassic (Pseudosuchia means "false crocodiles"). However, phylogenetic analyses performed in the 21st century place Euparkeriidae as a group of Archosauriformes, a position outside Pseudosuchia and close to the ancestry of both crocodile-line archosaurs and bird-line archosaurs (which include dinosaurs and pterosaurs). However, they are probably not direct ancestors of archosaurs.

Several other species apart from Euparkeria have been assigned to the family, but many are dubious or have been determined to have been placed in the family incorrectly. One study has suggested that Euparkeriidae may not represent a true evolutionary grouping or clade. Instead, the family may represent an evolutionary grade of small archosauriforms (making it paraphyletic) or a group of species that each evolved small body sizes through evolutionary convergence (making it polyphyletic). However, other studies consider the family valid, albeit difficult to diagnose. Euparkeriidae is defined as the most inclusive clade containing Euparkeria capensis but not Crocodylus niloticus (the nile crocodile) or Passer domesticus (the house sparrow).

Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemming from the last common ancestor of Proterosuchidae and Archosauria. Phil Senter (2005) defined it as the most exclusive clade containing Proterosuchus and Archosauria. Gauthier as part of the Phylonyms (2020) defined the clade as the last common ancestor of Gallus, Alligator, and Proterosuchus, and all its descendants. Archosauriforms are a branch of archosauromorphs which originated in the Late Permian (roughly 252 million years ago) and persist to the present day as the two surviving archosaur groups: crocodilians and birds.

Archosauriforms present several traits historically ascribed to the group Archosauria. These include serrated teeth set in deep sockets, a more active metabolism, and an antorbital fenestra (a hole in the skull in front of the eyes). Reptiles with these traits have also been termed "thecodonts" in older methods of classification. Thecodontia is a paraphyletic group, and its usage as a taxonomic category has been rejected under modern cladistic systems. The name Archosauriformes is intended as a monophyletic replacement compatible with modern taxonomy.