51-592: Archosauria ( lit. ' ruling reptiles ' ) or archosaurs ( / ˈ ɑːr k ə ˌ s ɔːr / ) is a clade of diapsid sauropsid tetrapods , with birds and crocodilians being the only extant representatives. Although broadly classified as reptiles , which traditionally exclude birds, the cladistic sense of the term includes all living and extinct relatives of birds and crocodilians such as non-avian dinosaurs , pterosaurs , phytosaurs , aetosaurs and rauisuchians as well as many Mesozoic marine reptiles . Modern paleontologists define Archosauria as
102-534: A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
153-493: A crown group that includes the most recent common ancestor of living birds and crocodilians, and all of its descendants. The base of Archosauria splits into two clades: Pseudosuchia , which includes crocodilians and their extinct relatives; and Avemetatarsalia , which includes birds and their extinct relatives (such as non-avian dinosaurs and pterosaurs). Older definitions of the group Archosauria rely on shared morphological characteristics, such as an antorbital fenestra in
204-545: A monophyletic grouping, thus forming a true clade. One of the first studies of archosaur phylogeny was authored by French paleontologist Jacques Gauthier in 1986. Gauthier split Archosauria into Pseudosuchia , the crocodilian line, and Ornithosuchia , the dinosaur and pterosaur line. Pseudosuchia was defined as all archosaurs more closely related to crocodiles, while Ornithosuchia was defined as all archosaurs more closely related to birds. Proterochampsids, erythrosuchids, and proterosuchids fell successively outside Archosauria in
255-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
306-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
357-534: A new tree in a phylogenetic study of basal archosaurs. As in Gauthier's tree, Benton and Clark's revealed a basal split within Archosauria. They referred to the two groups as Crocodylotarsi and Ornithosuchia. Crocodylotarsi was defined as an apomorphy -based taxon based on the presence of a "crocodile-normal" ankle joint (considered to be the defining apomorphy of the clade). Gauthier's Pseudosuchia, by contrast,
408-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
459-538: A simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The earliest avemetatarsalians, such as Teleocrater and Asilisaurus, retained "primitive mesotarsal" ankles. The ornithodirans differed from other archosaurs in other ways: they were lightly built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodirans were completely bipedal . The archosaurian fourth trochanter on
510-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
561-475: A valid grouping. Because they are considered a "basal stock", thecodonts are paraphyletic , meaning that they form a group that does not include all descendants of its last common ancestor: in this case, the more derived crocodilians and birds are excluded from "Thecodontia" as it was formerly understood. The description of the basal ornithodires Lagerpeton and Lagosuchus in the 1970s provided evidence that linked thecodonts with dinosaurs, and contributed to
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#1732779929783612-405: A wide range of taxa including dinosaurs , crocodilians , thecodonts , sauropterygians (which may be related to turtles), rhynchocephalians (a group that according to Cope included rhynchosaurs , which nowadays are considered to be more basal archosauromorphs , and tuataras , which are lepidosaurs ), and anomodonts , which are now considered synapsids. It was not until 1986 that Archosauria
663-435: Is a cladogram modified from Benton (2004) showing this phylogeny: † Hyperodapedon ( Rhynchosauria ) [REDACTED] † Prolacerta ( Prolacertiformes ) [REDACTED] † Proterosuchus ( Proterosuchidae ) [REDACTED] † Euparkeria ( Euparkeriidae ) [REDACTED] † Proterochampsidae [REDACTED] † Phytosauridae [REDACTED] Clade In biological phylogenetics ,
714-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
765-476: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
816-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
867-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
918-498: Is within the larger clade Archosauriformes , which includes some close relatives of archosaurs, such as proterochampsids and euparkeriids . These relatives are often referred to as archosaurs despite being placed outside of the crown group Archosauria in a more basal position within Archosauriformes. Historically, many archosauriforms were described as archosaurs, including proterosuchids and erythrosuchids , based on
969-539: The Anisian stage (247–242 Ma) of Tanzania , and include Asilisaurus (an early silesaurid ), Teleocrater (an aphanosaur ), and Nyasasaurus (a possible early dinosaur). Synapsids are a clade that includes mammals and their extinct ancestors . The latter group are often referred to as mammal-like reptiles, but should be termed protomammals, stem mammals, or basal synapsids, because they are not true reptiles by modern cladistic classification. They were
1020-592: The Early Triassic period, though the first archosauriforms and archosauromorphs (reptilians closer to archosaurs than to lizards or other lepidosaurs ) appeared in the Permian . Archosaurs quickly diversified in the aftermath of the Permian-Triassic mass extinction (~252 Ma ), which wiped out most of the then- dominant therapsid competitors such as the gorgonopsians and anomodonts , and
1071-473: The K-Pg extinction, rediversifying in the subsequent Cenozoic era. Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day. Archosaurs can traditionally be distinguished from other tetrapods on the basis of several synapomorphies , or shared characteristics, which were present in their last common ancestor . Many of these characteristics appeared prior to
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#17327799297831122-584: The Olenekian stage (247–251 Ma) of the Early Triassic . A few fragmentary fossils of large carnivorous crocodilian-line archosaurs (informally termed " rauisuchians ") are known from this stage. These include Scythosuchus and Tsylmosuchus (both of which have been found in Russia ), as well as the Xilousuchus , a ctenosauriscid from China . The oldest known fossils of bird-line archosaurs are from
1173-530: The Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus. The earliest fossils of Avemetatarsalia ("bird ankles") appear in the Anisian age of the Middle Triassic . Most Ornithodirans had "advanced mesotarsal" ankles. This form of ankle incorporated a very large astragalus and very small calcaneum, and could only move in one plane, like
1224-469: The Triassic . In their ankles, the astragalus was joined to the tibia by a suture and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs. Modern crocodilians are crurotarsans that can employ a diverse range of gaits depending on speed. Euparkeria and
1275-547: The last common ancestor of two or more taxa and all of its descendants. Ornithodira includes the last common ancestor of pterosaurs and dinosaurs (which include birds), while Crurotarsi includes the last common ancestor of living crocodilians and three groups of Triassic archosaurs: ornithosuchids , aetosaurs , and phytosaurs . These clades are not equivalent to "bird-line" and "crocodile-line" archosaurs, which would be branch-based clades defined as all taxa more closely related to one living group (either birds or crocodiles) than
1326-438: The 20th century. Thecodonts were considered the "basal stock" from which the more advanced archosaurs descended. They did not possess features seen in later avian and crocodilian lines, and therefore were considered more primitive and ancestral to the two groups. With the cladistic revolution of the 1980s and 90s, in which cladistics became the most widely used method of classifying organisms, thecodonts were no longer considered
1377-399: The basal split and thought that the crurotarsan ankle developed independently in these two groups, but in opposite ways. Cruickshank also thought that the development of these ankle types progressed in each group to allow advanced members to have semi-erect (in the case of crocodilians) or erect (in the case of dinosaurs) gaits. In many phylogenetic analyses, archosaurs have been shown to be
1428-442: The catastrophic Permian-Triassic extinction event . Unlike their close living relatives, the lepidosaurs, archosaurs lost the vomeronasal organ . Archosaurs are a subgroup of archosauriforms , which themselves are a subgroup of archosauromorphs . Both the oldest archosauromorph ( Protorosaurus speneri ) and the oldest archosauriform ( Archosaurus rossicus ) lived in the late Permian. The oldest true archosaurs appeared during
1479-492: The disuse of the term "Thecodontia", which many cladists consider an artificial grouping. With the identification of "crocodilian normal" and "crocodilian reversed" ankles by Sankar Chatterjee in 1978, a basal split in Archosauria was identified. Chatterjee considered these two groups to be Pseudosuchia with the "normal" ankle and Ornithosuchidae with the "reversed" ankle. Ornithosuchids were thought to be ancestral to dinosaurs at this time. In 1979, A.R.I. Cruickshank identified
1530-498: The dominant land vertebrates throughout the Permian , but most perished in the Permian–;Triassic extinction event . Very few large synapsids survived the event, but one form, Lystrosaurus (a herbivorous dicynodont ), attained a widespread distribution soon after the extinction. Following this, archosaurs and other archosauriforms quickly became the dominant land vertebrates in the early Triassic . Fossils from before
1581-452: The femur may have made it easier for ornithodirans to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodirans diversified to produce dinosaurs and pterosaurs . Archosauria is normally defined as a crown group , which means that it only includes descendants of the last common ancestors of its living representatives. In the case of archosaurs, these are birds and crocodilians. Archosauria
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1632-451: The fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
1683-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
1734-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
1785-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
1836-402: The main explanation for Mesozoic mammals being small. Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot. The Pseudosuchia appeared early in
1887-661: The mass extinction have only been found around the Equator, but after the event fossils can be found all over the world. Suggested explanations for this include: However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which tested) and the fact that desert dwelling mammals are as well adapted in this department as archosaurs, and some cynodonts like Trucidocynodon were large sized predators. A study favors competition amidst mammaliaforms as
1938-503: The middle of the femur shaft that serves as a muscle attachment, mainly for the musculus caudofemoralis longus , the main retractor tail muscle that pulls the thighbone to the rear. The fourth trochanter is considered homologous with the internal trochanter , an asymmetrical ridge-like structure that extends down from the femoral head and is edged by an intertrochanteric fossa in other reptiles such as lizards . The fourth trochanter can be characterized by its position further down
1989-780: The monophyly of both of these clades were questioned. Sereno and Arcucci incorporated archosaur features other than ankle types in their analyses, which resulted in a different tree than previous analyses. Below is a cladogram based on Sereno (1991), which is similar to the one produced by Sereno and Arcucci: † Proterosuchidae [REDACTED] † Erythrosuchidae [REDACTED] † Euparkeria [REDACTED] † Proterochampsidae [REDACTED] † Parasuchia [REDACTED] † Ornithosuchidae [REDACTED] Suchia [REDACTED] † ? Scleromochlus † Pterosauria [REDACTED] Dinosauromorpha [REDACTED] Ornithodira and Crurotarsi are both node-based clades, meaning that they are defined to include
2040-422: The origin of the clade Archosauria, as they were present in archosauriforms such as Proterosuchus and Euparkeria , which were outside the crown group . The most obvious features include teeth set in deep sockets, antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively), and a pronounced fourth trochanter (a prominent ridge on the femur ). Being set in sockets,
2091-399: The other. Benton proposed the name Avemetatarsalia in 1999 to include all bird-line archosaurs (under his definition, all archosaurs more closely related to dinosaurs than to crocodilians). His analysis of the small Triassic archosaur Scleromochlus placed it within bird-line archosaurs but outside Ornithodira, meaning that Ornithodira was no longer equivalent to bird-line archosaurs. Below
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2142-449: The presence of an antorbital fenestra. While many researchers prefer to treat Archosauria as an unranked clade , some continue to assign it a traditional biological rank. Traditionally, Archosauria has been treated as a Superorder, though a few 21st century researchers have assigned it to different ranks including Division and Class. Archosauria as a term was first coined by American paleontologist Edward Drinker Cope in 1869, and included
2193-518: The resulting tree. Below is the cladogram from Gauthier (1986): † Proterosuchidae [REDACTED] † Erythrosuchidae [REDACTED] † Proterochampsidae [REDACTED] † Parasuchia [REDACTED] † Aetosauria [REDACTED] † Rauisuchia [REDACTED] Crocodylomorpha [REDACTED] † Euparkeria [REDACTED] † Ornithosuchidae [REDACTED] Ornithodira [REDACTED] In 1988, paleontologists Michael Benton and J. M. Clark produced
2244-531: The shaft, symmetrical nature, and lack of an intertrochanteric fossa. The caudofemoralis attachment crest first separated from the femoral head in the Erythrosuchidae , large basal archosauriform predators of the early Triassic period. Shortly afterwards, eucrocopodan archosauriforms (such as Euparkeria ) evolved, losing the intertrochanteric fossa and acquiring a symmetrical fourth trochanter. This seemingly insignificant detail may have made
2295-458: The skull, serrated teeth, and an upright stance. Some extinct reptiles, such as proterosuchids and euparkeriids , also possessed these features yet originated prior to the split between the crocodilian and bird lineages. The older morphological definition of Archosauria nowadays roughly corresponds to Archosauriformes , a group named to encompass crown-group archosaurs and their close relatives. The oldest true archosaur fossils are known from
2346-529: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Fourth trochanter The fourth trochanter is a shared characteristic common to archosaurs . It is a knob-like feature on the posterior-medial side of
2397-468: The subsequent arid Triassic climate allowed the more drought -resilient archosaurs (largely due to their uric acid -based urinary system ) to eventually become the largest and most ecologically dominant terrestrial vertebrates from the Middle Triassic period up until the Cretaceous–Paleogene extinction event (~66 Ma). Birds and several crocodyliform lineages were the only archosaurs to survive
2448-490: The teeth were less likely to be torn loose during feeding. This feature is responsible for the name " thecodont " (meaning "socket teeth"), which early paleontologists applied to many Triassic archosaurs. Additionally, non-muscular cheek and lip tissue appear in various forms throughout the clade, with all living archosaurs lacking non-muscular lips, unlike most non-avian saurischian dinosaurs. Some archosaurs, such as birds, are secondarily toothless. Antorbital fenestrae reduced
2499-446: The weight of the skull, which was relatively large in early archosaurs, rather like that of modern crocodilians . Mandibular fenestrae may also have reduced the weight of the jaw in some forms. The fourth trochanter provides a large site for the attachment of muscles on the femur. Stronger muscles allowed for erect gaits in early archosaurs, and may also be connected with the ability of the archosaurs or their immediate ancestors to survive
2550-513: Was a stem-based taxon . Unlike Gauthier's tree, Benton and Clark's places Euparkeria outside Ornithosuchia and outside the crown group Archosauria altogether. The clades Crurotarsi and Ornithodira were first used together in 1990 by paleontologist Paul Sereno and A. B. Arcucci in their phylogenetic study of archosaurs. They were the first to erect the clade Crurotarsi, while Ornithodira was named by Gauthier in 1986. Crurotarsi and Ornithodira replaced Pseudosuchia and Ornithosuchia, respectively, as
2601-471: Was defined as a crown-clade, restricting its use to more derived taxa. Cope's term was a Greek-Latin hybrid intended to refer to the cranial arches, but has later also been understood as "leading reptiles" or "ruling reptiles" by association with Greek ἀρχός "leader, ruler". The term "thecodont", now considered an obsolete term, was first used by the English paleontologist Richard Owen in 1859 to describe Triassic archosaurs, and it became widely used in
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