Archosaurs are a group of diapsid amniotes whose living representatives consist of birds and crocodilians. This group also includes all extinct dinosaurs, extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most recent common ancestor of living birds and crocodilians and all of its descendants. It includes two main 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).
|Birds and crocodilians (in this case a yellow-billed stork and a Nile crocodile) are the only living archosaur groups.|
Arctopoda Haeckel, 1895
Archosaurs can be distinguished from other tetrapods on the basis of several synapomorphies, or shared characteristics, first found in a common ancestor. The simplest and most widely agreed synapomorphies of archosaurs include teeth set in sockets, antorbital and mandibular fenestrae (openings in front of the eyes and in the jaw, respectively), and a fourth trochanter (a prominent ridge on the femur). Being set in sockets, the teeth were less likely to be torn loose during feeding. This feature is responsible for the name "thecodont" (meaning "socket teeth"), which paleontologists used to apply to many Triassic archosaurs. Some archosaurs, such as birds, are secondarily toothless. Antorbital fenestrae reduced 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 the catastrophic Permian-Triassic extinction event.
There is some debate about when archosaurs first appeared. Those who classify the Permian reptiles Archosaurus rossicus and/or Protorosaurus speneri as true archosaurs maintain that archosaurs first appeared in the late Permian. Some taxonomists classify both Archosaurus rossicus and Protorosaurus speneri as archosauriforms (not true archosaurs but very closely related); these taxonomists maintain that archosaurs first evolved from archosauriform ancestors during the Olenekian stage of the Early Triassic. The earliest archosaurs were rauisuchians, such as Scythosuchus and Tsylmosuchus, both of which have been found from Russia and date back to the Olenekian.
Synapsids (a group including mammals and their extinct relatives, which are often referred to as "mammal-like reptiles") were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event. Very few large synapsids survived the event, although one form, Lystrosaurus (an herbivorous dicynodont), attained a widespread distribution soon after the extinction. Archosaurs quickly became the dominant land vertebrates in the early Triassic. Fossils from before the mass extinction have only been found around the Equator, but after the event fossils can be found all over the world. The two most commonly suggested explanations for this are:
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 as well adapted in this department as archosaurs, and some cynodonts like Trucidocynodon were large sized predators.
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 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 (most notably the Rauisuchia). Modern crocodilians are crurotarsans that can walk with their limbs sprawling or erect depending on speed of locomotion.
The earliest fossils of Avemetatarsalia ("bird ankles") appear in the Carnian age of the late Triassic, but it is hard to see how they could have evolved from crurotarsans — possibly they actually evolved much earlier, or perhaps they evolved from the last of the "primitive mesotarsal" archosaurs. Ornithodires' "advanced mesotarsal" ankle had a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The ornithodires 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 ornithodires were completely bipedal. The archosaurian fourth trochanter on the femur may have made it easier for ornithodires to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodires 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 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 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 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 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 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 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 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 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 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 the resulting tree. Below is the cladogram from Gauthier (1986):
In 1988, paleontologists Michael Benton and J.M. Clark produced 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, 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 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:
Ornithodira and Crurotarsi are both node-based clades, meaning that they are defined to include 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 the another.
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 is a cladogram modified from Benton (2004) showing this phylogeny:
In Sterling Nesbitt's 2011 monograph on early archosaurs, a phylogenetic analysis found strong support for phytosaurs falling outside Archosauria. Many subsequent studies supported this phylogeny. Because Crurotarsi is defined by the inclusion of phytosaurs, the placement of phytosaurs outside Archosauria means that Crurotarsi must include all of Archosauria. Nesbitt reinstated Pseudosuchia as a clade name for crocodile-line archosaurs, using it as a stem-based taxon. Below is a cladogram modified from Nesbitt (2011):
Crocodilians, pterosaurs and dinosaurs survived the Triassic–Jurassic extinction event about 200 million years ago, but other archosaurs became extinct.
Non-avian dinosaurs and pterosaurs perished in the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago, but birds (the only remaining dinosaur group) and crocodilians survived. Both are descendants of archosaurs, and are therefore archosaurs themselves under phylogenetic taxonomy.
Crocodilians (which include all modern crocodiles, alligators, and gharials) and birds flourish today in the Holocene. It is generally agreed that birds have the most species of all terrestrial vertebrates.
Like the early tetrapods, early archosaurs had a sprawling gait because their hip sockets faced sideways, and the knobs at the tops of their femurs were in line with the femur. In the early to middle Triassic, some archosaur groups developed hip joints that allowed (or required) a more erect gait. This gave them greater stamina, because it avoided Carrier's constraint, i.e. they could run and breathe easily at the same time. There were two main types of joint which allowed erect legs:
It has been pointed out that an upright stance requires more energy, so it may indicate a higher metabolism and a higher body temperature.
Most were large predators, but members of various lines diversified into other niches. Aetosaurs were herbivores and some developed extensive armor. A few crocodilians were herbivores, e.g., Simosuchus, Phyllodontosuchus. The large crocodilian Stomatosuchus may have been a filter feeder. Sauropodomorphs and ornithischian dinosaurs were herbivores with diverse adaptations for feeding biomechanics.
Archosaurs are mainly portrayed as land animals, but:
The metabolism of archosaurs is still a controversial topic. They certainly evolved from cold-blooded ancestors, and the surviving non-dinosaurian archosaurs, crocodilians, are cold-blooded. But crocodilians have some features which are normally associated with a warm-blooded metabolism because they improve the animal's oxygen supply:
So, why did natural selection favour the development of these features, which are very important for active warm-blooded creatures, but of little apparent use to cold-blooded aquatic ambush predators that spend the vast majority of their time floating in water or lying on river banks?
Paleontological evidence shows that the ancestors of living crocodilians were active and endothermic (warm-blooded). Some experts believe that their archosaur ancestors were warm-blooded as well. Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence and show that the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. Crocodilian embryos develop fully 4-chambered hearts at an early stage. Modifications to the growing heart form a pulmonary bypass shunt that includes the left aortic arch, which originates from the right ventricle, the foramen of Panizza between the left and right aortic arches, and the cog‐tooth valve at the base of the pulmonary artery. The shunt is used during diving to make the heart function as 3-chambered heart, providing the crocodilian with the neurally controlled shunting used by ectotherms. The researchers concluded that the ancestors of living crocodilians had fully 4-chambered hearts, and were therefore warm-blooded, before they reverted to a cold-blooded or ectothermic metabolism. The authors also provide other evidence for endothermy in stem archosaurs. It is reasonable to suggest that later crocodilians developed the pulmonary bypass shunt as they became cold-blooded, aquatic, and less active.
If the original crocodilians and other Triassic archosaurs were warm-blooded, this would help to resolve some evolutionary puzzles:
A recent study of the lungs of the American alligator has shown that the airflow through them is unidirectional, moving in the same direction during inhalation and exhalation. This is also seen in birds and many non-avian dinosaurs, which have air sacs to further aid in respiration. Both birds and alligators achieve unidirectional air flow through the presence of parabronchi, which are responsible for gas exchange. The study has found that in alligators, air enters through the second bronchial branch, moves through the parabronchi, and exits through the first bronchial branch. Unidirectional airflow in both birds and alligators suggests that this type of respiration was present in basal Triassic archosaurs and their non-dinosaurian descendants, including phytosaurs, aetosaurs, rauisuchians, crocodylomorphs, and pterosaurs. The use of unidirectional airflow in the lungs of archosaurs may have given the group an advantage over synapsids, which had lungs where air moved tidally in and out through a network of bronchi that terminated in alveoli, which were cul-de-sacs. The better efficiency in gas transfer seen in archosaur lungs may have been advantageous during the times of low atmospheric oxygen which are thought to have existed during the Mesozoic.
Most archosaurs are oviparous. Birds and crocodilians lay hard-shelled eggs, as did extinct dinosaurs, and crocodylomorphs. Hard-shelled eggs are present in both dinosaurs and crocodilians, which has been used as an explanation for the absence of viviparity or ovoviviparity in archosaurs. However, both pterosaurs and baurusuchids have soft-shelled eggs, implying that hard shells are not a plesiomorphic condition. The pelvic anatomy of Cricosaurus and other metriorhynchids and fossilized embryos belonging to the non-archosaur archosauromorph Dinocephalosaurus, together suggest that the lack of viviparity among archosaurs may be a consequence of lineage-specific restrictions.
Archosaurs are ancestrally superprecocial as evidenced in various dinosaurs, pterosaurs, and crocodylomorphs. However, parental care did evolve independently multiple times in crocodilians, dinosaurs, and aetosaurs. In most such species the animals bury their eggs and rely on temperature-dependent sex determination. The notable exception are Neornithes which incubate their eggs and rely on genetic sex determination – a trait that might have given them a survival advantage over other dinosaurs.
Adamanasuchus is an extinct genus of aetosaur. Fossils have been found from several localities from the Chinle Group in Arizona and date back to the late Carnian stage of the Late Triassic. The locality from which it was named after also lends its name to the Adamanian LVF (land vertebrate faunachron).Adzhosuchus
Adzhosuchus is an extinct genus of crocodylomorph in the family Shartegosuchidae. Fossils have been found from southwestern Mongolia that date back to the Late Jurassic period.Alligatoroidea
Alligatoroidea is a superfamily of crocodilians that evolved in the Late Cretaceous period. Cladistically, Alligatoroidea is Alligator mississippiensis (the American alligator) and all crocodylians more closely related to A. mississippiensis than to either Crocodylus niloticus (the Nile crocodile) or Gavialis gangeticus (the gharial).Arganasuchus
Arganasuchus is a genus of rauisuchid archosaur. Its fossils have been found in Upper Triassic rocks of the Argana Basin, Morocco. It is based on remains found in the 1970s and initially thought to belong to the rauisuchian Ticinosuchus. Bones recovered include part of the upper jaws, neck vertebrae, and hip bones. Arganasuchus was described in 2007 by Nour-Eddine Jalil and Karin Peyer. The type species is A. dutuiti. Arganasuchus had several anatomical details in common with the rauisuchids Batrachotomus, Fasolasuchus, and Postosuchus. It is thought Arganasuchus was a carnivore, due to dentary characteristics.Bromsgroveia
Bromsgroveia is an extinct genus of predatory archosaur from the Triassic of England. Ctenosauriscids were a group of rauisuchians that was related to the ancestors of modern crocodiles and alligators.Crocodyloidea
The Crocodyloidea superfamily of crocodilians evolved in the Late Cretaceous period. Cladistically, it is defined as Crocodylus niloticus (the Nile crocodile) and all crocodylians more closely related to C. niloticus than to either Alligator mississippiensis (the American alligator) or Gavialis gangeticus (the gharial).Ctenosauriscidae
Ctenosauriscidae is an extinct family of pseudosuchian archosaurs within the clade Poposauroidea. Ctenosauriscids existed in Africa, Asia, Europe and North America during the Early Triassic to the Middle Triassic period (latest Olenekian to Anisian stages). All species had large "sails" on their backs. Ctenosauriscids are among the earliest archosaurs and represent the first global radiation of the group.Dianosuchus
Dianosuchus is an extinct genus of protosuchid crocodylomorph. Fossils have been found from the Dark Red Beds of the Lower Lufeng Formation in Yunnan, China, dating back to the Sinemurian stage of the Early Jurassic. It is characterized by an unusually flattened snout compared to other protosuchians, conical isodont teeth that lacked striations, and very small antorbital fenestrae.Dolichobrachium
Dolichobrachium is a dubious genus of extinct poposaurid crurotarsan. Fossils have been found from the Popo Agie Formation in Wyoming and are of Late Triassic age. It was one of the first rauisuchians to have been named.Eucoelophysis
Eucoelophysis (meaning "true hollow form") is a genus of dinosauriform from the Late Triassic (Norian) period Chinle Formation of New Mexico. It was assumed to be a coelophysid upon description, but a study by Nesbitt et al. found that it was actually a close relative of Silesaurus, which was independently supported by Ezcurra (2006), who found it to be the sister group to Dinosauria, and Silesaurus as the next most basal taxon.However, the relationships of Silesaurus are uncertain. Dzik found it to be a dinosauriform (the group of archosaurs from which the dinosaurs evolved), but did not rule out the possibility that it represents a primitive ornithischian.Jushatyria
Jushatyria is an extinct genus of archosaur. Fossils have been found in the Koltaevo III Locality, district of Kumertau near the Ural Mountains in European Russia from the Bukobay Gorizont. The locality dates back to the Ladinian stage of the Middle Triassic. Additional material has been described from a locality on the banks of the Berdyanka River that was previously assigned to a rauisuchid-like archosaur. However, this material differed from the original specimens because it lacked slit-like antorbital openings accompanying the antorbital fossa. Nesbitt (2009) and Gower and Sennikov (2000) suggested that all material currently referred to Jushatyria most likely does not represent a single taxon. Thus, Jushatyria is known only from its holotype PIN 2867/5, an incomplete left maxilla. As the maxilla is damaged, many "rauisuchian" characters could not be verified. Jushatyria was reassigned as an indeterminate archosaur on the basis of the presence of an antorbital fossa on the lateral surface of the maxilla.Lewisuchus
Lewisuchus is a genus of archosaur that lived during the Middle Triassic (Ladinian); it was a silesaurid dinosauriform, a member of the group of reptiles which led to the dinosaurs. Lewisuchus was about 1 metre (3.3 ft) long. Fossils have been found in the Chañares Formation of Argentina. It exhibited osteoderms along its back.Mesoeucrocodylia
Mesoeucrocodylia is the clade that includes Eusuchia and crocodyliforms formerly placed in the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, and continues to the present day.Protosuchus
Protosuchus is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name Protosuchus means "first crocodile", and is among the earliest animals that resemble crocodilians. Protosuchus was about 1 metre (3.3 ft) in length and about 40 kilograms (88 lb) in weight.
As an early crocodile relative, its skull featured more crocodilian characteristics than its earlier ancestors; it had short jaws that broadened out at the base of the skull, providing a large surface to which its jaw muscles could attach. This increased the maximum gape of the animal's mouth and the force with which the jaws could be closed. The dentition of the animal also resembled modern crocodiles, including the teeth in the lower jaw that fitted into notches on either side of the upper jaw when the mouth was closed. It also possessed a powerful tail which later developed into a propulsion mechanism through water in its descendants.
The body was covered and reinforced by scales, and along of the back was a double row of bony plates. It was an unusual quadrupedal reptile whose legs were columnar, with the rear legs longer than the front legs. Its five toes were clawed and it is believed that they were good runners and good swimmers.Sichuanosuchus
Sichuanosuchus is an extinct genus of crocodylomorph from the Late Jurassic and possibly Early Cretaceous of China.Smok (archosaur)
Smok (meaning "dragon" in Polish) is an extinct genus of large carnivorous archosaur. It lived during the latest Triassic period (latest Norian to early Rhaetian stage, between 205–200 Ma). Its remains have been found in Lisowice, southern Poland. The type species is Smok wawelski (after the Wawel Dragon) and was named in 2012. It is larger than any other known predatory archosaur from the Late Triassic or Early Jurassic of central Europe. The relation of Smok to other archosaurs has not yet been thoroughly studied; it may be a rauisuchid, prestosuchid or ornithosuchid crurotarsan (part of the crocodile line of archosaurs) or a theropod dinosaur (part of the bird line of archosaurs).Ticinosuchus
Ticinosuchus is an extinct genus of pseudosuchian archosaur from the Middle Triassic (Anisian - Ladinian) of Switzerland and Italy.Ziphosuchia
Ziphosuchia is a clade of mesoeucrocodylian crocodyliforms that includes notosuchians and sebecosuchiaZosuchus
Zosuchus ("Zos [Canyon] crocodile") is a genus of basal, Late Cretaceous crocodyliform from the Mongolia.
The type species is Z. davidsoni, after Amy Davidson.
Extant chordate classes