Archosaurs Fossil range: Early Triassic - Recent
Living archosaurs include crocodilians and birds. The Early Triassic (also known as Lower Triassic, Buntsandstein, or Scythian) is the first of three epochs of the Triassic Crocodilia is an order of large Reptiles that appeared about 84 million years ago in the late Cretaceous Period ( Campanian stage Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs.
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Subclass: Diapsida Infraclass: Archosauromorpha (unranked) Archosauria Cope, 1869
Clades
Crurotarsi Aetosauria † Crocodilia (crocodiles) Phytosauria † Rauisuchia † Ornithodira Dinosauria (birds) Pterosauria †

Archosaurs (Greek for 'ruling lizards') are a group of diapsid reptiles represented by modern birds and crocodilians. Chordates ( Phylum Chordata) are a group of Animals that includes the Vertebrates together with several closely related Invertebrates Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Diapsids ("two arches" are a group of Reptiles that developed two holes ( Temporal fenestra) in each side of their Skulls about 300 Million Archosauromorpha ( Greek for "ruling lizard forms" is an Infraclass of Diapsid Reptiles that first appeared during the late Permian Edward Drinker Cope (July 28 1840&ndashApril 12 1897 was an American paleontologist and comparative anatomist, as well as a noted herpetologist A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor The Crurotarsi ("cross-ankles" are a group of archosaurs, whose name was erected as a node-based clade by Paul Sereno in 1991 The Aetosaurs (literally "Eagle Lizards" ( family) Stagonolepididae, Order Aetosauria) are an Extinct Clade of heavily In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Crocodilia is an order of large Reptiles that appeared about 84 million years ago in the late Cretaceous Period ( Campanian stage Phytosaurs - family Phytosauridae or Parasuchidae - were a group of large (2 to 12 meters long - average size 3 to 4 meters semi-aquatic predatory In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters Triassic Archosaurs Originally it was believed that they were related In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Ornithodira is a Clade within the larger group Archosauria In 1986 Jacques Gauthier coined the name for a node clade containing the last common Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. For other meanings see Pterodactyl (disambiguation. Pterosaurs (ˈtɛrəsɔr from the Greek πτερόσαυρος pterosauros In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. The Ancient Greek language is the historical stage in the development of the Hellenic language family spanning the Archaic (c Diapsids ("two arches" are a group of Reptiles that developed two holes ( Temporal fenestra) in each side of their Skulls about 300 Million Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Modern birds (subclass Neornithes) are the members of class Aves that have survived into recent times and have coexisted with Humans Modern birds are Crocodilia is an order of large Reptiles that appeared about 84 million years ago in the late Cretaceous Period ( Campanian stage This group also includes extinct non-avian dinosaurs, pterosaurs and relatives of crocodiles. For other meanings see Pterodactyl (disambiguation. Pterosaurs (ˈtɛrəsɔr from the Greek πτερόσαυρος pterosauros

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. The Permian is a geologic period and system that extends from 299 Those who classify both Archosaurus rossicus and Protorosaurus speneri as archosauriformes (not true archosaurs but very closely related) maintain that archosaurs first evolved from Archosauriform ancestors during the Olenekian (early Triassic Period). Archosauriformes ( Greek for 'ruling lizards' and 'form' are a clade of Diapsid Reptiles that developed from Archosauromorph ancestors some time Archosauriformes ( Greek for 'ruling lizards' and 'form' are a clade of Diapsid Reptiles that developed from Archosauromorph ancestors some time The Olenekian (also known as the Yongningzhenian) is a stage of the Early Triassic epoch. The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago

Contents 1 Distinguishing characteristics 2 Archosaur takeover in the Triassic 3 Main types of archosaurs 4 Hip joints and locomotion 5 Extinction and survival 6 Archosaur lifestyle 6.1 Diet 6.2 Land, water and air 6.3 Metabolism 7 Phylogeny 8 References 9 Further reading 10 External links

Distinguishing characteristics

The simplest and most widely-agreed synapomorphies of archosaurs are:

• Teeth set in sockets, which makes them less likely to be torn loose during feeding. In Evolutionary biology, a synapomorphy is a derived Character state shared by two or more terminal groups ( taxa included in a Cladistic analysis This feature is responsible for the name "thecodonts" ("socket teeth"), which paleontologists used to apply to all or most archosaurs. Thecodont ("socket-toothed" reptile now considered an obsolete term was formerly used to describe a diverse range of early Archosaurs that first appeared in the
• Antorbital fenestrae (openings in the skull in front of the eyes but behind the nostrils), which reduced the weight of the skull, a useful feature since most early archosaurs had long, heavy skulls, rather like those of modern crocodilians. Crocodilia is an order of large Reptiles that appeared about 84 million years ago in the late Cretaceous Period ( Campanian stage The preorbital fenestrae (sometimes called anteorbital fenestrae) are often larger than the orbits (eye sockets).
• Mandibular fenestrae (small openings in the jaw bones), which may have reduced the weight of the jaw slightly.
• A fourth trochanter (ridge for attaching muscles) on the femur. The Fourth trochanter is a shared characteristic common to Archosaurs It is a knob-like feature on the Femur that serves as a muscle attachment The femur is the thigh bone In Humans, it is the longest, most voluminous and strongest Bone. This seemingly insignificant detail may have made the evolution of dinosaurs possible (all early dinosaurs and many later ones were bipeds), and may also be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic Permian-Triassic extinction event. The Permian–Triassic (P–Tr extinction event, informally known as the Great Dying, was an Extinction event that occurred, and 70 percent of terrestrial

Archosaur takeover in the Triassic

The Synapsida (informally known as "mammal-like reptiles") were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event. Synapsids ('fused arch' also known as theropsids ('beast face' are a class of Animals that includes Mammals and everything closer to mammals than The Permian is a geologic period and system that extends from 299 The Permian–Triassic (P–Tr extinction event, informally known as the Great Dying, was an Extinction event that occurred, and 70 percent of terrestrial Lystrosaurus (a herbivorous mammal-like reptile) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time. Lystrosaurus (meaning 'shovel lizard' pronunciation in ˌlɪstrɒˈsɔrəs was a Genus of Late Permian and Early Triassic Period ^[1]

But archosaurs quickly became the dominant land vertebrates in the early Triassic. The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago The two most commonly-suggested explanations for this are:

• Archosaurs made quicker progress than mammal-like reptiles towards erect limbs, and this gave them greater stamina by avoiding Carrier's constraint. Carrier's constraint is the observation that air-breathing Vertebrates which have two Lungs and flex their bodies sideways during Locomotion find it very This is unconvincing since Archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of Lystrosaurus and other mammal-like reptiles. Lystrosaurus (meaning 'shovel lizard' pronunciation in ˌlɪstrɒˈsɔrəs was a Genus of Late Permian and Early Triassic Period
• The early Triassic was predominantly arid, because most of the earth's land was concentrated in the supercontinent Pangaea. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 In Geology, a supercontinent is a Landmass comprising more than one Continental core or Craton. Pangaea, Pangæa or Pangea (pænˈdʒiːə from παν pan, meaning entire, and Γαῖα Gaea, meaning Earth in Archosaurs were probably better at conserving water than mammal-like reptiles because:

• Modern diapsids (lizards, snakes, crocodilians, birds) excrete uric acid, which can be excreted as a paste. Diapsids ("two arches" are a group of Reptiles that developed two holes ( Temporal fenestra) in each side of their Skulls about 300 Million Uric acid (or urate) is an Organic compound of Carbon, Nitrogen, Oxygen and Hydrogen with the formula C5H4N4O3 It is reasonable to suppose that archosaurs (diapsids and ancestors of crocodilians, dinosaurs and birds) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.
• Modern mammals excrete urea, which requires a lot of water to keep it dissolved. Urea is an Organic compound with the Chemical formula ( N[[hydrogen H]]22 C[[oxygen O]] Their skins also contain many glands, which also lose water. Assuming that mammal-like reptiles had similar features, e. g. as argued in Palaeos [1], they were at a disadvantage in a mainly arid world. Palaeoscom is a web site on Biology, Paleontology, Cladistics and Geology and which covers the history of Earth The same well-respected site points out that "for much of Australia's Plio-Pleistocene history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic varanid lizards (Megalania) and land crocs. Monitor lizards are the family Varanidae, a group of carnivorous lizards which includes the largest living lizard the Komodo dragon. Megalania ("great roamer" Greek Μέγας "great" + ἀλαίνω "roam" the giant goanna is an Extinct giant "

It has also been suggested that the Triassic was low on oxygen and archosaurs had a more advanced respiratory system. ^[2]

Main types of archosaurs

Since the 1970s scientists have classified archosaurs mainly on the basis of their ankles. ^[3] 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. See Talus for other meanings of the word The talus bone or astragalus of the Ankle joint connects the leg to the foot The calcaneus, or the heel bone is the largest bone of the Human foot. The tibia, shinbone, or shankbone is the larger and stronger of the two Bones in the Leg below the Knee in vertebrates and connects In Anatomy, a suture is a fairly rigid joint between two or more hard elements of an animal without significant overlap

The Crurotarsi appeared early in the Triassic. The Crurotarsi ("cross-ankles" are a group of archosaurs, whose name was erected as a node-based clade by Paul Sereno in 1991 The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago 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). Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters Triassic Archosaurs Originally it was believed that they were related And modern crocodilians are "crurotarsans" which can walk with their limbs sprawling or erect depending on how much of a hurry they are in.

Euparkeria and the Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus. Euparkeria (/juˈpɑː(rkəriːə/ meaning "Parker's good animal" named in honor of W Ornithosuchidae is an Extinct family of quadrupedal and facultatively bipedal Crurotarsan Archosaurs These carnivores were

The earliest fossils of Ornithodira ("bird necks") appear in the Carnian age of the late Triassic, but it is hard to see how they could have evolved from the "crurotarsans" - possibly they actually evolved much earlier, or perhaps they evolved from the last of the "primitive mesotarsal" archosaurs. Ornithodira is a Clade within the larger group Archosauria In 1986 Jacques Gauthier coined the name for a node clade containing the last common The Carnian (less commonly Karnian) is the lowermost stage of the Upper Triassic series (or earliest age of the Late Triassic The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago 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 was only suitable for animals with erect limbs, but provided more stability when the animals were running. The ornothodires 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 ornothodires were completely bipedal. Bipedalism is a form of Terrestrial locomotion where an organism moves by means of its two rear limbs The archosaurian fourth trochanter on the femur may have made it easier for ornothodires to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic the ornithodires diversified to produce pterosaurs and dinosaurs. For other meanings see Pterodactyl (disambiguation. Pterosaurs (ˈtɛrəsɔr from the Greek πτερόσαυρος pterosauros ^[4]

Hip joints and locomotion

Like the early tetrapods, early archosaurs had a sprawling gait because:

• Their hip sockets faced sideways. Tetrapods ( Greek τετραποδη tetrapoda, Latin Quadruped, "four-footed" are Vertebrate Animals
• The knobs at the tops of their femurs were in line with the femur. The femur is the thigh bone In Humans, it is the longest, most voluminous and strongest Bone.

In the early to mid Triassic, some archosaur groups developed hip joints which allowed (or required) a more erect gait. The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago This gave them greater stamina, because it avoided Carrier's constraint, i. Carrier's constraint is the observation that air-breathing Vertebrates which have two Lungs and flex their bodies sideways during Locomotion find it very e. they could run and breathe easily at the same time. There were two main types of joint which allowed erect legs:

• The hip sockets faced sideways but the knobs on the femurs were at right angles to the rest of the femur, which therefore pointed downwards. Dinosaurs evolved from archosaurs with this hip arrangement.
• The hip sockets faced downwards and the knobs on the femurs were in line with the femur. This "pillar-erect" arrangement appears to have evolved more than once independently in various archosaur lineages, for example it was common in Rauisuchia and also appeared in some aetosaurs. Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters Triassic Archosaurs Originally it was believed that they were related The Aetosaurs (literally "Eagle Lizards" ( family) Stagonolepididae, Order Aetosauria) are an Extinct Clade of heavily

Extinction and survival

Crocodilians, pterosaurs, dinosaurs, and champsosaurs survived the Triassic-Jurassic extinction event about 195 million years ago, but other archosaurs became extinct. Choristodera is an order of semi-aquatic Diapsid Reptiles which ranged from the Middle Jurassic, or possibly Late Triassic, to

Non-avian dinosaurs and pterosaurs perished in the Cretaceous-Tertiary extinction event, but crocodilians, champsosaurs, and birds (last surviving dinosaur group) survived. The Cretaceous–Tertiary extinction event, which occurred approximately ( Ma) was a large-scale mass extinction of animal and plant species in a geologically Birds are descendants of archosaurs, and are therefore archosaurs themselves under phylogenetic taxonomy. Phylogenetic nomenclature (PN is an alternative to rank-based nomenclature.

Champsosaurs became extinct in the Early Miocene. The Miocene is a geological epoch of the Neogene period and extends from about 23

Crocodilians (which include all modern crocodiles, alligators, and gharials) and birds flourish today, and it is generally agreed that birds have the most species of all terrestrial vertebrates. A crocodile is any Species belonging to the family Crocodylidae (sometimes classified instead as the Subfamily Crocodylinae) for differences between alligators and crocodiles please see Crocodilia An Alligator is a Crocodilian in the Genus The gharial ( Gavialis gangeticus) sometimes called the Indian gavial or gavial, is one of two surviving members of the family

Archosaur lifestyle

Diet

Most were large predators, but members of various lines diversified into other niches:

• Aetosaurs were herbivores and some developed spectacular armor. The Aetosaurs (literally "Eagle Lizards" ( family) Stagonolepididae, Order Aetosauria) are an Extinct Clade of heavily
• A few crocodilians were herbivores, e. g. Simosuchus.
• The large crocodilian Stomatosuchus may have been a filter feeder. Stomatosuchus inermis ("Weaponless mouth crocodile" was an enormous 12 meter long Eusuchian crocodilian from the Cenomanian Filter feeders (also known as suspension feeders) are Animals that feed by straining suspended matter and food particles from water typically by passing the water

Land, water and air

Archosaurs are mainly portrayed as land animals, but:

• The crocodilians dominated the rivers and swamps and even invaded the seas (the Teleosaurs and Metriorhynchidae). The teleosaurids were marine Crocodilians similar to the modern Gharial that during the Early Jurassic to the Early Cretaceous. Metriorhynchids were a Clade of fully-aquatic Crocodilians that lived in seas of the Jurassic and Cretaceous periods The Metriorhynchidae were rather dolphin-like, with paddle-like forelimbs, a tail fluke and smooth, unarmoured skins.
• Their descendants the pterosaurs and the birds dominated the air. For other meanings see Pterodactyl (disambiguation. Pterosaurs (ˈtɛrəsɔr from the Greek πτερόσαυρος pterosauros

Metabolism

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:

• 4-chambered hearts. Mammals and birds have 4-chambered hearts. Non-crocodilian reptiles have 3-chambered hearts, which are less efficient because they allow oxygenated and de-oxygenated blood to mix and therefore send some de-oxygenated blood out to the body instead of to the lungs. Modern crocodilians' hearts are 4-chambered, but are smaller relative to body size and run at lower pressure than those of modern mammals and birds. They also have a bypass which makes them functionally 3-chambered when under water, conserving oxygen.
• a secondary palate, which allows the animal to eat and breathe at the same time. The secondary palate is an anatomical structure that divides the Nasal cavity from the oral cavity in many Vertebrates In human Embryology
• a hepatic piston mechanism for pumping the lungs. lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive This is different from the lung-pumping mechanisms of mammals and birds but similar to what some researchers claim to have found in some dinosaurs. ^[5][6]

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 which spend the vast majority of their time floating in water or lying on river banks?

Some experts believe that crocodilians were originally active, warm-blooded predators and that their archosaur ancestors were warm-blooded. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of Developmental studies indicate that crocodilian embryos develop fully 4-chambered hearts first and then develop the modifications which make their hearts function as 3-chambered under water. Using the principle that ontogeny recapitulates phylogeny, the researchers concluded that the original crocodilians had fully 4-chambered hearts and were therefore warm-blooded and that later crocodilians developed the bypass as they reverted to being cold-blooded aquatic ambush predators. The theory of recapitulation, also called the biogenetic law or embryological parallelism, and often expressed as Ontogeny recapitulates Phylogeny ^[7][8]

If the original crocodilians were warm-blooded and other Triassic archosaurs were also warm-blooded, this would help to resolve some evolutionary puzzles:

• The earliest crocodilians, e. The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago g. Terrestrisuchus, were slim, leggy terrestrial predators whose build suggests a fairly active lifestyle, which requires a fairly fast metabolism. Terrestrisuchus ('land crocodile' is an Extinct Genus of Crocodylomorph that was about 50 cm (1 ft 8 in long And some other "crurotarsan" archosaurs appear to have had erect limbs, while those of rauisuchians are very poorly adapted for any other posture. Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters Triassic Archosaurs Originally it was believed that they were related Erect limbs are advantageous for active animals because they avoid Carrier's constraint, but disavantageous for more sluggish animals because they increase the energy costs of standing up and lying down. Carrier's constraint is the observation that air-breathing Vertebrates which have two Lungs and flex their bodies sideways during Locomotion find it very
• If early archosaurs were completely cold-blooded and (as seems most likely) dinosaurs were at least fairly warm-blooded, dinosaurs would have had to evolve warm-blooded metabolisms in less than half the time it took for mammal-like reptiles to do the same. Note In this article "dinosaur" means "non-avian dinosaur" since most experts regard birds as an advanced group of dinosaurs

Phylogeny

     `--Archosauria [Crown group Archosauria = Avesuchia]
           |--Crurotarsi
           |     |-?Ctenosauriscidae
           |     `--Crocodylotarsi
           |           |--Ornithosuchidae
           |           `--+--Phytosauria
           |              `--Suchia
           |                    |--Prestosuchidae
           |                    `--Rauisuchiformes
           |                          |--Aetosauria
           |                          `--Rauisuchia
           |                                |--Rauisuchidae
           |                                `--+--Paracrocodylomorpha
           |                                   `--Crocodylomorpha (crocodiles and relatives)
           `--Ornithodira
                 |--Pterosauromorpha
                 |     |--Scleromochlus
                 |     `--Pterosauria
                 `--Dinosauromorpha
                    `--Dinosauriformes
                       `--Dinosauria
                              |--Ornithischia
                              `--Saurischia
                                 `--Aves (birds)

References

Further reading

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. This page is about the paleontologist Michael J Benton For the American Foreign Service Officer see Michael L Blackwell Science Ltd
• Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, W. Robert Lynn Carroll (born May 5 1938, Kalamazoo Michigan) is a vertebrate Paleontologist who specialises in Paleozoic and Mesozoic H. Freeman and Co. New York

External links

• UCMP
• Paleos reviews the messy history of archosaur phylogeny (family tree) and has an excellent image of the various archosaur ankle types.
• Mikko's Phylogeny Archive Archosauria

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