Archosaur

Archosaurs Temporal range: Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Birds and crocodilians (in this case, a great egret and a mugger crocodile) are the only living archosaur groups.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Clade:	Eucrocopoda
Clade:	Archosauria Cope, 1869
Subgroups
Avemetatarsalia (birds and their extinct relatives) Pseudosuchia (crocodilians and their extinct relatives) Archosaurs of uncertain affinity †Albisaurus †Avalonianus †Avipes †Incertovenator? †Jushatyria †Palaeosaurus †Picrodon †Protecovasaurus? †Sikannisuchus † Smok †Zanclodon † Phytosauria?(paraphyletic?)[2]
Synonyms
Avesuchia Benton, 1999

Archosauria (lit. 'ruling reptiles') is a

The most obvious features include teeth set in deep sockets,

Unlike their close living relatives, the lepidosaurs, archosaurs lost the vomeronasal organ.^[10]

Origins

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 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),^[11] as well as the Xilousuchus, a ctenosauriscid from China.^[4] The oldest known fossils of bird-line archosaurs are from the Anisian stage (247-242 Ma) of Tanzania, and include Asilisaurus (an early silesaurid), Teleocrater (an aphanosaur), and Nyasasaurus (a possible early dinosaur).^{[citation needed]}

Archosaurian domination in the Triassic

• Archosaurs made more rapid progress towards erect limbs than synapsids, and this gave them greater stamina by avoiding Carrier's constraint. An objection to this explanation is that archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of Lystrosaurus and other synapsids.^{[citation needed]}
• Archosaurs have more efficient^{[clarification needed]} respiratory systems featuring unidirectional air flow.^[14] The ability to breathe more efficiently in hypoxic conditions may have been advantageous to early archosaurs during the suspected drop in oxygen levels at the end of the Permian.^[14]
• The Early Triassic was predominantly arid, because most of the Earth's land was concentrated in the supercontinent Pangaea. Archosaurs were probably better at conserving water than early synapsids because:
  • Modern diapsids (lizards, snakes, crocodilians, birds) excrete uric acid, which can be excreted as a paste, resulting in low water loss as opposed to a more dilute urine. It is reasonable to suppose that archosaurs (the ancestors of crocodilians, dinosaurs and pterosaurs) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.^{[citation needed]}
  • Modern mammals excrete
    varanid lizards (Megalania) and land crocs."^[8]

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,[15] and some cynodonts like Trucidocynodon were large sized predators.^[16] A study favors competition amidst mammaliaforms as the main explanation for Mesozoic mammals being small.^[17]

Main forms

Euparkeria and 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 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 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.

Classification

Modern classification

Archosauria is normally defined as a

Archosaur ankle types: Adapted with permission from Palaeos
  

Calcaneum

Primitive mesotarsal ankle

Crocodilian form of crurotarsal ankle

Reversed crurotarsal ankle

"Advanced" mesotarsal ankle

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".^[25]

The term "thecodont", now considered an obsolete term, was first used by the English paleontologist

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.^[26]

Phylogeny

In many

Sauropsida	†Proterosuchidae †Erythrosuchidae †Proterochampsidae   Archosauria     Pseudosuchia   † Parasuchia † Aetosauria †Rauisuchia Crocodylomorpha   Ornithosuchia   †Euparkeria †Ornithosuchidae Ornithodira

In 1988, paleontologists

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.^[26][29] 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:^[26]

Archosauriformes	†Proterosuchidae †Erythrosuchidae †Euparkeria †Proterochampsidae  Archosauria   Crurotarsi  † Parasuchia †Ornithosuchidae Suchia   Ornithodira   †?Scleromochlus † Pterosauria Dinosauromorpha

Ornithodira and Crurotarsi are both

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:^[24]

Archosauria

† Rhynchosauria) † Prolacertiformes) †Proterosuchus (Proterosuchidae) †Euparkeria (Euparkeriidae) †Proterochampsidae Avesuchia  Crurotarsi  † Phytosauridae †Gracilisuchus †Ornithosuchidae  Suchia  † Stagonolepididae †Postosuchus Crocodylomorpha †Fasolasuchus  Prestosuchidae †Ticinosuchus †Prestosuchus †Saurosuchus  Avemetatarsalia  †Scleromochlus   Ornithodira   † Pterosauria  Dinosauromorpha †Lagerpeton   Dinosauriformes   †Marasuchus Dinosauria †Ornithischia  Saurischia  †Sauropodomorpha  Theropoda  †Herrerasaurus Neotheropoda  (Crown group Archosauria)

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):^[4]

Sauropsida

† Phytosauria   Archosauria     Pseudosuchia   †Ornithosuchidae Suchia †Gracilisuchus †Turfanosuchus †Revueltosaurus † Aetosauria †Ticinosuchus Paracrocodylomorpha †Poposauroidea   Loricata   †Prestosuchus †Saurosuchus †Batrachotomus †Fasolasuchus †Rauisuchidae Crocodylomorpha   Avemetatarsalia  / †Pterosauromorpha Dinosauromorpha †Lagerpetidae    Dinosauriformes    †Marasuchus †Silesauridae    Dinosauria    †Ornithischia   Saurischia   †Sauropodomorpha Theropoda Ornithodira * * Nesbitt did not include Scleromochlus in the analysis, meaning that Avemetatarsalia and Ornithodira occupy the same place in this cladogram

Extinction and survival

Crocodylomorphs, pterosaurs and dinosaurs survived the Triassic–Jurassic extinction event about 200 million years ago, but other archosaurs had become extinct at or prior to the Triassic-Jurassic boundary.

Non-avian dinosaurs and

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.^{[citation needed]}

Archosaur lifestyle

Hip joints and locomotion

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:

• 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 independently in various archosaur lineages, for example it was common in "Rauisuchia" (non-crocodylomorph paracrocodylomorphs) and also appeared in some aetosaurs.

It has been pointed out that an upright stance requires more energy, so it may indicate a higher metabolism and a higher body temperature.^[30]

Diet

Most were large predators, but members of various lines diversified into other niches. Aetosaurs were herbivores and some developed extensive armor. A few crocodyliforms were herbivores, e.g., Simosuchus, Phyllodontosuchus. The large crocodyliform Stomatosuchus may have been a filter feeder. Sauropodomorphs and ornithischian dinosaurs were herbivores with diverse adaptations for feeding biomechanics.

Land, water and air

Archosaurs are mainly portrayed as land animals, but:

• Many phytosaurs and crocodyliforms dominated the rivers and swamps and even invaded the seas (e.g., the teleosaurs, Metriorhynchidae and Dyrosauridae). The Metriorhynchidae were rather dolphin-like, with paddle-like forelimbs, a tail fluke and smooth, unarmoured skins.
• Two clades of ornithodirans, the
  pterosaurs
  and the birds, dominated the air after becoming adapted to a volant lifestyle.
• Some dinosaurs like
  penguins
  also adapted to this lifestyle.

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. Both birds and mammals have 4-chambered hearts, which completely separate the flows of oxygenated and de-oxygenated blood. Non-crocodilian reptiles have 3-chambered hearts, which are less efficient because they let the two flows mix and thus 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 birds and mammals. They also have a pulmonary 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.
• a
  lungs. 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.^[31][32]

Historically there has been uncertainty as to why

predators

that spend the vast majority of their time floating in water or lying on river banks.

Paleontological evidence^{[clarification needed]} shows that the ancestors of living crocodilians were active and endothermic (warm-blooded). Some experts^[who?] believe that their archosaur ancestors were warm-blooded as well. This is likely because feather-like filaments evolved to cover the whole body and were capable of providing thermal insulation.^[33] 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.^[34][35] It is reasonable to suggest that later crocodilians developed the pulmonary bypass shunt as they became cold-blooded, aquatic, and less active.

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

• The earliest crocodylomorphs, e.g., Terrestrisuchus, were slim, leggy terrestrial predators whose build suggests a fairly active lifestyle, which requires a fairly fast metabolism. And some other crurotarsan archosaurs appear to have had erect limbs, while those of rauisuchians are very poorly adapted for any other posture. Erect limbs are advantageous for active animals because they avoid Carrier's constraint, but disadvantageous for more sluggish animals because they increase the energy costs of standing up and lying down.
• 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 synapsids to do the same.

Respiratory system

A recent study of the lungs of Alligator mississippiensis (the

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 at the base of Archosauria and retained by both dinosaurs and non-dinosaurian archosaurs, such as aetosaurs, "rauisuchians" (non-crocodylomorph paracrocodylomorphs), crocodylomorphs, and pterosaurs.^[36] 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.^[37]

Reproduction

Most (if not all) archosaurs are

baurusuchids^[40] have soft-shelled eggs, implying that hard shells are not a plesiomorphic condition. The pelvic anatomy of Cricosaurus and other metriorhynchids^[41] and fossilized embryos belonging to the non-archosaur archosauromorph Dinocephalosaurus,^[42] together suggest that the lack of viviparity among archosaurs may be a consequence of lineage-specific restrictions.^{[clarification needed}

]

Archosaurs are ancestrally

Neornithes which incubate their eggs and rely on genetic sex determination – a trait that might have given them a survival advantage over other dinosaurs.^[45]

See also

• 2022 in archosaur paleontology

References

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Sources

• Benton, M. J.
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• ISBN 978-0-7167-1822-2
  .

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