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Temporal range: Early TriassicPresent, 251–0 Ma PreꞒ O S D C P T J K Pg NArchosaur_header_cell_0_0_0

Scientific classification ArchosauriaArchosaur_header_cell_0_1_0
Kingdom:Archosaur_cell_0_2_0 AnimaliaArchosaur_cell_0_2_1
Phylum:Archosaur_cell_0_3_0 ChordataArchosaur_cell_0_3_1
Class:Archosaur_cell_0_4_0 ReptiliaArchosaur_cell_0_4_1
Clade:Archosaur_cell_0_5_0 EucrocopodaArchosaur_cell_0_5_1
Clade:Archosaur_cell_0_6_0 Archosauria

Cope, 1869Archosaur_cell_0_6_1


Archosauria ("ruling reptiles") is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaur_sentence_0

Archosaurs are broadly classified as reptiles, in the cladistic sense of term which includes birds. Archosaur_sentence_1

Extinct archosaurs include non-avian dinosaurs, pterosaurs, and extinct relatives of crocodilians. Archosaur_sentence_2

Modern paleontologists define Archosauria as a crown group that includes the most recent common ancestor of living birds and crocodilians, and all of its descendants. Archosaur_sentence_3

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). Archosaur_sentence_4

Older definitions of the group Archosauria rely on shared morphological characteristics, such as an antorbital fenestra in the skull, serrated teeth, and an upright stance. Archosaur_sentence_5

Some extinct reptiles, such as proterosuchids and euparkeriids, possessed these features yet originated prior to the split between the crocodilian and bird lineages. Archosaur_sentence_6

The older morphological definition of Archosauria nowadays roughly corresponds to Archosauriformes, a group named to encompass crown-group archosaurs and their close relatives. Archosaur_sentence_7

The oldest true archosaur fossils are known from the Early Triassic period, though the first archosauriforms and archosauromorphs (reptiles closer to archosaurs than to lizards or other lepidosaurs) appeared in the Permian. Archosaur_sentence_8

Archosaurs quickly diversified in the aftermath of the Permian-Triassic mass extinction (~252 Ma), becoming the largest and most ecologically dominant terrestrial vertebrates from the Middle Triassic period up until the K-Pg mass extinction (~66 Ma). Archosaur_sentence_9

Birds and several crocodyliform lineages were the only archosaurs to survive the K-Pg extinction, rediversifying in the subsequent Cenozoic era. Archosaur_sentence_10

Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day. Archosaur_sentence_11

Distinguishing characteristics Archosaur_section_0

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. Archosaur_sentence_12

Many of these characteristics appeared prior to the origin of the clade Archosauria, as they were present in archosauriforms such as Proterosuchus and Euparkeria, which were outside the crown group. Archosaur_sentence_13

The most obvious features include teeth set in deep sockets, antorbital and (openings in front of the eyes and in the jaw, respectively), and a pronounced fourth trochanter (a prominent ridge on the femur). Archosaur_sentence_14

Being set in sockets, the teeth were less likely to be torn loose during feeding. Archosaur_sentence_15

This feature is responsible for the name "thecodont" (meaning "socket teeth"), which early paleontologists applied to many Triassic archosaurs. Archosaur_sentence_16

Some archosaurs, such as birds, are secondarily toothless. Archosaur_sentence_17

Antorbital fenestrae reduced the weight of the skull, which was relatively large in early archosaurs, rather like that of modern crocodilians. Archosaur_sentence_18

Mandibular fenestrae may also have reduced the weight of the jaw in some forms. Archosaur_sentence_19

The fourth trochanter provides a large site for the attachment of muscles on the femur. Archosaur_sentence_20

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. Archosaur_sentence_21

Origins Archosaur_section_1

Archosaurs are a subgroup of archosauriforms, which themselves are a subgroup of archosauromorphs. Archosaur_sentence_22

Both the oldest archosauromorph (Protorosaurus speneri) and the oldest archosauriform (Archosaurus rossicus) lived in the late Permian. Archosaur_sentence_23

The oldest true archosaurs appeared during the Olenekian stage (247-251 Ma) of the Early Triassic. Archosaur_sentence_24

A few fragmentary fossils of large carnivorous crocodilian-line archosaurs (informally termed "rauisuchians") are known from this stage. Archosaur_sentence_25

These include Scythosuchus and Tsylmosuchus (both of which have been found in Russia), as well as the Xilousuchus, a ctenosauriscid from China. Archosaur_sentence_26

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). Archosaur_sentence_27

Archosaur takeover in the Triassic Archosaur_section_2

Synapsids are a clade that includes mammals and their extinct ancestors. Archosaur_sentence_28

The latter group, which 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, were the dominant land vertebrates throughout the Permian, but most perished in the Permian–Triassic extinction event. Archosaur_sentence_29

Very few large synapsids survived the event, and one form, Lystrosaurus (a herbivorous dicynodont), attained a widespread distribution soon after the extinction. Archosaur_sentence_30

Instead, archosaurs and other archosauriforms quickly became the dominant land vertebrates in the early Triassic. Archosaur_sentence_31

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. Archosaur_sentence_32

The three most commonly suggested explanations for this are: Archosaur_sentence_33


  • 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.Archosaur_item_0_0
  • Archosaurs have more efficient respiratory systems featuring unidirectional air flow. Dr. Peter Ward suggests this may have proven advantageous in a suspected drop in oxygen levels at the end of the Permian.Archosaur_item_0_1
  • 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:Archosaur_item_0_2
    • 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.Archosaur_item_0_3
    • Modern mammals excrete urea, which requires a relatively high urinary rate to keep it from leaving the urine by diffusion in the kidney tubules. Their skins also contain many glands, which also lose water. Assuming that early synapsids had similar features, e.g., as argued by the authors of Palaeos, they were at a disadvantage in a mainly arid world. 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."Archosaur_item_0_4

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. Archosaur_sentence_34

Main forms Archosaur_section_3

Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles. Archosaur_sentence_35

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. Archosaur_sentence_36

The Pseudosuchia appeared early in the Triassic. Archosaur_sentence_37

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. Archosaur_sentence_38

Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs. Archosaur_sentence_39

Modern crocodilians are crurotarsans that can walk with their limbs sprawling or erect depending on speed of locomotion. Archosaur_sentence_40

Euparkeria and the Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus. Archosaur_sentence_41

The earliest fossils of Avemetatarsalia ("bird ankles") appear in the Anisian age of the late Triassic. Archosaur_sentence_42

Most Ornithodirans had "advanced mesotarsal" ankles. Archosaur_sentence_43

This form of ankle incorporated a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. Archosaur_sentence_44

This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. Archosaur_sentence_45

The earliest avemetatarsalians, such as Teleocrater and Asilisaurus, retained "primitive mesotarsal" ankles. Archosaur_sentence_46

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. Archosaur_sentence_47

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. Archosaur_sentence_48

In the late Triassic, the ornithodirans diversified to produce dinosaurs and pterosaurs. Archosaur_sentence_49

Classification Archosaur_section_4

Modern classification Archosaur_section_5

Archosauria is normally defined as a crown group, which means that it only includes descendants of the last common ancestors of its living representatives. Archosaur_sentence_50

In the case of archosaurs, these are birds and crocodilians. Archosaur_sentence_51

Archosauria is within the larger clade Archosauriformes, which includes some close relatives of archosaurs, such as proterochampsids and euparkeriids. Archosaur_sentence_52

These relatives are often referred to as archosaurs despite being placed outside of the crown group Archosauria in a more basal position within Archosauriformes. Archosaur_sentence_53

Historically, many archosauriforms were described as archosaurs, including proterosuchids and erythrosuchids, based on the presence of an antorbital fenestra. Archosaur_sentence_54

While many researchers prefer to treat Archosauria as an unranked clade, some continue to assign it a traditional biological rank. Archosaur_sentence_55

Traditionally, Archosauria has been treated as a Superorder, though a few 21st century researchers have assigned it to different ranks including Division and Class. Archosaur_sentence_56

History of classification Archosaur_section_6

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. Archosaur_sentence_57

It was not until 1986 that Archosauria was defined as a crown-clade, restricting its use to more derived taxa. Archosaur_sentence_58

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 "leader, ruler". Archosaur_sentence_59

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. Archosaur_sentence_60

Thecodonts were considered the "basal stock" from which the more advanced archosaurs descended. Archosaur_sentence_61

They did not possess features seen in later avian and crocodilian lines, and therefore were considered more primitive and ancestral to the two groups. Archosaur_sentence_62

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. Archosaur_sentence_63

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. Archosaur_sentence_64

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. Archosaur_sentence_65

With the identification of "crocodilian normal" and "crocodilian reversed" ankles by Sankar Chatterjee in 1978, a basal split in Archosauria was identified. Archosaur_sentence_66

Chatterjee considered these two groups to be Pseudosuchia with the "normal" ankle and Ornithosuchidae with the "reversed" ankle. Archosaur_sentence_67

Ornithosuchids were thought to be ancestral to dinosaurs at this time. Archosaur_sentence_68

In 1979, A.R.I. Archosaur_sentence_69 Cruickshank identified the basal split and thought that the crurotarsan ankle developed independently in these two groups, but in opposite ways. Archosaur_sentence_70

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. Archosaur_sentence_71

Phylogeny Archosaur_section_7

In many phylogenetic analyses, archosaurs have been shown to be a monophyletic grouping, thus forming a true clade. Archosaur_sentence_72

One of the first studies of archosaur phylogeny was authored by French paleontologist Jacques Gauthier in 1986. Archosaur_sentence_73

Gauthier split Archosauria into Pseudosuchia, the crocodilian line, and Ornithosuchia, the dinosaur and pterosaur line. Archosaur_sentence_74

Pseudosuchia was defined as all archosaurs more closely related to crocodiles, while Ornithosuchia was defined as all archosaurs more closely related to birds. Archosaur_sentence_75

Proterochampsids, erythrosuchids, and proterosuchids fell successively outside Archosauria in the resulting tree. Archosaur_sentence_76

Below is the cladogram from Gauthier (1986): Archosaur_sentence_77

In 1988, paleontologists Michael Benton and J.M. Clark produced a new tree in a phylogenetic study of basal archosaurs. Archosaur_sentence_78

As in Gauthier's tree, Benton and Clark's revealed a basal split within Archosauria. Archosaur_sentence_79

They referred to the two groups as Crocodylotarsi and Ornithosuchia. Archosaur_sentence_80

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). Archosaur_sentence_81

Gauthier's Pseudosuchia, by contrast, was a stem-based taxon. Archosaur_sentence_82

Unlike Gauthier's tree, Benton and Clark's places Euparkeria outside Ornithosuchia and outside the crown group Archosauria altogether. Archosaur_sentence_83

The clades Crurotarsi and Ornithodira were first used together in 1990 by paleontologist Paul Sereno and A.B. Archosaur_sentence_84

Arcucci in their phylogenetic study of archosaurs. Archosaur_sentence_85

They were the first to erect the clade Crurotarsi, while Ornithodira was named by Gauthier in 1986. Archosaur_sentence_86

Crurotarsi and Ornithodira replaced Pseudosuchia and Ornithosuchia, respectively, as the monophyly of both of these clades were questioned. Archosaur_sentence_87

Sereno and Arcucci incorporated archosaur features other than ankle types in their analyses, which resulted in a different tree than previous analyses. Archosaur_sentence_88

Below is a cladogram based on Sereno (1991), which is similar to the one produced by Sereno and Arcucci: Archosaur_sentence_89

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. Archosaur_sentence_90

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. Archosaur_sentence_91

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. Archosaur_sentence_92

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). Archosaur_sentence_93

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. Archosaur_sentence_94

Below is a cladogram modified from Benton (2004) showing this phylogeny: Archosaur_sentence_95

In Sterling Nesbitt's 2011 monograph on early archosaurs, a phylogenetic analysis found strong support for phytosaurs falling outside Archosauria. Archosaur_sentence_96

Many subsequent studies supported this phylogeny. Archosaur_sentence_97

Because Crurotarsi is defined by the inclusion of phytosaurs, the placement of phytosaurs outside Archosauria means that Crurotarsi must include all of Archosauria. Archosaur_sentence_98

Nesbitt reinstated Pseudosuchia as a clade name for crocodile-line archosaurs, using it as a stem-based taxon. Archosaur_sentence_99

Below is a cladogram modified from Nesbitt (2011): Archosaur_sentence_100

Extinction and survival Archosaur_section_8

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. Archosaur_sentence_101

Non-avian dinosaurs and pterosaurs perished in the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago, but crown-group birds (the only remaining dinosaur group) and many crocodyliforms survived. Archosaur_sentence_102

Both are descendants of archosaurs, and are therefore archosaurs themselves under phylogenetic taxonomy. Archosaur_sentence_103

Crocodilians (which include all modern crocodiles, alligators, and gharials) and birds flourish today in the Holocene. Archosaur_sentence_104

It is generally agreed that birds have the most species of all terrestrial vertebrates. Archosaur_sentence_105

Archosaur lifestyle Archosaur_section_9

Hip joints and locomotion Archosaur_section_10

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. Archosaur_sentence_106

In the early to middle Triassic, some archosaur groups developed hip joints that allowed (or required) a more erect gait. Archosaur_sentence_107

This gave them greater stamina, because it avoided Carrier's constraint, i.e. they could run and breathe easily at the same time. Archosaur_sentence_108

There were two main types of joint which allowed erect legs: Archosaur_sentence_109


  • 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.Archosaur_item_1_5
  • 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.Archosaur_item_1_6

It has been pointed out that an upright stance requires more energy, so it may indicate a higher metabolism and a higher body temperature. Archosaur_sentence_110

Diet Archosaur_section_11

Most were large predators, but members of various lines diversified into other niches. Archosaur_sentence_111

Aetosaurs were herbivores and some developed extensive armor. Archosaur_sentence_112

A few crocodyliforms were herbivores, e.g., Simosuchus, Phyllodontosuchus. Archosaur_sentence_113

The large crocodyliform Stomatosuchus may have been a filter feeder. Archosaur_sentence_114

Sauropodomorphs and ornithischian dinosaurs were herbivores with diverse adaptations for feeding biomechanics. Archosaur_sentence_115

Land, water and air Archosaur_section_12

Archosaurs are mainly portrayed as land animals, but: Archosaur_sentence_116


  • 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.Archosaur_item_2_7
  • Two clades of ornithodirans, the pterosaurs and the birds, dominated the air after becoming adapted to a volant lifestyle.Archosaur_item_2_8

Metabolism Archosaur_section_13

The metabolism of archosaurs is still a controversial topic. Archosaur_sentence_117

They certainly evolved from cold-blooded ancestors, and the surviving non-dinosaurian archosaurs, crocodilians, are cold-blooded. Archosaur_sentence_118

But crocodilians have some features which are normally associated with a warm-blooded metabolism because they improve the animal's oxygen supply: Archosaur_sentence_119


  • 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.Archosaur_item_3_9
  • a secondary palate, which allows the animal to eat and breathe at the same time.Archosaur_item_3_10
  • a hepatic piston mechanism for pumping the 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.Archosaur_item_3_11

Historically there has been uncertainty as to why natural selection favored 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? Archosaur_sentence_120

Paleontological evidence shows that the ancestors of living crocodilians were active and endothermic (warm-blooded). Archosaur_sentence_121

Some experts believe that their archosaur ancestors were warm-blooded as well. Archosaur_sentence_122

This is likely because feather-like filaments evolved to cover the whole body and were capable of providing thermal insulation. Archosaur_sentence_123

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. Archosaur_sentence_124

Crocodilian embryos develop fully 4-chambered hearts at an early stage. Archosaur_sentence_125

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. Archosaur_sentence_126

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. Archosaur_sentence_127

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. Archosaur_sentence_128

The authors also provide other evidence for endothermy in stem archosaurs. Archosaur_sentence_129

It is reasonable to suggest that later crocodilians developed the pulmonary bypass shunt as they became cold-blooded, aquatic, and less active. Archosaur_sentence_130

If the crocodilian ancestors and other Triassic archosaurs were warm-blooded, this would help to resolve some evolutionary puzzles: Archosaur_sentence_131


  • 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.Archosaur_item_4_12
  • 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.Archosaur_item_4_13

Respiratory system Archosaur_section_14

A recent study of the lungs of Alligator mississippiensis (the American alligator) has shown that the airflow through them is unidirectional, moving in the same direction during inhalation and exhalation. Archosaur_sentence_132

This is also seen in birds and many non-avian dinosaurs, which have air sacs to further aid in respiration. Archosaur_sentence_133

Both birds and alligators achieve unidirectional air flow through the presence of parabronchi, which are responsible for gas exchange. Archosaur_sentence_134

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. Archosaur_sentence_135

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. Archosaur_sentence_136

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. Archosaur_sentence_137

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. Archosaur_sentence_138

Reproduction Archosaur_section_15

Most archosaurs are oviparous. Archosaur_sentence_139

Birds and crocodilians lay hard-shelled eggs, as did extinct dinosaurs, and crocodylomorphs. Archosaur_sentence_140

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. Archosaur_sentence_141

However, both pterosaurs and baurusuchids have soft-shelled eggs, implying that hard shells are not a plesiomorphic condition. Archosaur_sentence_142

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. Archosaur_sentence_143

Archosaurs are ancestrally superprecocial as evidenced in various dinosaurs, pterosaurs, and crocodylomorphs. Archosaur_sentence_144

However, parental care did evolve independently multiple times in crocodilians, dinosaurs, and aetosaurs. Archosaur_sentence_145

In most such species the animals bury their eggs and rely on temperature-dependent sex determination. Archosaur_sentence_146

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. Archosaur_sentence_147

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