What Are Archosaurs?

Archosaurs are a large evolutionary group of vertebrates that includes dinosaurs, birds, crocodilians, pterosaurs and many extinct animals of the Triassic and later Mesozoic. Dinosaurs are archosaurs, but not every archosaur is a dinosaur. The living members of the group are birds and crocodilians, including alligators, caimans and gharials.

The name is sometimes misspelled as “archiosaurs”. The standard term is archosaurs, from Greek roots commonly translated as “ruling reptiles”. The name is historical; it does not mean that every member was large or dominant.

Archosaurs at a glance

What does the word archosaur mean?

In modern phylogenetic classification, Archosauria is defined through the relationships of living animals. It includes the last common ancestor of living birds and crocodilians and all of that ancestor’s descendants.

This definition matters because it does not depend on outward resemblance. A bird and a crocodile look very different, yet both descend from an archosaur ancestor. Dinosaurs and pterosaurs were on the bird side of the tree; ancient crocodile relatives belonged to the other side.

A simplified outline is:

• Archosauria
• Bird line: pterosaurs, dinosaurs, birds and close relatives
• Crocodile line: ancient pseudosuchians and living crocodilians

This outline describes ancestry, not the familiar appearance of the animals.

When did archosaurs appear?

Close relatives of archosaurs were present in the Late Permian. True members of Archosauria are known from the Early Triassic, after the largest mass extinction in Earth’s history, about 252 million years ago.

The Permian–Triassic crisis left many ecological roles open. During the Early and Middle Triassic, archosaurs diversified into lineages that occupied terrestrial hunting, plant-eating, semiaquatic and aerial habitats, and later some marine ecosystems. Their early history was not a simple progression from one primitive animal to dinosaurs. Many related branches lived at the same time; some disappeared while others gave rise to the major Mesozoic groups.

The two main archosaur lines

Two principal branches are recognised near the base of Archosauria. They are often called the crocodile line and the bird line.

The crocodile line

The crocodile branch is called Pseudosuchia. It includes living crocodilians and all archosaurs more closely related to them than to birds.

Triassic pseudosuchians were much more diverse than living crocodilians. They included large terrestrial predators, small agile forms, animals with bony armour, plant-eating species and early crocodylomorphs. Many ancient pseudosuchians did not resemble today’s semiaquatic crocodiles. Some had long limbs and occupied ecological roles later associated with dinosaurs.

Modern crocodilians are only one surviving part of a once much broader branch. The ancient crocodylomorph catalogue follows some of that variety, including forms adapted to life on land and in water.

The bird line

The bird branch of archosaurs is called Avemetatarsalia. Its members are more closely related to birds than to crocodilians. It includes dinosaurs, birds as living dinosaurs, pterosaurs and close relatives of dinosaurs and pterosaurs.

“Bird line” does not mean that all of its members could fly or had feathers. It indicates relationship to modern birds. Dinosaurs originated within this branch in the Late Triassic. The overview of the closest relatives of dinosaurs explains how birds, pterosaurs and crocodilians occupy different positions on the tree.

How are archosaurs different from dinosaurs?

The key difference is the size of the groups. Archosaurs are a broad group with two major evolutionary branches. Dinosaurs are one branch within the bird line of archosaurs.

For that reason, the phrase “dinosaurs and archosaurs” can be confusing if the groups are treated as equal and separate. “Dinosaurs and other archosaurs” is more precise. A separate guide compares dinosaurs with other reptiles.

Pterosaurs were archosaurs, but not dinosaurs

Pterosaurs are often called flying dinosaurs, but the label is inaccurate. They were a separate branch of the bird-line archosaurs. Pterosaurs and dinosaurs shared an older relative but evolved separately. The first pterosaurs appeared in the Triassic and had already evolved active flight long before birds appeared.

A pterosaur wing was supported by an extremely elongated fourth finger. A bird wing formed differently, within the feathered theropod lineage. The useful distinctions are simple: a pterosaur is a flying archosaur; a bird is a living dinosaur and archosaur; a bat is a flying mammal, not an archosaur. The guide to why pterosaurs were not dinosaurs looks more closely at this difference.

Crocodilians are archosaurs too

Modern crocodiles, alligators, caimans and gharials belong to Archosauria. They represent the living crocodile branch of the group.

Despite their superficial resemblance to large lizards, crocodilians are evolutionarily closer to birds than to lizards and snakes. A bird and a crocodilian share a more recent common ancestor with each other than either does with a monitor lizard or iguana.

At the end-Cretaceous extinction, non-avian dinosaurs disappeared, while birds and several crocodylomorph lineages survived. The fossil record documents this different outcome, although the ecological reasons need to be considered separately for each lineage.

Features of early archosaurs

Palaeontologists recognise archosaurs from a combination of skeletal features. There is no single simple external feature shared unchanged by birds, crocodilians and every extinct member. Many early archosaurs and close relatives had an opening in the skull in front of the eye socket, an additional opening in the lower jaw, teeth set in separate sockets, changes in the ankle joint, and features of the pelvis and vertebral column associated with different limb postures.

Not every feature remained in all later descendants. Bird skulls changed greatly, for example, and teeth disappeared entirely in some lineages. Researchers therefore infer relationships from many anatomical characters and phylogenetic analyses, rather than from one trait alone.

Did all archosaurs walk with their legs under the body?

An upright limb posture is often presented as the defining archosaur advantage, but the real picture is more varied. Archosaurs moved with limbs held nearly under the body, in intermediate positions, or more widely splayed. Some walked on two legs and others on four.

Dinosaurs generally had limbs positioned under the trunk. Crocodilians can use a low, sprawling walk or a higher posture. Triassic pseudosuchians had their own forms of upright limb placement, which did not simply copy the dinosaur arrangement. “All archosaurs walked upright” is a useful simplification, not a strict diagnostic rule.

Archosauromorphs, archosauriforms and archosaurs

These similar names refer to groups of different sizes. Archosauromorpha is a broad reptile lineage that includes archosaurs and more distant extinct relatives. It contrasts with Lepidosauromorpha, the lineage associated with lizards, snakes and tuatara.

Archosauriformes is a narrower group within archosauromorphs. It includes true archosaurs and several close branches that existed before the bird and crocodile lines split. In the strict modern sense, Archosauria begins with the common ancestor of living birds and crocodilians.

As classification has changed, older books sometimes called the same animals “primitive archosaurs” even when modern analyses place them outside Archosauria but within Archosauriformes.

Were thecodonts archosaurs?

Older literature used “thecodonts” for various Triassic reptiles with teeth set in sockets. The group Thecodontia is now considered obsolete because it combined animals that do not form one natural branch: early archosauriforms, members of the crocodile line, close relatives of dinosaurs and other forms.

The statement “dinosaurs evolved from thecodonts” therefore needs qualification. Dinosaurs arose among early members of the bird-line archosaurs, not from one recognised group called thecodonts. Older ideas also appear in accounts of the first dinosaurs, so their historical context matters.

Which archosaurs lived in the Triassic?

Triassic archosaur diversity was much wider than the familiar trio of dinosaurs, crocodilians and pterosaurs. It included aetosaurs, often plant-eating or omnivorous animals with armour; large predators on the crocodile line; early crocodylomorphs; silesaurids and other close dinosaur relatives; the first true dinosaurs; and the first pterosaurs.

Some forms traditionally called archosaurs may lie just outside Archosauria in a stricter sense. The position of phytosaurs, for example, has shifted among analyses. This is one reason the classification of early forms continues to be refined.

Why did archosaurs become successful?

No single feature automatically made archosaurs dominant. Their success resulted from a combination of evolutionary changes and historical circumstances. Possible factors include varied limb structure, efficient movement, respiratory changes in some branches, the ability to occupy open ecological roles, a wide range of sizes and diets, and survival or expansion after mass extinctions.

Archosaurs did not immediately dominate after the Permian–Triassic catastrophe. During the Triassic they lived alongside therapsids, large amphibians and other reptiles. Their advantage developed over time and differed from one lineage and ecosystem to another.

Were all large ancient reptiles archosaurs?

No. Size and geological age do not establish membership in Archosauria. Dimetrodon and other early synapsids, marine ichthyosaurs, plesiosaurs, mosasaurs, lizards, snakes, turtles and many early diapsids were not archosaurs.

Archosaurs are often confused with dinosaurs and marine reptiles. Palaeontological classification follows ancestry and skeletal structure, not the broad impression of a “large ancient lizard”.

What is established and what remains under discussion?

Several broad relationships are well established: birds and crocodilians are living archosaurs; dinosaurs and pterosaurs belong to the bird line; dinosaurs are only one branch of Archosauria; and the bird and crocodile lines had separated by the Triassic. Archosaurs also became widespread in the aftermath of the Permian–Triassic extinction.

Researchers continue to refine the exact position of some early archosauriforms, the relationships among Triassic groups, the placement of phytosaurs relative to strict Archosauria, the appearance of the earliest common ancestor of birds and crocodilians, and why some lineages survived environmental crises more successfully than others.

The colours of most extinct archosaurs, behaviour in a particular scene, sounds, social relationships and the precise shape of unpreserved soft tissues are artistic reconstructions. Fossils can constrain these depictions, but they rarely specify every detail.