Crocodiles and birds are extant archosaurs

The Archosauria are a group diapsider amniotes, which the crocodiles ( Crocodilia ) and birds ( Aves ) and various fossil groups, including the pterosaurs ( Pterosauria ) and the dinosaurs includes. Archosaurs were the dominant vertebrates on land and in the air during much of the Mesozoic. Even today, they account for more than 10,000 species a significant part of the vertebrate fauna.

Etymology and definitions

The zoologist Edward Drinker Cope coined the term Archosauria because of the temporal arches (Latin singular: arcus temporalis ), which owns the Archosaurierschädel due to the openings in its rear part. However, it is usually with reference to the ancient Greek. Ἀρχός, archos, " leader, prince " interpreted as "the highest standing Reptiles".

The term Archosauria referred to a crown group of crocodiles and birds, so include the last common ancestor of crocodiles and birds as well as all its descendants. This corresponds to the standard in biology Archosauria - term.

A second, by some paleontologists preferred definition, however, includes other, more primitive and older groups outside the crown group with a how Erythrosuchidae that Proterochampsidae that Proterosuchidae and Euparkeria, that refers to the so-called Pan - group, which includes all Diapsiden closer with are related to modern birds and crocodiles than with the present-day lizards. This is in order to distinguish the nomenclature of the crown Archosaurs, also referred to as Archosauriformes. The other way round, the crown archosaurs, to distinguish them from the pan archosaurs, as Avesuchia called.


The most important common features ( synapomorphies ) of the archosaurs are found in the structure of the skull. This initially has a total of three skulls window and is referred for this reason as triapsider skull. Here, the Antorbitalfenster ( Präorbitalfenster ) in front of the eye socket (orbit ); two more skull window, the so-called temporal window (Tempo Ralf Rochester ), located behind the eye socket. Show the two temporal windows that archosaurs are among the Diapsiden.

For the extant archosaurs but this is not true. They have relatively highly specialized skull that are no longer triapsid. Although crocodiles have clearly distinct temporal window, but with them the Antorbitalfenster is closed. In birds the Antorbitalfenster is indistinct, partly not separated from the eye socket and the pace Ralf Most are small and also fused with the eye socket.

The lower jaw ( mandible ) has also originally a window, which is called Mandibularfenster and the crocodiles is still available. The attachment of the teeth in the Kieferknochem is thecodont, ie, the teeth sit in an own tooth socket ( socket), where they are joined by connective tissue to the bone, and do not have a multi-part tooth roots. In modern birds and some dinosaur groups, the teeth are regressed.

Many purely fossil Archosauriergruppen ( Aetosaurier, Phytosaurier, Ankylar ) and the fossil and currently living crocodiles had or have one with embedded into the skin bone plates ( osteoderms ) armored back.


The first occurrence of archosaurs is dated for the crown group ( Archosauria or Avesuchia ) on the upper Lower Triassic ( Olenekian ) for the Pan - group ( Archosauria or Archosauriformes ) on the upper Permian. One of the oldest representative of the Pan - Archosauria is Archosaurus rossicus from the Upper Permian.

In the Lower Triassic basal Archosauriformes took over the role of the extinct at the Permian - Triassic boundary Synapsids Apex predators ( carnivores at the top of the food chain) from the groups Gorgonopsia and Titanosuchia. Another Radiation was carried out in the Upper Triassic with the emergence of dinosaurs and pterosaurs ( Ornithodira ). In Jurassic and Cretaceous particularly the dinosaurs produced a huge variety. The archosaurs in this period were the dominant group of land vertebrates. After the mass extinction at the boundary of Cretaceous and Paleogene ( Cretaceous-Tertiary boundary) about 65 million years, with the birds at least one group of dinosaurs - line survives to this day.

The crocodile - line ( Crurotarsi ) took a completely different development. Their representatives were increasingly displaced during the Upper Triassic of the dinosaurs from their ancestral habitats on land and retired from the law back into the water, which they trained the primal, lizard-like habit, which is typical for today's crocodiles.


For the layout of the crown group Archosauria is the training of the ankle joint of central importance. So the Crurotarsi ( crocodiles and fossil relatives ) are due to their typical Crurotarsalgelenks in which the Gelenkung between the tarsal bones Astragalus and calcaneus occurs, compared with the Ornithodira ( pterosaurs and dinosaurs including birds) with their Mesotarsalgelenk in the astragalus and calcaneus, the joint together form, with the distal (closer to toe lying ) tarsal bones.

The formerly named " Thecodontia " combined basal pan and crown archosaurs provide contrast, no natural kinship group ( clade ) but a Paraphylum dar.

The probable kinship relationships are the following cladogram (after Benton, 2007) again (, Archosauria ' denotes the Pan - group):

† Proterosuchidae

† Erythrosuchidae

† Euparkeria

† Phytosauridae

† Stagonolepididae

† Ornithosuchidae

† Rauisuchia

Crocodylomorpha including crocodiles ( Crocodylia )

† Scleromochlus

† pterosaur ( Pterosauria )

† Lagerpeton


Dinosaurs ( Dinosauria ) including birds ( Aves)

Footnotes: 1) archosaurs Pan group or Archosauriformes; 2) crown group archosaurs

Pictures of Archosaur