Diapsida : Archosauromorpha : Archosauriformes : Archosauria
Archosauria is defined as the most recent common ancestor of birds and crocodiles and all of its descendants. By definition then, Archosauria consists of two big clades that are sister taxa (i.e., closest relatives) of each other: the croc-line archosaurs, or pseudosuchians, and the bird-line archosaurs, or ornithosuchians. Ornithosuchia includes a lot of animals that are very different from birds, including pterosaurs, huge sauropods like Brachiosaurus, and bizarre plant-eating dinosaurs like Stegosaurus and Triceratops. Similarly, most pseudosuchians were not very similar to living crocs. In a paper on spinosaurs, Tom Holtz pointed out that the evolution of theropod dinosaurs was more than just a "bird factory" (Holtz 19981). Likewise, the evolution of pseudosuchians was more than just a croc factory.
A cast of the reconstructed skull of Postosuchus kirkpatricki, a Late Triassic poposaurid rauisuchian from Apache County, AZ.
Pseudosuchia includes these fairly distinct groups:
- Ornithosuchidae (not to be confused with Ornithosuchia!), bipedal predators that superficially resembled primitive meat-eating dinosaurs
- Phytosauria, the very crocodile-like phytosaurs
- The armored and herbivorous Aetosauria
- Rauisuchidae, mostly quadrupedal, mostly predatory animals that nevertheless included a few bipeds and a few herbivores
- Crocodylomorpha, which is essentially composed of living crocs and all of the animals that are more closely related to them than to any other pseudosuchians (and that still includes a large number of very un-croc-like animals)