What Is an Archosaur? The Group Holding Crocodiles and Birds Together

The group is diagnosed by a set of skeletal features, most conspicuously additional openings in the skull, including one in front of the eye socket and one in the lower jaw, which reduce weight and provide muscle attachment. Teeth set in sockets rather than fused to the jaw is another, which is the condition in crocodiles and in the toothed birds and dinosaurs. Modifications to the ankle and the hip appear early and matter enormously, since they permit the limbs to be held under the body rather than sprawling, which changes how an animal walks, how fast it can move and whether it can breathe while running. Several of those features are hard to see in living members, since birds have lost the teeth and modified the skull heavily, which is part of why the relationship was not recognised from anatomy alone for a long time.

The group divided early and the two lines are distinguished mainly by the ankle:

• •The crocodile line, which includes living crocodilians and a large number of extinct forms far more varied than the survivors suggest
• •The bird line, which includes pterosaurs, dinosaurs and therefore birds
• •Extinct crocodile-line animals including fully terrestrial running forms, armoured herbivores and marine predators with flippers
• •Extinct bird-line animals including everything commonly called a dinosaur plus the flying reptiles, which were not dinosaurs
• •A large set of early forms near the split that are difficult to assign and that occupy the Triassic
• •Two very unlike survivors, since the living members of the two branches could hardly look less similar

The group rose to dominance in the Triassic, immediately after the end-Permian extinction removed the previously dominant land animals, which were mostly relatives of mammals. That sequence is one of the clearest large-scale examples of incumbent replacement, since the mammal line had held the large-bodied roles for tens of millions of years and lost them abruptly. Why the archosaurs prevailed is debated, with proposals involving the low oxygen levels of the early Triassic and the efficiency of the air sac breathing system, the upright limb posture, or simply contingency, since extinction survival is partly a matter of luck. The Triassic that followed was strange, containing a wide variety of large archosaurs of both branches occupying roles that later went to dinosaurs, and a further extinction at the end of that period removed most of them and left the dinosaurs in possession.

The period after the group rose to dominance produced animals that have no modern equivalent and that are among the strangest vertebrates known. Crocodile-line forms included long-legged running predators that looked superficially like later theropods, armoured plant eaters with spiked shoulders, animals with beaks and no teeth, and heavily built ambush predators at the top of their food webs. Others took to water early and produced marine forms. Alongside them sat early dinosaurs, which for much of the period were modest in size and numbers rather than dominant, and pterosaurs, which achieved powered flight well before any bird existed. The variety is such that reconstructing the period requires setting aside expectations formed from the Jurassic and Cretaceous, and the resemblances between unrelated forms in that assemblage are a much-cited demonstration of how readily similar ways of life produce similar animals.

The grouping was proposed on anatomical grounds in the nineteenth century, when the resemblance between certain dinosaur and bird features was already being noticed, and it was contested for a long time afterwards. Detailed comparative anatomy accumulated evidence, particularly from the skull, the ankle and later from the discovery of feathered dinosaurs, which removed any remaining doubt about birds. Molecular evidence from living animals confirmed that crocodilians are the closest living relatives of birds, which surprises people and is not seriously disputed. The consequence for classification is substantial, since it means the traditional category of reptiles is not a natural group unless birds are included in it, and the modern arrangement places birds inside the reptile radiation rather than alongside it. The same reasoning that makes birds dinosaurs makes them reptiles, and both follow from classifying by ancestry.

Extra skull openings, teeth in sockets and modified ankles and hips that bring the limbs under the body define the group. It split early into a crocodile line and a bird line, with pterosaurs and dinosaurs on the latter. It took over after the end-Permian extinction from the relatives of mammals. Crocodilians are the closest living relatives of birds, which puts birds inside the reptiles.