Crocodiles and birds are the last survivors of the ruling reptiles
A pigeon and a crocodile look nothing alike, yet they are each other's closest living relatives. Both belong to the archosaurs, the group that also produced dinosaurs and pterosaurs and dominated life on land for over 150 million years after a catastrophe wiped out their rivals.
Palaeontologists define Archosauria as the last common ancestor of birds and crocodilians plus all its descendants. Near the base the family tree forks: one branch, Pseudosuchia, leads to crocodiles and their extinct kin; the other, Avemetatarsalia, or bird ankles, leads to pterosaurs, dinosaurs and birds. Hallmarks include teeth anchored in deep sockets, which earned early forms the label thecodont, openings in the skull in front of the eyes that lightened a heavy head, and a prominent ridge on the thigh bone for anchoring muscle. All living archosaurs have also lost the vomeronasal organ that lizards use to smell.
The oldest true archosaurs date from the Early Triassic, between about 251 and 247 million years ago, though close relatives go back to the late Permian. Their big chance came with the Permian-Triassic extinction around 252 million years ago, which wiped out most of the dominant synapsids, the lineage leading to mammals. Several reasons are offered for why archosaurs then took over. They may have had one-way airflow through their lungs, useful if oxygen levels had dropped. In the arid world of the supercontinent Pangaea, they probably excreted uric acid as a paste and had glandless skin, both of which save water. Critics note that desert mammals conserve water just as well, so the case is not settled.
Since the 1970s, researchers have sorted archosaurs largely by their ankles. The crocodile line evolved a peg-and-socket joint, while the bird line developed a simple hinge suited to animals standing fully upright, and those lightweight, long-necked, often two-legged forms gave rise to dinosaurs and pterosaurs in the late Triassic.
Edward Drinker Cope coined the name in 1869, originally for a jumble of groups including some now classed with mammals; the modern definition dates from 1986. Only birds and some crocodile relatives survived the extinction 66 million years ago.
Source: Archosaur
