Figure 23.50: Archosaurs (A) Caimans, alligators, and their relatives live in tropical and warm temperate climates. The spectacled caiman is the most common crocodilian throughout much of tropical Central and South America. (B) Birds are the other living archosaur group. This common ostrich is a palaeognath, a group that includes several flightless and weakly flying birds of the Southern Hemisphere. Flightless members of this early-diverging group lack the sternum keel structure that anchors the flight muscles of other bird groups.