Fossils of some forms of these birds have been found in western South Dakota. Have well- developed flight apparatus, skull with teeth, and a primitive pelvic region. Formerly thought that they represented early forms of many modern lineages, but they actually represent a side-branch that became extinct at the end of the Cretaceous. Passerines appear during the Oligocene.
Flight is expensive to maintain, both in terms of metabolism and embryology.
When costs outweigh benefits, flightlessness may evolve. Flightlessness has evolved many times among 13 avian Orders, always from flying ancestors. Usually its evolution is associated with geographic isolation and a relative absence of terrestrial predators. Sternum remains unkeeled in flightless birds. Obtuse angle between scapula and coracoid.