A group comprising the flightless birds, including the ostrich, kiwi, cassowary, rhea, and emu, plus the extinct moa and elephant bird. They are typically large-bodied, have long legs, heavy bones and small wings, and lack a keel to the sternum. It is thought that these birds originated in the northern hemisphere during the Cretaceous period, between 115 and 105 million years ago, and their ancestors subsequently spread into the southern hemisphere. Following the mass extinction at the end of the Cretaceous, they diversified and independently lost the ability to fly; all except the kiwi then assumed the large body size. The partridge-like flighted tinamous of South America are also now regarded as ratites.