Editing Carinatae (section)

==Definition==
Traditionally, Carinatae were defined as all birds whose [[sternum]] (breast bone) has a [[keel (bird)|keel]] (''carina''). The keel is a strong median ridge running down the length of the sternum. This is an important area for the attachment of flight muscles. Thus, all flying birds have a pronounced keel. [[Ratite]]s, all of which are flightless, lack a strong keel. Thus, living birds were divided into carinatae (keeled) and ratites (from ''ratis'', "raft", referring to the flatness of the sternum). The difficulty with this scheme [[phylogenetics|phylogenetically]] was that some flightless birds, without strong keels, are descended directly from ordinary flying birds possessing one. Examples include the [[kākāpō]], a flightless [[parrot]], and the [[dodo]], a columbiform (the [[pigeon]] family). Neither of these birds are a ratite. Thus, this supposedly distinctive feature was easy to use, but had nothing to do with actual phylogenetic relationship.{{cn|date=October 2020}}

Beginning in the 1980s, Carinatae was given several [[phylogenetic]] definitions. The first was as a node-based [[clade]] uniting ''[[Ichthyornis]]'' with modern birds.<ref name = "Cracraft"/> However, in many analyses, this definition would be synonymous with the more widely used name [[Ornithurae]]. An alternate definition was provided in 2001, naming Carinatae an [[apomorphy]]-based clade defined by the presence of a keeled sternum.<ref name="Gauthier"/>

The most primitive known bird relative with a keeled breastbone is ''[[Confuciusornis]]''. While some specimens of this stem-bird have flat breastbones, some show a small ridge that could have supported a cartilaginous keel.<ref name = "Chiappe"/>