| Carinatae | |
|---|---|
 | |
| Chestnut-bellied sandgrouse (Pterocles exustus) in flight | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Clade: | Ornithurae |
| Clade: | Carinatae |
Carinatae is the group of allbirds and their extinct relatives to possess akeel, or "carina", on the underside of thebreastbone used to anchor large flight muscles.
Traditionally, Carinatae were defined as all birds whosesternum (breast bone) has akeel (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.Ratites, all of which are flightless, lack a strong keel. Thus, living birds were divided into carinatae (keeled) and ratites (fromratis, "raft", referring to the flatness of the sternum). The difficulty with this schemephylogenetically was that some flightless birds, without strong keels, are descended directly from ordinary flying birds possessing one. Examples include thekākāpō, a flightlessparrot, and thedodo, a columbiform (thepigeon 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.[citation needed]
Beginning in the 1980s, Carinatae was given severalphylogenetic definitions. The first was as a node-basedclade unitingIchthyornis with modern birds.[1] However, in many analyses, this definition would be synonymous with the more widely used nameOrnithurae. An alternate definition was provided in 2001, naming Carinatae anapomorphy-based clade defined by the presence of a keeled sternum.[2]
The most primitive known bird relative with a keeled breastbone isConfuciusornis. While some specimens of this stem-bird have flat breastbones, some show a small ridge that could have supported a cartilaginous keel.[3]
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