Carinatae

Carinatae is the group of all birds and their extinct relatives to possess a keel, or "carina", on the underside of the breastbone used to anchor large flight muscles.

Traditionally, Carinatae were defined as all birds whose sternum (breast bone) has a keel (carina).

Thus, living birds were divided into carinatae (keeled) and ratites (from ratis, "raft", referring to the flatness of the sternum).

An alternate definition was provided in 2001, naming Carinatae an apomorphy-based clade defined by the presence of a keeled sternum.

While some specimens of this stem-bird have flat breastbones, some show a small ridge that could have supported a cartilaginous keel.