Carinatae is the group of all

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s and their extinct relatives to possess a

keel The keel is the bottom-most longitudinal structural element of a watercraft, important for stability. On some sailboats, it may have a fluid dynamics, hydrodynamic and counterbalancing purpose as well. The keel laying, laying of the keel is often ...

, or "carina", on the underside of the

breastbone The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood ve ...

used to anchor large flight muscles.

Definition

Traditionally, Carinatae were defined as all birds whose

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

(breast bone) has a

(''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 Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...

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

phylogenetically In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

was that some flightless birds, without strong keels, are descended directly from ordinary flying birds possessing one. Examples include the

kākāpō The kākāpō (; : ; ''Strigops habroptilus''), sometimes known as the owl parrot or owl-faced parrot, is a species of large, nocturnal, ground-dwelling parrot of the superfamily Strigopoidea. It is endemic to New Zealand. Kākāpō can be u ...

, a flightless

parrot Parrots (Psittaciformes), also known as psittacines (), are birds with a strong curved beak, upright stance, and clawed feet. They are classified in four families that contain roughly 410 species in 101 genus (biology), genera, found mostly in ...

, and the

dodo The dodo (''Raphus cucullatus'') is an extinction, extinct flightless bird that was endemism, endemic to the island of Mauritius, which is east of Madagascar in the Indian Ocean. The dodo's closest relative was the also-extinct and flightles ...

, a columbiform (the

pigeon Columbidae is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with small heads, relatively short necks and slender bills that in some species feature fleshy ceres. ...

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. Beginning in the 1980s, Carinatae was given several

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

definitions. The first was as a node-based

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

uniting ''

Ichthyornis ''Ichthyornis'' (meaning "fish bird", after its fish-like vertebrae) is an extinct genus of toothy seabird-like ornithuran from the late Cretaceous period of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansa ...

'' with modern birds. However, in many analyses, this definition would be synonymous with the more widely used name

Ornithurae Ornithurae (meaning "bird tails" in Greek) is a natural group that includes modern birds and their very close relatives such as the ichthyornithines and the hesperornithines. This clade is defined in the ''PhyloCode'' by Juan Benito and collea ...

. An alternate definition was provided in 2001, naming Carinatae an

apomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

-based clade defined by the presence of a keeled sternum. The most primitive known bird relative with a keeled breastbone is ''

Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, ''Confuciusornis'' had a toothless beak, bu ...

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

References

{{reflist , refs = {{cite journal , last1=Chiappe , first1=Luis M. , last2=Ji , first2=Shu' An , last3=Ji , first3=Qiang , last4=Norell , first4=Mark A. , year=1999 , title=Anatomy and systematics of the Confuciusornithidae (Theropoda, Aves) from the late Mesozoic of northeastern China , journal=Bulletin of the AMNH , volume=242 , url=https://www.researchgate.net/publication/270509827 {{cite journal , last=Cracraft , first=Joel , year=1986 , title=The origin and early diversification of birds , journal=

Paleobiology Paleobiology (or palaeobiology) is an interdisciplinary field that combines the methods and findings found in both the earth sciences and the life sciences. An investigator in this field is known as a paleobiologist. Paleobiology is closely ...

, volume=12 , issue=4 , pages=383–399 , doi=10.1017/s0094837300003122, bibcode=1986Pbio...12..383C , s2cid=89557960 {{cite conference , last1=Gauthier , first1=Jacques , last2=de Queiroz , first2=Kevin , year=2001 , title=Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name "Aves" , conference=New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrum , pages=7–41 , publisher = Peabody Museum of Natural History, Yale University , url=https://www.researchgate.net/publication/40662575 Euornithes