The uncinate processes of the ribs are extensions of

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

that project

caudally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

from the vertical segment of each

rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ch ...

. (Uncinate means ''hooked'' from Latin ''uncinatus'', from ''uncinus'', barb, from ''uncus'', hook.) They are found in

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s (except for

screamer The screamers are three South American bird species placed in family Anhimidae. They were thought to be related to the Galliformes because of similar bills, but are more closely related to ducks (family Anatidae),Todd, F. (1991) and most closel ...

s), reptiles, and the early amphibian ''

Ichthyostega ''Ichthyostega'' (from el, ἰχθῦς , 'fish' and el, στέγη , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of Greenland. It was among the earliest four-limbed vertebrates in the fossil record, and was o ...

''. These processes can serve to attach

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...

muscles, and help to strengthen the

rib cage The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi ...

overlapping with the rib behind them. They are also shown to have a role in respiration by increasing the effectiveness of muscles involved in inspiration including the appendicocostal muscles. The processes are short in walking birds and long in diving species and are of intermediate length in non-specialist birds. The screamers ( Anhimidae) are unique in lacking this process. The process has also been noted in some

enantiornitheans The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cl ...

.Fucheng, Zhang, Zhou Zhoghe, Hou Lianhai & Gu Gang 2001. Early diversification of birds: Evidence from a new opposite bird. Chinese Science Bulletin. 46(11):945-94
PDF
/ref> Although among living vertebrates, bony uncinate processes are unique to birds, cartilaginous uncinate processes are present in crocodiles. The uncinate process has also been reported in ''

Sphenodon Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and ...

'' and fossil vertebrates including ''

Caudipteryx ''Caudipteryx'' (which means "tail feather") is a genus of peacock-sized theropod dinosaurs that lived in the Barremian age of the early Cretaceous (about 124.6 million years ago). They were feathered and extremely birdlike in their overall app ...

'',

oviraptorids Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between on ...

, dromaeosaurids, ''

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, b ...

'', '' Chaoyangia'', and ''

Longipteryx ''Longipteryx'' is a genus of prehistoric bird which lived during the Early Cretaceous (Aptian stage, 120.3 million years ago). It contains a single species, ''Longipteryx chaoyangensis''. Its remains have been recovered from the Jiufotang Forma ...

''; however it apparently does not occur in ''

Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...

'', though Codd ''et al''. (2007) reported uncinate processes in ''Archaeopteryx''.

References

{{reflist Bird anatomy Bones