The uncinate processes of the ribs are extensions of bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 that in bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s project caudally from the vertical segment of each rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

. (Uncinate means hooked from Latin uncinatus, from uncinus, barb, from uncus, hook.) These processes help to strengthen the rib cage

Rib cage

The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....

 of birds by 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 Enantiornithines. Although bony uncinate processes are unique to birds, cartilaginous uncinate processes are present in crocodiles. The uncinate process has also been reported in Sphenodon, Caudipteryx

Caudipteryx

Caudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....

, oviraptorids, Confuciusornis

Confuciusornis

Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...

and Chaoyangia

Chaoyangia

Chaoyangia is an extinct ornithuran bird genus from the Early Cretaceous of Liaoning, China. Only one species is described to date, Chaoyangia beishanensis....

however it does not occur in Archaeopteryx

Archaeopteryx

Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

.