Uncinate processes of ribs

They are found in birds (except for screamers), reptiles, and the early amphibian Ichthyostega.

[1][2] They are also shown to have a role in respiration by increasing the effectiveness of muscles involved in inspiration including the appendicocostal muscles.

[4] The screamers (Anhimidae) are unique in lacking this process.

[6] 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 and fossil vertebrates including Caudipteryx, oviraptorids, dromaeosaurids,[7][8] Confuciusornis, Chaoyangia, and Longipteryx; however it apparently does not occur in Archaeopteryx,[6] though Codd et al. (2007) reported uncinate processes in Archaeopteryx.