Thursday, February 7, 2019

Avian Uncinate Process

In comparison to other vertebrates such as reptiles and mammals, the avian skeleton shows extensive fusion of bones into rigid structures that are both lightweight and strong enough to hold up to the severities of flight. The most distinctive of skeletal adaptations are found in the thorax and pelvis.

In the thorax, the thoracic vertebrate is tightly bound in a rigid structure that resists twisting and bending forces associated with wing flapping. The individual ribs of flying birds have a unique structure of backward-pointing bony extensions called the uncinate process. These small extensions are lateral braces that attach the ribs to each other in addition to the attachments to the spine and sternum, forming a strong supporting “basket” around the lungs and heart (source: Manual of Ornithology, Proctor N.S. 1993.).

American Goldfinch (Spinus Tristis) Uncinate Process
Photo Credit: Paul Cianfaglione
The uncinate processes are functionally linked to movements of the vertebral ribs and sternum during breathing; acting as a lever arm for movement of the ribs in a fixed plane about their articulation on the vertebral column (source; Codd J.R. 2004).

Given the demands of flight, it is not too hard to imagine how an evolutionary trait like the uncinate process could transpire in modern day birds. 

Mallard (Anas platyrhynchos) Uncinate Process
Photo Credit: Paul Cianfaglione
American Crow (Corvus brachyrhynchos) Uncinate Process
Photo Credit: Paul Cianfaglione
What I don’t quite understand is why bony extensions would have evolved on the ribs of 120 million-year-old fossil oviraptorids and basal birds, many of which possessed questionable flight capabilities.

Did the uncinate process develop in bipedal non-avian dinosaurs and early birds to help assist in dynamic breathing during the inception of forearm-flapping or running? That is a good question. 

The fossils also reveal a puzzling feature to the process that is often mentioned, but rarely given further details.

If we look closely at the fossil images, we will notice that the processes are not actually ossified to the ribs, and may have been attached by cartilage or ligaments instead. 

Confuciusornis Uncinate Process
Image Credit: Qingjin Meng

If that is the case, should we still be calling these ancient traits “processes”? Or is their presence in the fossil record more in line with sesamoid bones?

In anatomy, a sesamoid bone is a bone embedded within a tendon or a muscle. Although they are a special structure that is technically independent of the main skeleton, sesamoids begin their development much like regular bony tissue. Specialized bone-generating cells gather at the appropriate point and are nurtured by a locally enhanced blood supply. After initial mineralization, there is a period of remodeling so that the new bone can meet any special role it might play in the movement of neighboring bones (Source: Kaiser, G. 2007. The Inner Bird, Anatomy and Evolution).

The fossil of Citipati osmolskae, from the Late Cretaceous of Mongolia, has a single isolated uncinate process visible in top righthand corner of the specimen indicating how easily these small bones could be lost in otherwise complete specimens (see source below).

Citipati osmolskae with isolated uncinate process 
Image Credit: Codd J.R. 2004

Even stranger than the fossils unossified processes is that of the Southern Screamer (Chauna torquata), the only bird which lack the uncinate process entirely. 

Southern Screamer (Chauna torquata) ribs lacking the uncinate process
Photo Credit: Paul Cianfaglione

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