Wednesday, April 24, 2019

Avian Cnemial Crest

Birds have several mechanisms for aquatic locomotion. Two types of propulsion are employed by divers, this by use of their wings or legs. Underwater use of wings is seen in species such as penguins and auks, whereas foot-propulsion is utilized by loons, grebes and some waterfowl. 

Suggestions of an aquatic lifestyle are often noted in a bird’s skeletal features, especially when fossil interpretation is concerned. 

An Early Cretaceous taxon for which a more aquatic lifestyle is strongly established is in Gansus yumenensis, from the lacustrine deposits of the Xiagou Formation in Gansu Province in China (source: Mayr G. Avian Evolution. 2017).

Gansus is commonly mentioned to possess long feet, which were used aptly for hindlimb propulsion. But the strongest evidence for this type of aquatic behavior may lie in the elongated cnemial crest of the tibiotarsus.

The cnemial crest is a crest-like prominence located at the front side of the head of the tibiotarsus or tibia in the legs of birds and other dinosaurs.

Avian Cnemial Crest Diagram
From Manual of Ornithology, N.S. Proctor 1993.
The crest is most highly developed in the loons and grebes in which it partly encircles the knee joint (Source: Avian Osteology). The greatly elongated cnemial crest serves to enlarge the insertion sites of the hypertrophied leg muscles (source: Mayr G. Avian Evolution. 2017). The main extensor muscle of the thigh is attached to this ridge.

Cnemial crest of an extant loon.
From; Mayr G Avian Evolution. 2017
Surprisingly, the evolutionary relationship between loons, grebes and fossil birds is often found, and better understood, within the development of the crest itself. 

Paleontologist Sankar Chatterjee explains this in his 2006 paper; The Morphology and Systematics of Polarornis, a Cretaceous Loon (Aves: Gaviidae) from Antarctica.  

“The long cnemial crest of Polarornis clearly indicates its foot-propelled diving adaptation. Although Polarornis superficially resemble grebes and hesperornithiforms in the mechanical design of the hindlimbs, the way in which the cnemial crest is developed among different foot-propelled diving birds may be an important phylogenetic character. The cnemial crest of the loon is derived solely from the tibiotarsus, but the patella is lacking. In Hesperornis, the cnemial crest evolved from the development and expansion of the patella. In grebes, on the other hand, both patella and tibiotarsus contribute to the formation of the cnemial crest. If these diving birds were closely related, it is unlikely that the formation of this structure would have evolved along such a very different pathway. It is likely that loons, grebes, and hesperornithiform birds are purely convergent in their adaptations and did not share a common ancestry.”

Morphological variations of cnemial crest among diving birds. 
From; The Morphology and Systematics of Polarornis. Chatterjee, 2006 
Name: Extensor digitorum longus. Origin: Anterior surface of the proximal tibiotarsus. Insertion: Distal phalanges of the digits. Action: Extends the digits.
Proctor, N.S. (1993) Manual of Ornithology: Avian Structure and Function. (See below)

Avian cnemial crest 
From Proctor N.S. (1993) Manual of Ornithology

Avian extensor digitorum longus muscle 
From Proctor N.S. (1993) Manual of Ornithology

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