The clavicular region of some cursorial Cavioidea (Rodentia: Mammalia)

The clavicle plays a particular role in the shoulder girdle of mammals. In Rodentia the clavicle can be rudimentary in some families. In Artiodactyla, the clavicle is completely absent. We provide data on the myology and osteology of the clavicular region of caviomorph rodents (five species) and the clavicular region of one artiodactyla. The complete mammalian clavicle usually provides both a spoke and a strut effect on shoulder movements. In the spoke effect, the acromion and the manubrium maintain a fixed distance assuring an arcuate movement between these two structures. In the strut effect, the clavicle, which is under compression, is prevented from passing very close to the median plane. All analysed rodents examined in this paper showed a very reduced clavicle, resembling a thin rod. We can suppose that they are more under spoke effect rather than strut effect. Therefore, they are able to perform forelimb excursions closer to the median plane, a fact that could be interpreted as a tendency to cursorial adaptations. The grade of reduction is independent of the size. Clavicle reduction is apparently associated with adaptation to running because it appears in the forms adapted to running and because we can give a biomechanical interpretation of this reduction. Thus, Tragulus did not possess any clavicle although a thin and long ligamentous or connective tissue was found in the place of the clavicle. All caviomorph rodents examined here are primarily adapted to cursorial activity. In fact, we showed that the clavicle presents considerable reduction in five species of caviomorphs which which implies convergence with cursorial mammals of others that lack a clavicle.