| Abstract: | The shoulder joint of the Microchiroptera shows a remarkable morphological variation that has been studied in 20 individual bats from 15 species and 11 families. The basic morphology of the shoulder joint, with a globular humeral head and a corresponding glenoid cavity, is found in the Megachiroptera and, within the Microchiroptera, in the Rhinopomatidae. Besides this basic shoulder joint, there are two derived joint types: the derived and specialized shoulder joint with a single articular surface on the scapula and a more-or-less oblong humeral head, and the derived and specialized shoulder joint with secondary articular surfaces on the trochiter and on the dorsal aspect of the scapula. The first type of derived joint is most strikingly developed in the Mormoopidae and the Noctilionidae, the second one in the Vespertilionidae and the Molossidae. It is suggested that both types of derived shoulder joints have the functional significance of reducing the pronatory movements of the abducted forearm during the downstroke of the wing-beat cycle. This suggested function of the secondary shoulder joint is a new approach to understanding this very peculiar structure. In species with these specialized shoulder joints, the downstroke musculature is comparatively better developed and the M. serratus ant. post. div. comparatively less well developed. A hypothesis is offered to explain and combine the osteological and myological findings. Each of the derived types of shoulder joints has developed independently more than once through parallel evolution. |
