Investigations on Cetacea (1993)


Of the 79 recent species of cetaceans 51 have been examined relative to the osteology of the flipper and the scapula and, in addition to this, video-recordings of 13 different species have been analyzed. In order to get an accurate inside into the functional aspect of these bones, the musculature of the pectoral girdle of some exemplary species of both Mysticeti and Odontoceti has been examined.
The articulation between scapula and humerus is the only one of the forearm which is still functioning. Within the same species the humerus shows a very constant morphology. There are small differences between species of the same family, whereas there are great differences between species of different families. An identification key with characteristics of humerus, radius and ulna has been developed, with the help of which part of the species can be determined.
In contrast to the humeral bones of the Mysticeti those of the Odontoceti are characterized by a less conspicuous and more radial lying tuberculum proximale humeri. With the possession of a big caput humeri having the form of a hemisphere the Balaenidae differ from the Balaenopteridae.
In regard to shape and size of the humerus the Odontoceti form a heterogeneous group. The humerus of the Physeteridae can definitely be recognized by its big, forward projecting Tuberculum mediale humeri. It’s lack the Monodontidae, which have the most slender humerus of the cetaceans.
The Humeri of the Ziphiidae still show the strongest similarity to those of the oldest fossil cetaceans. There is no expansion at the distal end, a difference to all other Odontoceti. Especially in the Delphinidae, this distal part is rather wide. In this family the shortest and most compact humeri can be found, and in addition to this, size and position of their tuberculum proximale humeri are rather conspicuous.
In contrast to the humeri of the Delphinidae those of the Phocoenidae are marked by a slimmer shape and those of the Platanistidae show a smaller tuberculum proximale humeri.
The distances measured on the skeletal elements have been used in some cluster analyses. The cluster analysis, where only the characteristical features of the humerus were taken into account, clearly represents the relationship given by todays system.
Those species showing strong derivations in their morphology of the humerus compared to other representatives of the family have been placed exactly in those groups where we find them in some newest literature. So with the help of the humeral bone it is possible to classify the species concerned.
As shown by the film analyses, there are strong variations in the different species concerning the movements of the flipper, and they are much more complex than generally assumed. Whereas the slender flippers of the fast-swimming species are seldom used for steering, the wider ones of slow-swimming species are employed rather often, and their movements are much stronger.
The mobility of the flipper can be concluded from the morphology of the humeral bone, where especially length of the humerus, size and position of the tuberculum proximale humeri has to be taken into account.