Meeting Abstract
We used linear measurements to study the changes in the proportions of the humerus and scapula of swimming mammal groups, within interest to glean information about the evolution of terrestrial mammals back to the water. We found differences between homologous structures of the shoulder by analyzing photographs of humeri and scapulae from 26 mammalian species (n = 413) that we classified as terrestrial, semi-aquatic, or aquatic in locomotory mode. Our data about skeletal morphology were used to reconstruct the position and function of the muscles of the shoulder joint.
We found highly significant differences (p < 0.0001) in the relative size of the length of the greater tuberosity and the narrowest width of the humerus for the three locomotory modes. We found significant differences (all p < 0.02) among the relative lengths of (1) the scapular spine and (2) the metacromion, (3) the distance between the metacromion and the acromion. These differences remained in the face of some intraspecies variation associated with body size. For example, in terrestrial mammals the greater tuberosity is less than half the total length of the humerus, but in aquatic mammals it is more than half. In terrestrial mammals, the posterior margin of the scapula is nearly straight, but is more rounded in swimmers, particularly those that use pectoral oscillation. Semi-aquatic species are intermediate in shape and proportions for these characteristics. The size of the deltoid’s insertion is the primary influence on humeral measurements, but the scapular shape reflects the action of many more muscles for both locomotion and posture.
