Didelphis

A 100-micron thick section from the calcaneus (heel bone) of Didelphis virginiana, an opossum. Evolutionary biologists often study opossums because their skeletal structure is thought to be similar to that of the ancestor to marsupials and other mammals. In the image seen here, circularly polarized light is used to highlight differences in the orientation of the collagen fibers present in the bone, as represented by different shades of gray. The patterns of collagen fiber orientation reflect the forces the bones are subjected to during life, which vary depending on the way members of a species move. By comparing the patterns we see in fossil bones to those in living species, we can reconstruct the locomotion of extinct animals.