Tachy

This image is a color-coded map representing variability in bone tissue types across a single 100-micron-thick section of the femur (thigh bone) of an adult echidna (Tachyglossus aculeatus). The echidna represents a group of mammals referred to as Monotremes. They are of considerable interest to biologists because they shed light on the early evolutionary history of mammals. They retain many details of their skeletal structure and reproductive biology that more closely resemble that of reptiles than other mammals; for instance, they lay eggs.

The particular bone tissue types deposited at different times during life is a reflection of individual growth patterns, as well as the loads borne by the skeleton resulting from the way organisms move (i.e. posture and locomotion). By comparing the patterns we see in fossil bones to those in living species, we can begin to reconstruct the evolution of particular growth patterns and habits observed in modern mammals.