Skeletochronology is used to determine the chronological age of a species of animal by counting the concentric growth rings found in a cross section of bone. This technique cannot be used for all species of bony animals.
Amphibians and reptiles are commonly aged using this method because they undergo discrete annual activity cycles (i.e. winter dormancy). Cross-sections can be taken from any long bone, including femurs and phalanges, and sectioned using a microtome. Once stained with hematoxylin and viewed through a microscope, growth rings are apparent.