This photo illustrates an extreme case of osteopenia. Note the loss of dermal (shell) bone, exposing the rib cage. (This is about as bad as it gets.) Osteopenia of the shell is common in captive chelonians and likely due to suboptimal nutrition. Surprisingly, this tortoise was a wild animal. Lizards with fibrous osteodystrophy replace lost bone with fibrous tissue (fibrous osteodystrophy), but chelonians have a much less prominent fibrous response, and osteopenia may be a more accurate term. Photo courtesy University of Florida, Dept of Pathobiology.