Many of our Chihuahuan Desert lizards, including such types as skinks and the Gila Monster, possess osteoderms. For those of you unversed in the esoteric language of biology, "osteoderms" refers to small plates or lumps of bone buried within the skin and quite different in their development from the bones that make up the skeleton. Although this may seem strange, even mammals like ourselves retain two different types of bone arising evolutionarily from different sources.
Cartilage bone, quite reasonably, receives its name because it appears
first in the form of cartilage and only later ossifies into bone. Most of our body
skeleton consists of this type. The other kind of bone usually is called dermal bone,
and this forms directly, without being preformed in cartilage. Dermal bone apparently
is the evolutionary offspring of the armor that covered the body and skull of primitive
fishes. In ourselves, it remains as major bones of the skull, with our brains being
largely protected on the top, sides, and back by bones directly descended from the bony
head plates of Paleozoic fishes.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Romer, A. S. 1962. The vertebrate body. 3rd Ed., W. B. Saunders Company, Philadelphia. 627 pp.