Raptor Lewis wrote:
In this case, the bones would be fossils because during fossilization, the tissue of the bone is replaced with minerals and sediments from the earth when it was quickly buried after death. So, the original tissue is gone and is replaced with a stone "copy" of the structure of the bone. So, in a sense it's not the "real" bone, but a mere carbon copy of the original.
I caught this while perusing some of the images recently and I'd like to add my two cents. Raptor Lewis is correct in several points--that the process of fossilization reduces the bone tissue and probably occurs very quickly. However, there's one common misconception here that I'd like to dispel.
As bone passes through initial decay, proteins in the bone (mostly collagen and osteocalcin) break down by hydrolysis and are lost. This constitutes between 15 and 35 wt% of most bone mass. The mineral component, a calcium phosphate complex called hydroxylapatite, is not destroyed or removed. Fossils of bone are not copies (carbon or otherwise), they are a residual component of the original material. This is particularly true of enamel, which even in life is less that 1 wt% collagen and as nearly mineralogically unliving a tissue as the vertebrate body can produce. Bone fossils are almost always the original, real mineral from the skeleton of the animal.
This is not to say that bones don't change as they fossilize. The collagen that escapes during pre-fossilization is a binding matrix for the nanometer-size mineral grains, so there may be some minute reorganization of the mineral crystals. The typical mode of fossilization for vertebrate skeletal tissue is the addition of calcite, barite, siderite, clay, or sediment into voids in the original material, a condition called permineralization. Remember that when minerals produce psuedomorphs--such as pyrite cubes replaced by limonite, or quartz replacing shell material in turritella gastropods--the internal structure is substantially reduced if not lost altogether. If dinosaur bone were a psuedomorph (a mineral 'carbon copy' ) we would see little or no internal structure. In fact, the original mineral structure is retained even to the point of preserving cellular structure of the original tissue such that "Haversian osteons" (millimeter-wide columns of bone mineral sheathing a tiny blood vessel) can be easily seen under the microscope. I've done a bit of work with this sort of detail in hadrosaurs, so I'm going to try attaching a picture of some of these osteons as they appear in the neural spine on a caudal vertebra.
(The magnification here is 150x; osteons appear as ovate whorls surrounded by a dark band; the pit in the interior of each is the cross-section track of the vessel; the tiny stipples in the whorl are the holes left by bone-precipitating cells called osteocytes. As bone matures, the older osteons wear out and are overprinted by new ones. There are strong similarities between this bone tissue and that of birds and large mammals.)
The take-home message here is, if you're handling a vertebrate fossil, odds are extremely good that you are touching original material from the animal.
