Anyone who has handled a fossilized

Anyone who has handled a fossilized bone knows that it is usually not exactly like
its modern counterpart, the most obvious difference being that it is often much heavier.
Fossils often have the quality of stone rather than of organic materials, and this has led
to the use of the term "petrifaction" (to bring about rock). The implication is that bone,
Line (5) and other tissues, have somehow been turned into stone, and this is certainly the
explanation given in some texts. But it is a wrong interpretation; fossils are frequently
so dense because the pores and other spaces in the bone have become fiIled with
minerals taken up from the surrounding sediments. Some fossils bones have all the
interstitial spaces filled with foreign minerals, including the marrow cavity, if there is
Line (10) one, while others have taken up but little from their surroundings. Probably all of the
minerals deposited within the bone have been recrystallized from solution by the action
of water percolating through them. The degree of mineralization appears to be determined
by the nature of the environment in which the bone was deposited and not by the
antiquity of the bone. For example, the black fossil bones that are so common in many
Line (15) parts of Florida are heavily mineralized, but they are only about 20.000 years old,
whereas many of the dinosaur bones from western Canada, which are about 75 million
years old, are only partially filled in. Under optimum conditions the process of
mineralization probably takes thousands rather than miIlions of years, perhaps
considerably less.
Line (20) The.amount of change that has occurred in fossil bone, even in bone as old as that of
dinosaurs, is often remarkably small. We are therefore usually able to see the
microscopic structure of the bone, including such fine details as the lacunae where the
living bone cells once resided. The natural bone mineral, the hydroxyapatite, is
virtuaIly unaltered too - it has the same crystal structure as that of modern bone.
Line (25) Although nothing remains of the original collagen, some of its component amino acids
are usually still detectable, together with amino acids of the noncollagen proteins of bone.