Although untold numbers of organisms have lived on the earth in past
ages, only a minute fraction of these have left any record of their
existence. Even if the basic requirements of fossilization have been
fulfilled, there are still other reasons why some fossils may never be
found.
For example, large numbers of fossils have been destroyed by erosion or
their hard parts have been dissolved by underground waters. Others were
entombed in rocks that were later subjected to great physical change,
and fossils enclosed in these rocks are usually so damaged as to be
unrecognizable.
Then, too, many fossiliferous rocks cannot be studied because they are
covered by water or great thicknesses of sediments, and still others are
situated in places that are geographically inaccessible. These and many
other problems confront the paleontologist as he attempts to catalog the
plants and animals of the past.
The missing pages in the fossil record become more obvious and more
numerous in the older rocks of the earth’s crust. This is because the
more ancient rocks have had more time to be subjected to physical and
chemical change or to be removed by erosion.