Chapter 18 Fossils and Geologic Time

ally the soft parts of an organism are eaten or decay before
they can be preserved.
Bones, teeth, and shells are far more likely to become fossilized
than soft tissue. For example, a homeowner enlarging
a pond in the Hudson Valley of New York discovered the bones
of another ice age elephant, a mastodon, in 1999. At the invitation
of the landowner, scientists and students from several
universities excavated a nearly complete skeleton of this relative
of modern elephants. This is just one of three very recent
finds of mastodon skeletons in New York State. Like mammoths,
mastodons became extinct about 10,000 years ago.
How did the scientists know what animal left these bones?
The bones of every animal are different. For this reason, an
experienced paleontologist can often identify fossil animals
from a single bone or tooth.
Replacement is another form of fossil preservation. It occurs
when minerals in groundwater gradually take the place
of organic substances. Petrified wood is a common example.
In the process of replacement, minerals take the shape and
may even show the internal structures of the original living
organism. However, these fossils are composed of mineral
material such as quartz rather than the wood or other organic
substance that made up the original organism.
A mold is created when fine sediments surround a fossil
organism or part of an organism, preserving its shape. Leaf
impressions are molds. Other sediments that fill in the hole
left by the organism are known as casts. In fact, filling a hollow
mold makes a cast. Fossils of seashells found in rocks
that extend from the lower Hudson valley through western
New York State are often molds and casts.
Trace Fossils
WHAT ARE FOSSILS? 415
Traces fossils are signs that living organisms were present.
However, they do not include or represent the body parts of
an organism. For example, the only dinosaur fossils found in
New York State are footprints found in the lower Hudson Valley.
Worm burrows are common in some sedimentary rocks in
western New York State. Coprolites can be described as “the
only material a fossil animal intended to leave behind.” They