Geology Reference
In-Depth Information
By far, the most common type of fossil preservation is in the form of a mold
or a cast. A
mold
forms when an organism is buried in sediments, the sediments
are hardened into a rock, and the remains of the organism decay or dissolve away.
What remains in the rock is a mold of the organism but no actual parts of the or-
ganism. A
cast
forms when sediments fill the cavity of a shell or bone and harden,
preserving the inner details of the body part.
Just passing through: Trace fossils
Some fossils provide evidence of an animal's activity in the distant past without pre-
serving any part of the organism itself. These kinds of fossils are called
ichnofossils
or
trace fossils
because the organism has left only a trace, or small indicator, of its life and
behavior. Trace fossils are any preserved indication of an organism's activity and in-
clude the following:
Burrows:
Burrows
indicate how an organism lived. Small burrows of ocean-dwelling
organisms, for example, are easily preserved in the soft sediments at the bottom
of the sea.
Tracks and trails:
Organisms that move across land or along the ocean bottom
may leave
footprints, tracks,
and
trails
in the sediments. They may be buried and
become trace fossils, indicating movement and motion.
Coprolites:
Coprolites
are fossilized droppings, or feces, from an organism.
Figure 17-1 illustrates burrow and track trace fossils.
Figure 17-1:
Trace
fossils include
tracks and bur-
rows.
