For many, fossils are a fascination that starts when we are young. How many times have you
marveled at the skeletal remains of a dinosaur, or admired the shiny surface of a fossil fern?
Have you ever wondered how these forms wound up trapped in rocks for us to find? Well,
today we’re going to tell you about how fossils form.
First, what is a fossil? Fossils are the remains of an organism or traces of their behavior
which have been preserved in the rock record. Fossils are generally at least 5000 years
old. There are two main kinds of fossils. The first is body fossils which preserve some
feature of the plant or animal itself. These include shells, bones, and petrified wood.
The second type are trace fossils. Trace fossils preserve something from an animal’s behavior,
such as footprints, tracks, or fossil dung, also known as coprolites.
There are many ways that a fossil can be made. First, there is preservation of the organisms'
soft parts. This means that it died in such a way that the original flesh was preserved.
This is the rarest of all fossilizations and is most famous for frozen mammoths or insects
trapped in amber. Next there are unaltered hard parts, when
bones, shells, and exoskeletons of creatures are preserved in their original material,
but without any of the softer flesh. This is more common in younger deposits, such as
tar pits. Finally, there is the most common kind of
fossilization, altered hard parts. Altered hard parts mean that, when the organism died
and was buried, the original material making up their bones or shells was either replaced
by other minerals or the shape of the hard part is preserved as a mold in the rock. These
kinds of fossils can be created in several different ways.
The first way is permineralization, where empty space in the pores of the hard part
are filled with minerals precipitated from mineral-rich water, which is often seen with
bones and petrified wood. The second way is carbonization, where all that is left of the
organism is a carbon film. Carbonization is especially common with fossil plants. Thick
accumulations of carbonized wood are called coal. The third way is recrystallization,
where hard parts made of amorphous or unstable crystals are altered to more stable crystalline
material of the same chemical composition. The fourth way is replacement, which occurs
when a shell, bone, or other hard part is replaced with another mineral. Replacement
fossilization may occur so gradually and at such fine scales that microstructural features
are preserved despite the total loss of original material.
The final type of altered hard part fossils is the preserved impression of an organism,
which often looks like other kinds of fossils. This is when the original remains are completely
dissolved, leaving an impression of the hard part in the rock, a kind of ghost fossil.
The outside impression is called an external mold. If this external mold cavity is later
filled with sediment or other minerals, it is a cast that forms an impression of the
organism itself. Now that you know a bit more about how fossils
form, next time you drop by the museum, you can impress your family and friends by sharing
what you learned!