What is a fossil?

Fossils provide us with an important picture of ancient life. But how is a fossil created, exactly? Marshall Brain breaks down the various natural processes behind fossilization in this episode.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstaff Friends, stuff works dot Com where smart
happens him Marshall Brain with a cold and with today's question,
what is a fossil? And if you think about it,
there's something very odd about the fact that a bone,

(00:23):
or a plant or a tree trunk can turn into
a piece of rock. So what's actually going on there?
The term fossil describes a wide range of natural artifacts. Generally,
speaking of fossil is any evidence of plant or animal
life that's preserved in the material of the Earth's crust.

(00:43):
But when most people talk about fossils they mean a
specific subsection of this group fossils, in which the shape
of the animal or plant has been preserved while the
actual organic matter of its body has disappeared. These amazing remnants,
which date to pre as historic times, were formed very
slowly by dynamic geological processes. In most cases, the fossilization

(01:07):
process began when a plant or animal died and was
quickly covered up with sediments, usually at the bottom of
a body of water. The loose sediments protected the bodily
remains from the elements, but more importantly, they protected them
from bacteria and other forces that caused weathering. In decay.
This slowed the decaying process down so that some of

(01:30):
the remains. In most cases, only the hard materials like
bone or shell were preserved for thousands of years without
getting eroded away or eating away or crushed by passing
hoofs or whatever. During this time, sediment layers continued to
build up above that piece of bone. Eventually, these sediment

(01:51):
layers became hard solid rock. Sometime after this hard rock
layer formed, water percolated down through the rock and washed
the preserved remains away. Since the rock above was hard
and rigid, it didn't fall down into the empty space
where the remains used to be. This empty space formed
a natural mold of the animal, perfectly preserving the shape

(02:14):
of the original remains. In some cases, percolating water carried
minerals into the mold. These minerals hardened to make a
natural cast of the form, just as an artist might
make a sculpture cast. By filling a mold with plaster,
all the original organic material disappeared, but nature left a
precise mineral reproduction of that plant or animal. In cases

(02:39):
where minerals did not fill the mold, paleontologists may fill
it themselves, creating an artificial cast. This is just one
scenario for fossilization. There are all sorts of other ways
nature might form a fossil. A lot of prehistoric insects,
for example, have been fossilized in amber. This sort of
fossilization occurred when the insect was enveloped in the liquid

(03:02):
sap from a tree, Just like the sediments at the
bottom of a body of water. The sap material protected
the insect from decay and eventually hardened into a rock
like material. Animal fossils are also found in tar pits, bogs, quicksand,
and volcanic ash. Another interesting fossil type is petrified wood.

(03:23):
Petrified wood generally forms when trees fall into a river.
They become saturated and then buried in mud, ash, silts,
or other materials. Minerals such as the silica and volcanic ash,
seep into the tree trunk and fill tiny pores in
the wood cells. This changes the overall composition of the wood,

(03:44):
turning it into stone while preserving the original structure of
the wood. The variety of minerals in petrified wood creates
strikingly vivid colors. In addition to fossilized plant and animal
body remains. Paleontologists study fossilized animal footprints and trails, and
even fossilized animal dung called corporate lights. These fossils are

(04:09):
enlightening because they reveal something about how prehistoric animals moved
and what they ate. The fossil record, the total collection
of fossils in the world is extraordinarily important to our
understanding of the Earth's history. Fossils tell us which plants
and animals existed in prehistoric times and where they lived.
They also tell us something about when they lived. Based

(04:31):
on the position of fossils in the layers of the
Earth's crust, paleontologists can determine which animals predate other animals
and which animals lived at the same time. Using carbon dating,
paleontologists can sometimes estimate the age of fossils. This provides
the age of the rock layer where the fossil was found,

(04:52):
which helps scientists date all the other material at that level.
Without fossils, we would have a much more incomplete picture
of the Earth's early history and life's early history. For
more on this and thousands of other topics, visit how
stuff works dot com.