Video Transcript
Complete the following. A fossil that can tell us the age
of the sedimentary rocks that surround it is called blank. (A) An index fossil, (B) a trace
fossil, (C) a time fossil, (D) an atypical fossil, (E) a reference fossil.
This question is asking us about
fossils, so what is a fossil exactly? A fossil is the preserved remains,
such as bones or shells, impression, or trace of an organism that lived many years
ago. Let’s look at how fossils can form
by using an example of a couple of shelled organisms. We’ll call these organisms A and
B.
In order for these organisms to be
turned into fossils, they need to be buried. So let’s say a giant sand dune
suddenly falls on top of them. Now our organisms are buried in
sand. Millions of years pass, and another
shelled organism that we’ll call organism C is living its life on top of the buried
organisms A and B. Suddenly, a nearby volcano erupts
and fills the sky with volcanic ash. This volcanic ash falls to the
ground and sadly buries organism C. Millions of years go by, and more
layers accumulate like a layer of mud, for example. And over the course of hundreds of
millions of years, more layers build up, and this puts a lot of weight on the bottom
layers. This weight compacts the bottom
layers.
Over time, these layers become a
special type of rock that we call sedimentary rock. Everything inside this rock
decomposed a long time ago and became a fossil. A fossil can be a rock version of
our organism or just a rock version of the shape of the organism. We can use these different layers
to give an idea of when things happened. Because fossil A and fossil B are
found in the same layer of sedimentary rock, we know that they’re from the same
time. And because fossil C is in a layer
that is on top, fossils A and B must be older.
In fact, we can use special
laboratory techniques to date these rocks so we have a better idea of how old these
things really are. But rather than heading to the
laboratory every time to date these rocks, we can approximate their age using index
fossils. An index fossil is a fossil that
can be used to define and identify different periods of time. Let’s pretend fossil A is an index
fossil. Index fossils are very common
fossils that are found around the world and are frequently found during certain
periods of time. So every time we find fossil A,
it’s always around 200 million years old.
So let’s pretend that we have no
idea what the dates of these layers of sedimentary rock are. And if we find fossil A in one of
these layers, then we know that the surrounding rock and all the fossils it contains
were formed about 200 million years ago. Therefore, the option that
completes the blank in the statement “A fossil that can tell us the age of the
sedimentary rocks that surround it is called blank” is given by answer choice (A),
an index fossil.
