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Paleontologists want to know the history of evolution and extinction
of fossil species through geologic time. To do that, they try to study
all of the fossils that have been preserved in sedimentary rocks. That's
called the fossil record. Paleontologists have been collecting fossils
from sedimentary rock layers around the world for over two hundred years.
Their goal is to figure out the succession of species through all of geologic
time. Once that succession is known, it serves as a scale of geologic
time. Then, if you find a particular fossil in a rock, you know where
that rock fits into the geologic time scale.
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Over tens of millions of years,
characteristics of the trilobites change in appearance.
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© Oklahoma University
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Left to Right
Fossil of a trilobite, an extinct arthropod, that existed
in the seas of the Paleozoic era. (Credit)
Fossil of a trilobite, an extinct arthropod, that existed
in the seas of the Middle Cambrian era. (Credit)
Fossil of a trilobite, an extinct arthropod, that existed
in the seas of the Middle Silurian era. (Credit)
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There's a big problem in figuring out the succession of species through
geologic time. You don't know beforehand what the succession of species
is! All you have are many stacks of sedimentary rocks (called stratigraphic
sections) around the world to look at. No single stack spans all of geologic
time, and no single stack has nearly all of the species that ever lived.
You have to compare all of the stacks against one another to get the best
approximation to the real succession. Paleontologists are still refining
their ideas about the succession, as new fossils are found.
Matching up stratigraphic sections from around the world can be very difficult.
If there were no fossils and you could only use the characteristics of
the rock layers it would be even harder! This is because at any given
time, very different types of sediments can be deposited in different
places. It is these sediments that will eventually become the sedimentary
rock layers making up the stratigraphic sections. At any given time, mud
may be slowly collecting in some places while in other places sand is
piling up rapidly. In other places, maybe there is nothing collecting
at all! So you see, very different looking rock layers may mark the same
time interval in different stratigraphic sections! The process of matching
up equivalent "time layers" of rocks in different places is
called stratigraphic correlation. One of the best (and oldest) tools for
correlating strata around the world is the use of special fossils called
index fossils.
Index fossils of organisms have two important characteristics. First,
they must have been widely distributed around the world. Second, they
must have only existed for relatively short periods of geologic time before
becoming extinct. Consider a fossil of an organism that only lived in
one place, or that existed for very long periods of geologic time. It
would be of little use in matching up layers of rock that were deposited
far from one another over the same limited span of time.
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