Geology Reference
In-Depth Information
Chapter 22
And Then There Were None:
Major Extinction Events in
Earth's History
In This Chapter
Looking for causes of mass extinctions
Finding out about five major extinctions in earth's history
Focusing on extinctions in human history
More than once in earth's long history, geologic events have led to the demise of multiple
species. Sometimes whole families of organisms disappeared, putting an end to that par-
ticular path of evolution and leaving room for surviving animals to spread into new habit-
ats. Each of these extinctions is well-documented by changes in the fossils preserved in
the geologic record.
To be
extinct
means to no longer exist. Technically speaking, at the end of your lifetime
you will be extinct, though your species
(Homo sapiens sapiens)
will not be extinct be-
cause many other people will still be living. When scientists talk about extinction, they
talk about the extinction of every member of a whole species, or even a
genera
(group of
species) or a
family
(group of genera).
When the term
mass extinction
is used, it indicates that a very large number of
species cease to exist. In geologic time, a mass extinction event may occur over
several hundreds of thousands — or even millions — of years. While this seems
like a long time relative to a human lifespan, remember that geologically speaking
it's not very long at all.
