Everyone knows about the mass extinction at the end of the Cretaceous Period, when dinosaurs died out.

You may not have thought about how we know that it happened.

Scientists spot mass extinctions by following species through time.  There are many species of animals and plants which are found in the Cretaceous Period which are not found in the Tertiary Period which followed it.  It is this pattern which identifies a mass extinctions.

The late Cretaceous mass extinction is only one of five which have occurred since the start of the Cambrian Period 570 million years ago.
 

Can you spot a mass extinction?

Let's look at some of the fossils we find in Tennessee, and follow them through geologic time.  (Click on the pictures to see a larger image.)
 

Brachiopod	Periods in which it is found
Hebertella	Cambrian, Ordovician, Silurian, Devonian
Lepidoculus	Ordovician, Silurian
Platystrophia	Cambrian, Ordovician, Silurian, Devonian
Rafinesquina	Cambrian, Ordovician, Silurian, Devonian
Resserella	Cambrian, Ordovician, Siluriuan, Devonian Carboniferous, Permian
Rhynchotrema	Ordovician, Silurian, Devonian
Zygospira	Ordovician, Silurian, Devonian
Discomyorthis	Devonian
Gypidula	Silurian, Devonian
Leptaena	Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic Jurassic
Levenea	Silurian, Devonian
Meristella	Silurian, Devonian
Pseudoatrypa	Devonian
Schuchertella	Ordovician, Silurian, Devonian, Carboniferous, Permian
Sphaerirhynchia	Silurian, Devonian
Uncinulus	Silurian, Devonian, Carboniferous, Permian

Take this data and build a histogram (bar graph)  on the chart form attached.

Can you spot what might be a mass extinction?
 

If you guessed the Devonian Period, you're right.

The five mass extinctions occurred:
 

• The end of the Ordovician Period
• The end of the Devonian Period
• The end of the Permian Period
• The end of the Triassic Period
• The end of the Cretaceous Period

There are several excellent websites on the great mass extinctions

The BBC has a report on mass extinctions, which gives a brief summary of each major extinction.

The Hooper Virtual Paleontological Museum in Canada has a similar summary.