Cambrian-Ordovician extinction
At the end of the Cambrian Period, there was a mass extinction event. Theories of how this may have happened range from glaciation to depletion of oxygen in water. However it happened, a very large amount of life on earth at this time went extinct. For example, many Brachiopods want extinct as well as a large number of Trilobites.
Theories
Glaciation- This is basically a fancy word for another ice age. If there had been an ice age, glaciers may have moved farther toward the equator. This may have caused dropping temperatures, which is something life at this point in time was not evolved to handle.
Oxygen Depletion- This theory is much more straightforward. What happens when this type of extinction occurs is the organisms in the water use more oxygen than is able to be supplied. This could definitely been the reason why so many species went extinct because of all the creatures in the water.