Trilobites florished during the Paleozoic Era. Eras are geologic time periods that have one overriding characteristic that sets it apart from all the others.
For the Paleozoic Era, that characteristic was Life in the Water. All the life on the planet was in the ancient seas. The Paleozoic Era lasted about 295 million years, but with such a long time span, it wouldn’t have kept the exact same characteristics throughout the entire 295 million years. So, geologists further divided the Era into Periods that had some particular unifying characteristic.
Trilobites make their entrance onto the earth's stage during the late Cambrian Period. This is the first and oldest division of the Paleozoic Era, was named for the location in which the rock layers were first identified: Cambria or Wales. It is followed by the Ordovician Period, the Silurian Period, the Devonian Period, and finally the Permian Period. Each of these periods also took their names from locations associated with the types of rock layers found there.
The numbers of trilobite species increased dramatically through the end
of the Cambrian Period and into the Early Ordovician Period. However,
from the Middle Ordovician until their final extinction at the end of
the Permian Period, the numbers of trilobite species declined steadily.
The greatest decline occurred as the Silurian Period began,
approximately 440 MYA, with a drop in the numbers of species from 42 to
It is generally believed that the Ordovician Catastrophe was a change in climate, due to an ice age, which lowered ocean levels and thus causing a change in land mass. During this climactic change, more than half the species of life on earth became extinct.