Silurian Period

Part of a coral reef illustration by Henrich Harder
Part of a coral reef illustration by Heinrich Harder, circa 1916

438-408 Million Years Ago

During the Silurian Period, the Midwest was south of the equator and supported coral reefs in a tropical marine environment. Trilobites continued to proliferate, preserved as fossils in northern and southwestern Illinois. By the end of the period, however, trilobites became less diverse and abundant.