During the Late Ordovician Period, much of the Midwestern United States was covered by a shallow sea. The entire region lay south of the Equator. The tropical conditions in the area created an environment thriving with life. Some of this life came in the form of corals. Corals occupy a very important role in the oceans.
Most corals build large reefs that provide habitats for many other forms of life.
They are also very useful paleontologically as indicators of climate and latitude. Corals only live in shallow marine environments because they require clear, constantly moving water, oxic conditions, and sunlight. They are restricted to tropical climates because the warm water facilitates the precipitation of their calcium carbonate skeletons. These corals are called hermatypic corals, meaning that they incorporate a symbiotic algae called zooxanthellae into their tissues. These algae live off of the carbon dioxide in the water around the corals and release oxygen as a by-product, giving the coral polyps one of the key ingredients they need for survival.