A fossil colonial coral. Photo by Jean Prior. These fossils are the remains of a multitude of creatures that inhabited a warm tropical seaway covering Iowa about 375 million years ago, during the Devonian period. Most of the fossils seen are from animals that inhabited the sea bottom and filtered small food particles from the water. Some limestone beds are crowded with their shells.
Crinoids are also present.
Most were fragmented into pieces by scavengers and bottom currents before fossilization, and their stem segments are abundant. However, some exceptional specimens at the site were buried intact, displaying their stem, head, and arms as they would have appeared in life. Fossil corals are the namesake for the City of Coralville, and it is certainly fitting that spectacular accumulations of fossil corals are so well displayed at the spillway. Delicate sponges and more massive sponge-like stromatoporoids also occur. Fossils of trilobites, pillbug-like scavengers that crawled across the sea bottom, can be recognized by visitors who carefully scrutinize the limestone beds.