Butler County is in an area known to scientists as the Cincinnati Dome, a large area in North America defined by a bulging of the earth's crust into a dome shape. The elevation of this dome caused the oldest fossils to be found at the peak of the dome. The farther away from the peak, the younger the fossils are.
The bedrock under the majority of the county is from the Ordovician period of the earth's history. [MAP] It is believed that this period occurred 505-440 million years ago. Our county was completely submerged under a shallow sea during this time.
Although there were no land-dwelling animals or plants during this period, life was abundant in the sea! The sea was home to plant life, in the form of algae, and to aquatic animals. Isotelus brachycephalus, Ohio's oldest fossil, was a living trilobite in the sea that covered Butler County in that distant past. Isotelus is Ohio's official State Fossil. Trilobites were not alone in the sea. Other creatures included ostracods, the clam-like brachiopods, graptolites, corals, bryozoans, squid-like nautiloids, and jawless fishes called agnatha. Fossils of these earliest known Butler countians have been deposited in the bedrock that was formed during the Ordovician period
