In 1985, Ohio made Isotelus the state’s official fossil. Isotelus is a trilobite that existed between 430 and 480 million years ago. At that time, an ocean covered much of what is now Ohio. A trilobite was an invertebrate marine creature having a hard outer shell or skeleton. Two lines crossed the body of the trilobite, making it appear to be in three parts. Trilobite means “three-lobed creature.” Isotelus primarily lived during the Ordovician Age and was one of the largest trilobites, with some of them reaching more than two feet in length.