From Ohio History Central
Phyllocarids are crustaceans with a two-part shell and a tail. Some reached two or more inches in length and they are found in marine rocks in Ohio ranging from Devonian through Pennsylvanian age. The most abundant genus is Echinocaris, known from Devonian rocks. A decopod crustacean, Palaeopalaemon, related to shrimps and lobsters, is known from Devonian rocks in northeastern Ohio. Eurypterids were aquatic predatory arthropods that looked superficially like scorpions. Most had large pincers and, although most were small, some reached lengths of several feet. They were Paleozoic creatures and became extinct by the end of the Permian. They are rare fossils in Ohio being known from Ordovician, Silurian, and Devonian rocks.