Placoderms were armored fishes and among the early jawed fishes. They were a diverse group and some reached very large sizes, perhaps nearly 20 feet in length. They first appeared in the Silurian and were extinct by the end of the Devonian. Remains of placoderms are well known from some Devonian rocks in Ohio.
Placoderms had a series of thick, interlocking, bony plates that protected the head and thorax. The remainder of the skeleton was cartilaginous and rarely preserved. The bony jaws did not have teeth but the jawbones were modified to function like teeth with sharp spikes, shearing blades, or crushing surfaces. Many placoderms were active predators. One of the largest, Dunkleosteus terrelli, is estimated to have a bite force greater than any fossil or living fish and among the most powerful of any animal.
Placoderm remains consisting of head shields of Macropetalithys, are found in Middle Devonian rocks in Ohio. However, the greatest abundance and diversity of placoderm remains are known from the Upper Devonian Ohio Shale and particularly the Cleveland Shale Member of this unit in the Cleveland area. Most of these placoderm remains are found in large carbonate concretions but some bones are found in the shale. The Cleveland Museum of Natural History has a large collection of these specimens and many are on display.