From Ohio History Central
Cephalopods (Class Cephalopoda) are probably familiar to most people as the octopus and the squid. However, in the fossil record, shelled cephalopods known as nautiloids and ammonoids were the dominant forms. Today, the chambered nautilus survives as the only surviving representative of these once diverse groups. Cephalopods are relatively common in some marine rocks in Ohio, ranging from Ordovician to Pennsylvanian.
Cephalopods were swimming, predatory marine animals that had either a straight or coiled, chambered shell. The chambers were divided by walls (septa) and in life filled with gas. This enabled the cephalopod to move up and down in the water column by changing the pressure in the chambers. The last chamber housed the body of the cephalopod. These organisms had tentacles for capturing prey, well-developed eyes, and a beak used to kill, dismember, and ingest prey. They were also capable of swimming by aid of a tube for jet propulsion.
Many cephalopods were comparatively large animals. Ordovician strait-shelled (orthocone) cephalopods have been found in the Cincinnati area that are nearly seven feet in length. Most coiled forms were a few inches to perhaps 10 inches in diameter. In many examples the outer shell of calcium carbonate has been leached away, revealing the surface of the septa that divide the chambers. Those cephalopods that have straight-walled septa are called nautiloids whereas those that have convoluted or wiggly septal walls are called ammonoids. Nautiloids occur throughout the marine rocks in Ohio whereas ammonoids are known from Devonian and later rocks.