Fossils of cephalopods (sef'-al-oh-pods) have been found in rocks of many ages, and numerous representatives are alive today. Squids, octopuses, cuttlefish, and the chambered nautilus are among the cephalopods living in modern seas.
Cephalopods are the most advanced of all animals without backbones. They have a highly developed nervous system and have eyes much like those of humans.
The cephalopod's mouth is surrounded by long tentacles commonly armed with suckers. Beneath the tentacles is a tube through which the animal can force a jet of water and thus move about by jet propulsion.
Coiled cephalopods live today only in the South Pacific, but in the geologic past they were found scattered throughout the world. Modern squids live in shallow coastal waters over much of the globe.
Most of the cephalopods we find as fossils had a calcareous outer shell. Some were loosely coiled, some were tightly coiled, and others were shaped like a tapered tube. As the shelled forms grew, they periodically made new and larger shell chambers to fit their bodies and sealed off the old part of their shells with a wall of pearly calcareous material—hence, the name "chambered nautilus."
During the Ordovician Period, about 460 million years ago, some straight cephalopods grew to be as long as 19 feet, although most were much shorter. Straight cephalopods were common in Ordovician and Silurian times; coiled ones became fairly common only by later Paleozoic times. We find both kinds in Pennsylvanian rocks in Illinois.