The name Gastropod means “stomach footed” and refers to mollusks such as snails, which have a coiled or conical shell. Fossil snails occur throughout Illinois, especially in Ordovician or Pennsylvanian rock strata. Ice Age snail fossils can be found in loess, deposits of wind-blown silt cemented by calcium carbonate, along the banks of major rivers. Gastropod fossils date as early as the Cambrian Period, 490 million years ago.
The name Cephalopod means “head-footed”, and refers to mollusks like squid, octopus, cuttlefish, and nautilus. Most fossil cephalopods had a shell of calcium carbonate. All cephalopods with shells are extinct except for the nautilus. Their shells were either coiled or in the form of a tapered tube which could grow up to nineteen feet in length. Cephalopods in Illinois are commonly found in Ordovician, Silurian, and Pennsylvanian rocks.
Pelecypods are aquatic bivalves like mussels, clams, oysters, and scallops. They have two shells that are mirror images of each other. Unlike brachiopods, bivalves remained highly diversified following the Permian extinction. Fossil bivalves are found in Cambrian marine rocks more than 500 million years old and are still abundant. Fossil bivalves are found mostly in central and northern Illinois, in Pennsylvanian and Ordovician rocks.