Gastropods are the most abundant mollusks: There are approximately 40,000 living species and 50,000 fossil species. Once confined to the oceans, gastropods now live in streams, lakes, and even on land. Some modern species are voracious predators equipped with poison. However, the snails that lived in Wisconsin’s Paleozoic seas were probably peaceful grazers.
The shell of a gastropod is usually coiled and is used for protection, as it is for most mollusks (A). Gastropods can retreat into their shells and close the opening with a cover called the operculum. The shape of the shell can be highly varied because of the way it coils. Each 360° revolution of the shell is called a whorl. The body whorl is the bottom and largest whorl; it contains the aperture, the opening of the shell. The shell spire includes the whorls above the body whorl. Whorls come in contact with one another at the shell sutures.
Gastropods have highly developed sensory organs, including tentacles and eyes (B). The head is attached to the foot, a muscular organ used for creeping and feeding. Many gastropods have a radula, a rasp-like structure used for scraping algae and other food off the seafloor. Some carnivorous gastropods use the radula for boring through the shells of other animals.
Gastropods originated in the Cambrian, but few are found in rock of that age in Wisconsin; more are found in Silurian and Devonian rock. Wisconsin’s Ordovician rock contains abundant and beautiful gastropod fossils.