Gastropod Mollusks: Snails

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.

Two drawings showing gastropod anatomy. A) is labeled with the parts of the shell, and B) is labeled with the parts of the body.
Gastropod mollusk anatomy. A) Apertural view of gastropod shell. B) Anatomy of living gastropod.

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.

Eleven drawings of different types of gastropod mollusks
Gastropod mollusk fossils. A) Helicotoma, an Ordovician gastropod with raised, angular sutures [2 cm]. B) Liospira, an Ordovician gastropod with a flattened shell [1.5 cm]. C) and D) Sinuites, a common Middle Ordovician gastropod with a large, smooth shell [2.5 cm]. E) Clathrospira, a large and common Ordovician gastropod with a cone-shaped spire [4.5 cm]. F) Lophospira, a gastropod with angular ridges similar to Trochonema (H), but taller and with an unwound body whorl [4.5 cm]. G) Bottom view of Maclurites, an Ordovician gastropod with one side flattened where it rested on the seafloor [4.5 cm]. H) Trochonema, an Ordovician gastropod with angular ridges [3.5 cm]. I) Hormotoma, a high-spired gastropod found in Ordovician rock [2 cm]. J) Phragmolites, an Ordovician gastropod possessing suture-like ornamentation [1.5 cm]. K) Subulites, a tall, high-spired Ordovician and Silurian gastropod [5 cm].

Gastropod mollusk photos

A circular whorled snail shell fossil
The underside of snail shell
A partial snail shell fossil
A small gastropod fossil protruding from a rock

(Photos courtesy the Milwaukee Public Museum.)