Gastropods
Gastropods (gas'-troh-pods) are commonly called snails.
Snails carry their shells on their backs and retreat into it whenever danger threatens. As a snail grows, the shell expands and lengthens. Most commonly the shell is coiled in a spiral, but some are shaped like a pointed turban hat.
Gastropods are the largest group of molluscs and contain about 70% of all mollusc speciesm with about 77 000 described living species and 15 000 fossil species, and can range in size from just a few mm to 600 mm, or even to 1 meter in the case of the sea hare Aplysia sp., some fossil species are 2 m long. They are bilaterally symmetrical,
Gastropod is a Greek word meaning 'stomach foot'. Although gastropods do move around on a large, sliding foot, it is not actually their stomach. The stomach is protected inside their shell. Most gastropods carry a single shell for protection and feed using a kind of rasping tongue with teeth called a radula. See below.
There are many kinds of gastropods. Some lilve in the sea, some live in rivers, and still others live on land. The ones that live in water have gills like fish, but those that breathe air have simple lungs.
Gastropods have a distinct head, feelers, eyes, and a mouth. Some of the snails have a rasp-like tongue and may use it for boring into other shellfish, which they eat.
Fossils
The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian.
By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats.
By the Carboniferous period many of the shapes we see in living gastropods can be matched in the fossil record, but it was during the Mesozoic era that the ancestors of many of the living gastropods evolved.
Gastropod fossils are sometimes confused with ammonites or other shelled cephalopods.
Growing up in Illinois, I was particularly fond of Trepospira, and I still have a sample of it in our family collection. Here's one from Texas.
I also have a nice Euomphalus. Here's a picture of a poor quality specimen, taken by T. Hunt.
As you can see, this marine gastropod looks very much like an ammonite. |
