June 27. Gastropods

Bellerophon gibsoni, Mississippian gastropod

We haven’t said much about gastropods, snails, in our journey. They have a long history, extending back at least to the Cambrian, and while they have a wide variety of shell forms, by Mississippian time many of the shapes that survive to the present day had been established.  

As with other marine invertebrates, gastropods thrived in the warm, shallow Mississippian seas. Some are known that were three inches or more across, relatively large for Mississippian gastropods – but today’s conchs are also gastropods and some of their shells grow to a foot long or more.

Gastropods are a class of the phylum Mollusca, which includes cephalopods – octopuses and squids – and bivalves including clams and scallops. Gastropods comprise about 80% of all mollusk species, and there are more than 60,000 known species today. More than 15,000 species are extinct and are known only from the fossil record.

During the Mississippian most or all gastropods were aquatic. Some of the earliest land-dwelling snails are known from the coal measures of Europe, the late Carboniferous or Pennsylvanian Period, which we will get to next month. Even then, land snails were rare for many more millions of years.

—Richard I. Gibson

Drawing of Mississippian gastropod from an old textbook (public domain)