Mollusk Fossils

From Ohio History Central

Mollusks (Phylum Mollusca) are a diverse group of organisms that include such familiar animals as clams and snails but other, less familiar ones as well. They range from Cambrian to Recent and, although they are primarily marine organisms, some groups have conquered freshwater and terrestrial environments.

Clams, sometimes called pelycopods (Class Pelecypoda) are filter-feeding animals with two identical shell halves. Some burrowed in bottom mud, whereas others rested on the sea bottom and a few were capable of swimming. Marine species are abundant and diverse in Ordovician through Permian rocks in Ohio and are commonly well preserved. Pennsylvanian and Permian rocks yield species that lived in fresh or brackish waters. Clams are common in some sediments deposited in lakes or streams during the latter part of the Pleistocene Ice Age.

Particularly interesting to fossil collectors are the large clams found in Middle Silurian rocks that are called “beef heart” clams because of their size and resemblance to the heart of a cow. This species, Megalomoidea canadensis (formerly called Megalomas) can reach almost 10 inches in length. They are commonly preserved as internal molds of the shell as the original calcium carbonate of the shell has been leached away. Clams from Mississippian and Pennsylvanian rocks have rarely been found with the original color patterns preserved on the shell. These occurrences remind us that in life many of the fossil organisms we find in Ohio rocks were not dull gray in color as they appear as fossils but had bright color patterns.

Snails (Class Gastropoda), or gastropods, are familiar to most everyone who has collected shells along a seashore or observed a terrestrial snail in the garden. They have been a very successful, diverse, and long-ranging group, having originated in the Cambrian. Shells of snails are well-represented in Ordovician through Pennsylvanian marine rocks and freshwater species are known from Pennsylvanian and Permian rocks and from deposits of the Pleistocene Ice Age. Most specimens are small, less than an inch long.

Snails are characterized by a single shell that is coiled, either in a flat plane (planisprial) or in a cone shape (conispiral). They have a foot that is used to crawl across the bottom in search of food and a mouth with a rasp-like tongue. Some modern species are predatory and are noted for boring circular holes in clam shells in order to get to the soft parts.

Many Ordovician and Silurian snail shells are internal molds of the shell (steinkerns) as the original calcium carbonate shell has been removed. These specimens are sometimes difficult to identify, as the external shell ornamentation is absent.

See Also


  1. Feldmann, R. M., 1996. "Introduction," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 1-25.