Fossil Clams and Other Bivalves

color photo of rock slab with numerous fossils; one scallop-like bivalve in center of photo, slightly larger than the penny used for scale

Pennsylvanian bivalves in limestone, collected near Bonner Springs, Kansas (note bryozoan fragments on right side of photo).

Description: Of the fossils commonly found in Kansas rocks, clams may be the easiest to recognize because they closely resemble the shells scattered along modern seashores. Clams and their relatives (oysters, scallops, and mussels) are often called bivalves (or bivalved mollusks) because their shell is composed of two parts called valves.

Bivalves have a long history. Their fossils first appear in rocks that date to the middle of the Cambrian Period, about 510 million years ago. Although the group became increasingly abundant about 400 million years ago during the Devonian Period, bivalves really took off following the massive extinction at the close of the Permian Period.

Modern bivalves live in a variety of marine and freshwater environments, from the shallow waters near shore to great depths in the ocean. Fossils indicate that bivalves have occupied most of these environments for more than 450 million years, but during the Paleozoic Era they were especially common in near-shore environments.

Inside the bivalve's hard shell, the body consists of two lobes, one lining each valve. A muscular structure called a foot, present in most bivalves, is used for locomotion and burrowing. Most bivalves feed on microorganisms suspended in the water, though some feed on the materials found in sediments of the sea floor and a few are even predatory.

Over their long history, bivalves have evolved a variety of lifestyles. Some live on the surface of the seafloor, attached to rocks and other objects or to the sediment. Oysters are an example of this mode of life. Other bivalves use their muscular foot to burrow into the substrate, sometimes quite deeply. These burrowing forms rely on tubelike structures that they can extend to the surface of the sea floor, allowing them to feed while remaining protected below the surface of the sediment. Some bivalves bore into rock and coral reefs and live in these cavities, and others simply lie on the substrate, unattached. A few unattached species such as scallops swim by clapping their valves together, propelled by the rapid expulsion of water.

Like their living descendants, fossil bivalves come in many different shapes and sizes. Externally, the valves have a wide range of markings. Typically bivalves are bilaterally symmetrical with the right and left valves being symmetrical, in contrast to the bilateral symmetry of individual brachiopod valves. In typical brachiopods, the plane of symmetry that divides the animal into mirror-image halves passes vertically down the middle of each valve. Some bivalves, such as oysters, do not have symmetrical valves.

Drawing illustrating typical bivalve symmetry, where one valve mirrors the other

The oldest fossil clams are generally the smallest; most Cambrian species are tiny, just large enough to see without magnification. Over time, larger species evolved. The largest--inoceramid clams from western Kansas--are as much as 6 feet in diameter. These extinct clams lived in colonies on the sea floor of the shallow ocean that covered the interior of North America during the Cretaceous Period (from about 145 to 65 million years ago) and are preserved in great numbers in the rocks of the Niobrara Chalk. Some of these huge fossils are covered with encrusting oysters. Others have been found with a variety of fish fossils between their shells, indicating that the fish used the giant clam as a safe feeding place.

Color photo of smooth interior of large inoceramid clam Color photo of oyster-encrusted exterior of large inoceramid clam

This fossil of Inoceramus (Volviceramus) grandis shows the larger sizes of some of the inoceramid clams from the Cretaceous (note penny for scale in each photo). The top photo is the interior view of valve; the lower photo shows the exterior valve, covered with fossils of encrusting oysters. This specimen is from the Smoky Hill Chalk Member, Niobrara Chalk, Trego County, Kansas.

In addition to the huge inoceramid clams, smaller clams and oysters are also common in the Cretaceous rocks of western Kansas, particularly in the Greenhorn Limestone. In the eastern part of the state, both marine and freshwater bivalves turn up as fossils in Pennsylvanian and Permian limestones and shales.

Color photo of three different specimens of Myalina

Several specimens of Myalina, a thick-shelled clam commonly found in the Pennsylvanian rocks of eastern Kansas. The middle specimen, showing the outside of the clam, is from the Stanton Limestone, Montgomery County; the other two, showing the interior of the shell, are from the Kanwaka Shale, Coffey County, Kansas.

Stratigraphic Range: Lower or Middle Cambrian to Holocene.

Taxonomic Classification: Bivalves belong to the Kingdom Animalia, Phylum Mollusca, Class Bivalvia.

Sources

Boardman, Richard S., Cheetham, Alan H., and Rowell, Albert J., 1987, Fossil Invertebrates: Boston, Blackwell Scientific Publications, 713 p.

Clarkson, E. N. K., 1979, Invertebrate Palaeontology and Evolution, 3rd Edition: London, Chapman and Hall, 434 p.

Cox, L. R., 1969, General Features of Bivalvia; in, Treatise on Invertebrate Paleontology, Bivalvia, Mollusca 6, Part N, Vol. 1, Moore, R. C., ed.: Boulder, Colorado and Lawrence, Kansas, Geological Society of America and The University of Kansas, p. N3-N129.

Johnson, Kirk B., and Stuckey, Richard K., 1995, Prehistoric Journey--A History of Life on Earth: Boulder, Colorado, Denver Museum of Natural History and Roberts Rinehart Publishers, 144 p.

Moore, Raymond C., Lalicker, Cecil G., and Fischer, Alfred G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.

Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.

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