Unionid clams

Clams look like they have not changed much since the Jurassic. Nevertheless this mollusk tells us a lot about the environment of the time. Scale bar is 1 inch.

About unionid clams:

Unionid clams live in freshwater rivers and are related to bivalves such as clams, oysters, and scallops. They are filter feeders and sessile, which means they stayed in place their whole lives. They do not have internal skeletons that could be fossilized. However, their hard shells are either fossilized or sediment fills the empty shell cavity to make a mold of the shell.

 

Why are unionid clams superstars?

Unionid clams give us valuable information about the past environment. Since they need water, we know there was a permanent river as opposed to flooding from rain. The water must have been clear, since they were filter feeders that would not want sediments interfering with feeding. All unionid clam larvae are parasitic on fish and use them to travel around river drainages. Their abundance in the Carnegie Quarry indicates that fish were abundant, even though their remains are unknown from that layer.

 

If you look closely, you can see clams like these in the Morrison Formation on the Fossil Discovery Trail. Often the shell has eroded away leaving just a mold of the shell.

Jurassic Fact: Unionoid clams, in their thousands, are by far the most abundant organism preserved in the Carnegie Quarry.

For more information: Visit the Quarry Exhibit Hall to see an extremely well-preserved clam on exhibit.You can also hike the Fossil Discovery Trail to see unionid clams still in the rock.