Above: Valves of Candona candida (left) and Cyclocyris ampla (right), two freshwater ostracodes

Ostracodes are tiny crustaceans, put together in many ways like a lobster or crab. In ostracodes, however, the shell (carapace) covers the entire body, and is divided into 2 valves hinged along the back. Living freshwater ostracodes (pronounced "ostra-COD", not "ostra-CODE") have been described as looking like a "lima bean riding a bicycle."

Ostracodes are helpful in figuring out temperatures and water chemistry of ancient lakes in much the same way that clams and other ancient life (that are still living today) can be helpful. If you have ostracodes preserved in a layer of sediment, and you are trying to work out the climate at the time those sediments were deposited, simply see where those same species of ostracodes are living today, figure out what the climate is like at that spot currently, and Viola!, you have your answer. This is called finding a "modern analogue".

Of course, it's never quite that simple. Ostracodes, like many other animals, can sometimes live in a wide range of habitats, so you often have to look at other features of sediments to narrow down the answers to your questions.

Photos courtesy of Alison Smith, Kent State University Department of Geology