of Candona candida (left)
and Cyclocyris ampla (right), two
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