Ostracodes (aws'-trah-cods) are very small animals that are common as fossils but are rarely large enough to be seen by the naked eye.
Ostracodes have been present on Earth since the early part of the Ordovician Period and occur today in great numbers in lakes, rivers, and seas. Ostracodes prefer shallow water and live in vast hordes, crawling over the bottom or swimming near the surface.
Ostracodes have two shells, one on each side of the body, so that some look much like small clams. But the animal inside looks much like a shrimp or an insect with jointed legs and feelers. As the animal grows, it sheds its shells and forms a new pair. The shells may be smooth or ornamented with pits, bumps, ribs, or spines.