Countless tiny crustacean 'shellfish' crop up in the fossil record - then, as now, vital links in the food chain for larger creatures. One group is the ostracods, also called mussel-shrimps or seed-shrimps. They look superficially like very small bivalve shellfish, but they are not molluscs or shrimps - they form their own major group, Ostracoda, within the Crustacea.