Ostracods are tiny crustaceans (relatives of shrimps, crabs and water-fleas), distinguished by having a shell that is easily fossilised. As microfossils, by virtue of a long and rich fossil record, ostracods are extremely useful for determining the age of the sedimentary strata in which they are found, as well as providing clues to the nature of the environments and climates in which those deposits were formed. The first ostracods lived in shallow continental shelf seas during the early Ordovician period nearly 500 million years ago, later spreading and diversifying into deep oceanic as well as continental environments such as lakes and rivers. Today, as living organisms, they are globally widespread and diverse, inhabiting almost every kind of aquatic environment from the abyssal depths of the oceans to freshwater ponds.