They are rarely numerous, but nearly always present in marine, brackish and fresh water. Some are even terrestrial. Fresh water forms are spread from lake to lake by living in the mud that is stuck to the feet of waterfowl. They are often abundant in the fossil record and it is easy to collect large numbers of them. This minimizes the possibility of missing rare species.
Some ostracodes have long geologic ranges and apparently narrow envirnomental tolerances.
Thus they are quite useful for paleoenvirnomental work. They have a bivalve larval stage that looks roughly like an ostracod, but the adult is sessile and grows a series of calcite plates around its body. The fossil record begins in the Silurian, but they are rare until the Mesozoic.