There are five orders that Ostracoda have been divided into both extant (living) and extinct groups.
- The extant groups
- Podocopida - Abundant in Mesozoic and Cenozoic sediments.
forms mainly distinguished by their soft parts. Valves are of unequal
- Myodocopida - Found from the Ordovician to the present day, they do not have a valve overlap. Contains large numbers of pelagic types and have a poorly calcified carapace.
- The extinct groups
- Phosphatocopida - Thought to have first appeared in the Cambrian Period.
- Leperditicopida - Large in size (5 to 30 mm) smooth shell with unequal valves.
- Palaeocopida - From the Palaeozoic and the Triassic Period and can be recognised by a long straight hinge line and often display distinctive sexual dimorphism.
(it has been highlighted that the latter, in a biological sense may not
actually be ostracods).