Mostly smaller than a fingernail, their two-part body casings, or carapaces, are familiar components of mud and ooze in today's oceans, lakes and ponds, and also in preserved sediments from these habitats. Ostracods have thrived since Ordovician times, almost 500 million years ago, at first in the sea, and then from the Carboniferous Period in fresh waters, with genera such as Darwinula.