An order of marine organisms that forms an important part of the class Ostracoda (subphylum Crustacea) and comprises three suborders. It has a long geological history that extends back at least to the Early Silurian. However, the lightly calcified carapaces of the species in this order are a factor in their having a sparse fossil record. Indeed, of the sixteen families of myodocopids, three are known only from modern marine environments. Species of those families that have a fossil record are quite rare and discontinuous in their stratigraphical distribution. Myodocopids are only abundant as fossils from organically rich shale that was deposited in deep, anoxic environments, especially those of the Devonian seas. Such environments were devoid of benthic (bottom-dwelling) life, but the valves and carapaces of the nektonic (free-swimming) myodocopids, both immature forms (termed instars) and adults, sank into such environments and were preserved. The rapid evolution of myodocopids during the Devonian Period has led to their use as guide fossils for dating these otherwise poorly fossiliferous marine rocks.