P-T境界
Cycloids are known from deposits ranging from Carboniferous ("Cyclus" spp.) to Maastrichtian ("Maastrichtocaris rostrata"). They are one of only three groups of "generally Palaeozoic" arthropods to survive the Permian–Triassic extinction event, the others being the freshwater-living Euthycarcinoida and the marine Thylacocephala.