A woodlouse (plural woodlice) is a crustacean from the monophyletic suborder Oniscidea within the isopods. This name is descriptive of their being found in old wood. The first woodlice were marine isopods which are presumed to have colonised land in the Carboniferous, though the oldest known fossils are from the Cretaceous period. They have many common names and although often referred to as "terrestrial Isopods" some species live semiterrestrially or have recolonised aquatic environments. Woodlice in the families Armadillidae, Armadillidiidae, Eubelidae, Tylidae and some other genera can roll up into a roughly spherical shape as a defensive mechanism; others have partial rolling ability but most cannot conglobate at all.