The earliest certain coleoids are known from the Mississippian sub-period of the Carboniferous Period, about 330 million years ago. Some older fossils have been described from the Devonian,[3] but paleontologists disagree about whether they are coleoids.[4] Other cephalopods with internal shells, which could represent coleoids but may also denote the independent internalization of the shell, are known from the Silurian.[5] It is possible that the Early–Middle Cambrian fossil Nectocaris represents a coleoid (or other cephalopod) that lost its shell, possibly secondarily.[6][7]