"global chronostratigraphic units"
   (Eon) Phanerozoic Eon (541.0 Ma - present)
     (Era) †Paleozoic Era (541.0 - 251.902 Ma)
       1 (Period) †Cambrian Period (541.0 - 485.4 Ma)
       2 (Period) †Ordovician Period (485.4 - 443.8 Ma)
       3 (Period) †Silurian Period (443.8 - 419.2 Ma)
       4 (Period) †Devonian Period (419.2 - 358.9 Ma)
       5 (Period) †Carboniferous Period (358.9 - 298.9 Ma)
       6 (Period) †Permian Period (298.9 - 251.902 Ma)