"global chronostratigraphic units"
   (Eon) Phanerozoic Eon (541.0 Ma - present)
     (Era) †Paleozoic Era (541.0 - 251.902 Ma)
       (Period) †Silurian Period (443.8 - 419.2 Ma)
         (Epoch) †Ludlow Epoch (427.4 - 423.0 Ma)
           1 (Stage) †Gorstian Stage (427.4 - 425.6 Ma)
           2 (Stage) †Ludfordian Stage (425.6 - 423.0 Ma)