stratigraphic intervals
   (System) †Ordovician (485.4 - 443.8 Ma)
     (Series) †Middle (470.0 - 458.4 Ma)
       1 (Stage) †Caradocian
       2 (Stage) †Llandeilian
       3 (Stage) †Llanvirnian
       4 (Stage) †Arengian
       5 (Stage) †Tremadocian (485.4 - 477.7 Ma)