stratigraphic intervals
   (System) †Ordovician (485.4 - 443.8 Ma)
     (Series) †Middle (470.0 - 458.4 Ma)
       (Stage) †Tremadocian (485.4 - 477.7 Ma)
         1 (Substage) †heres-serratus Zones
         2 (Substage) †flabelliforme-incipiens Zones