The Tremadocian is the lowest stage of Ordovician. Together with the later Floian stage it forms the Lower Ordovician epoch. The Tremadocian lasted from {Geologic Ages Inline|Tremadocian} to {Geologic Ages Inline|Floian} million years ago. The base of the Tremadocian is defined as the first appearance of the conodont species Iapetognathus fluctiva...