Ammonites and the mid-Cretaceous saga

Ever since the introduction of the Cretaceous System, nearly two centuries ago, the terms ‘mid-Cretaceous’ and ‘middle Cretaceous’ keep appearing in the literature, implying a three-fold division rather than the more generally accepted two-fold division of the system/period. The frequent and persistent use of these informal terms proves that they fulfil a need in stratigraphic communication; consequently, formalisation of a middle series/epoch...