Abstract
Radiometric data and phylogenetic analysis of Cretaceous planktonic foraminifera suggest that the Cretaceous be divided into three epochs rather than two. Primitive globigerines occur in the Lower Cretaceous, ingle-keeled Rotalipora faunas characterize the Middle Cretaceous, and single-keeled and double-keeled Globotruncanafaunas typifv the Upper Cretaceous. Keels arose in three independent generic lines of development in the Upper Cretaceous. Long-ranging tropical species are restricted to very short stratigraphic ranges toward the tropical boundaries defined by Globotruncana-Rotalipora lines. Cretaceous planktonic foraminiferal zonation
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