THE TERTIARY PERIOD (ca. 66.4 to 1.8 million years ago [Ma]) was an interval of enormous geologic, climatic, océanographie, and biologic change. It spans the transition from a globally warm world of relatively high sea levels to a world of lower sea levels, polar glaciation, and sharply differentiated climate zones. Over the past decade, however, it has become increasingly clear that Tertiary climatic history was not a simple unidirectional cooling driven by a single cause but a much more complicated pattern of change controlled by a complex and dynamic linkage between changes in atmospheric CO2 levels and ocean circulation, both probably ultimately driven by tectonic evolution of ocean-continent geometry. Although satisfactory explanations for many [Page 957]aspects of Tertiary climate history are available, many areas remain ...