INTRODUCTION

The Cephalopoda are an exclusively marine class of the Phylum Mollusca with a history extending back to Cambrian times. Their fossil record is remarkable for both its abundance and the diversity of shell form displayed. Thus the cephalopods can be divided into three informal groups, the nautiloids, ammonoids and coleoids. These were formerly regarded as three orders or subclasses, but the nautiloids include a very diverse array of forms that nowadays are divided into several orders. The nautiloid cephalopods reached their peak of diversity during Early Palaeozoic times and by the beginning of the Mesozoic most groups had long become extinct (Flower, 1988, fig. 1). Conversely, the Ammonoidea were a Late Palaeozoic to Mesozoic group while the Coleoidea are predominantly a Mesozoic to Recent subclass.

This chapter firstly reviews briefly the main changes in cephalopod distribution over the last 145 million years, in particular showing their varying response to the K/T boundary event. It goes on to concentrate on the Cretaceous interval (145–65 Ma), when two cephalopod groups, the ammonites and the belemnites, were among the most common marine macrofossils. Both groups appear to have been very sensitive to several parameters of global change. Thus, interpreting their ecology and determining the changing patterns in their distribution through space and time provides fundamental clues to the nature of Cretaceous palaeogeography, climate and oceanography. The influence of sea-level change and ocean currents is highlighted.