Abstract : In the early 1960s, R.P.S. Jefferies conducted an investigation of the microfossils found in the Actinocamax plenus Marls of Southern England. This was the first investigation of its kind, using closely-spaced samples and providing a detailed analysis of a relatively short interval of geological time. The Plenus Marls Member, as it is now known, is the on-shore representation of Oceanic Anoxic Event ll (=Bonarelli Event or Cenomanian/Turonian Boundary Event [CTBE]). Jefferies’ detailed analysis of the changes over this major extinction event in Earth history has provided the foundation for much later research. Anoxic events – including the CTBE – characterise the mid-Late Cretaceous interval and all are associated with stable isotope excursions, geochemical changes and distinctive patterns of both extinction and appearance of various taxa. Some of these (e.g., calcareous dinoflagellates, radiolaria) are now regarded as ‘disaster taxa’ and their appearance, in variable numbers, from a range of locations can be used to inform judgements about the nature of these events.