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Stage-level analysis of extinctions and originations of bryozoan genera of the orders Cyclostomata and Cheilostomata for the Upper Cretaceous and Cenozoic yields some predictable and some unexpected results. Extinction rates in cyclostomes and cheilostomes track one another except for some lower Upper Cretaceous stages in which apparently random extinctions among the small numbers of cheilostome genera generated ‘noise’. Both orders experienced high rates of extinction during or at the end of the Maastrichtian (our data cannot resolve the position of extinctions within a stage) but, surprisingly, Danian extinction rates were essentially equal to Maastrichtian rates. High extinction rates for the Danian are attributed to loss of the ‘chalk’ sea of northern Europe, which was a centre of bryozoan diversity from early in the Late Cretaceous until its disappearance at the end of the Danian. Origination rates of cyclostomes and cheilostomes were similar and relatively high during the Late Cretaceous, but following the K-T extinction event, cyclostome origination rates dropped and remained at low levels through the Cenozoic, while cheilostome origination rates rebounded by the Eocene and then declined through the Pleistocene. The different Cenozoic diversity trajectories of cyclostomes and cheilostomes appear to derive primarily from differences in origination rather than extinction rates.

Frank K. McKinney. Department of Geology, Appalachian State University, Boone, North Carolina 28608, USA. Honorary Research Fellow, Department of Palaeontology, The Natural History Museum, Cromwell Road, London, UK
Paul D. Taylor. Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.

KEY WORDS: extinctions, originations, Cretaceous-Tertiary, K-T, benthic invertebrates