Dinoflagellates, single-celled protists at or near the base of the food chain, showed remarkable patterns in species diversity in the Mesozoicndash;Cenozoic. First appearing in the mid Triassic, they rapidly increased to a Jurassic maximum of 420 species in the Kimmeridgian. After a minor decline, diversity rose to an all-time peak of 584 in the Albian, followed by a significant drop to 315 in the Coniacian: this may in part reflect the short duration of that stage. Diversity then recovered to a second Cretaceous peak of 568 in the Maastrichtian, followed by a major decline in the Early Paleocene (361 species), although relatively few of these species appear to have become extinct at the Cretaceousndash;Tertiary boundary. Other than a major peak of 518 in the Early Eocene, there is a continuing decrease through the Tertiary. Our plots are based on over 30ensp;000 records of age ranges from a major database. Although previous studies show similar plots, these were based on manually compiled records of substantially fewer species. Nevertheless, the resemblance between our plots and earlier ones is striking, supporting the premise that the patterns reflect natural events and processes rather than artifacts of literature or methodology. The present species diversity plot shows some similarity to the long-term sea-level curve, which may reflect the preference of dinoflagellates for shelf environments.Keywords: dinoflagellates, diversity, plankton, extinctions, fossils, paleoecology.
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