Dinoflagellates are protists that live primarily in the marine plankton. They are characterized by a distinctive nucleus, a duo of flagella and, commonly, by a cell wall involving an "armour" of cellulosic plates. The arrangement of these plates is fundamental to the classification of the group. Many dinoflagellates produce fossilizable cysts (dinocysts) as part of their life cycle, and organic-walled dinocysts are found in the thousands in many palynological preparations. Dinocysts have been used since the 1960s as biostratigraphic index fossils, but recent studies have shown that fossil dinocysts have interest and significance beyond this traditional application. As representatives of past microplankton, study of dinocyst evolutionary patterns may reflect aspects of ecosystem evolution. For example, the radiation of modern dinoflagellates in the early Mesozoic probably relates to the global recovery from the devastating end-Permian extinction event. And their apparent diversity decline in the later Cenozoic may reflect global cooling and/or lowering of sea level. As with other fossil groups, the dinocyst fossil record is also our only evidence for some now-extinct taxa, such as the Jurassic genus Nannoceratopsis, which appears to be a missing link between two modern subclasses. The fossil record can uniquely reveal relationships that are testable by molecular phylogenetics, the two disciplines thus jointly contributing to a more meaningful phylogeny of the group. An aspect of the is the calibration of molecular phylogenies by known fossil events to produce "molecular clocks".

Fossil dinocysts are also providing new insights into paleoenvironmental changes, such as those involving paleoceanic circulation patterns and climatic regimes. The second author, with Brinkhuis and others, has documented major changes in Paleogene assemblages from ODP Leg 189, off Tasmania.. The changes are directly related to the opening of the Tasman Gateway at the end of the Eocene, triggering the birth of the Antarctic Circumpolar Current and the change from a "hothouse" to an "icehouse" world. Huge insights from little bugs!