published 8 June 2022
Dinoflagellates (or dinophytes) are protists in which the motile (swimming) life-cycle stage is characterized by two distinctive flagella: a longitudinal flagellum that trails posteriorly from the cell and a transverse flagellum that encircles the cell, generally in a more or less equatorial position1,2. The combined action of the flagella propels the organism forward in a corkscrew motion (hence the Greek dinos ―”whirling” and the Latin flagellum ―”whip”). Dinoflagellates also have other distinctive cell characteristics3, including permanently condensed chromosomes and the presence of vesicles in the outer part (cortex) of the cell4, known as amphiesmal vesicles or alveolae, a feature that affiliates dinoflagellates to the alveolates, a major protist group that also includes ciliates5. In many motile forms, amphiesmal vesicles are filled with cellulosic, or thecal, plates whose arrangement (tabulation) is critically important taxonomically1,6.
Dinoflagellates come in many shapes and have varied life cycles, which may even include parasitic and multicellular stages. During the sexual phase of their life cycle, about 13–16 % of known modern motile species produce resting cysts (dinocysts)7, a dormancy stage capable of being preserved in the sedimentary record. Dinoflagellate cysts may be organic-, calcareous- or, rarely, siliceous-walled. Organic-walled dinoflagellate cysts, which are found as palynomorphs, are composed of a resistant biomacromolecule termed dinosporin1.
Evolution & Morphology
The earliest confirmed dinocysts appear in the fossil record in the Middle Triassic and show a classic radiation of morphologies during the Late Triassic through the Middle Jurassic8. More modern-looking forms proliferate during the later Jurassic and Cretaceous. This trend continues into the Cenozoic, albeit with some loss of diversity9. The rapid evolution and morphological variation, especially from the Middle Jurassic on, make them ideal index fossils for biostratigraphy10–12. Dinocysts may be recognized on the basis of criteria such as overall shape and reflection of motile cell features in the cyst morphology, such as tabulation and the presence of a cingulum and sulcus (which accommodate the transverse and longitudinal flagella respectively in the motile cell)13. Dinocyst taxa are differentiated most commonly on the basis of tabulation, position of excystment opening (archeopyle), and ornamentation. In many cases the ornamentation and archeopyle partially reflect the tabulation6. Major groups in the fossil record include representatives of the families Gonyaulacaeae, Peridiniaceae, and Ceratiaceae. The former two groups are characterized by tabulation patterns that have been distinct from one another but remarkably conservative within each group from the Jurassic to the present day; the Ceratiaceae, although having a variant of gonyaulacacean tabulation, are distinctive in shape with three to five projections, or horns, emanating from a flat central body. The ceratiaceans appeared in the Late Jurassic and are prominent in the Cretaceous record, but are absent (or almost so) from the fossil record through the Cenozoic despite having many species in modern aquatic environments.
Although modern dinoflagellates dwell in various aquatic environments, the vast majority of fossil dinocyst species are found in marine sedimentary rocks. However, different taxa tend to live in different marine environments: for example species of Impagidinium generally occur in open-ocean settings well away from shore, whereas the presence of Nyktericysta denotes marginal marine (and possibly freshwater) settings14. Dinocysts can also be used in conjunction with other palynomorphs, for example terrestrially derived spores and pollen, to assess paleoenvironments. Dinocysts are especially useful as sensitive environmental indicators in modern freshwater15,16, estuarine17,18 and marine settings19–28, providing information on sea surface conditions such as temperature, salinity, productivity, upwelling and sea ice cover20,29,30.
Fossil dinoflagellate nomenclature is governed by the International Code of Nomenclature for Algae, Fungi, and Plants31,32, which allows for a dual nomenclature of taxa based on living and fossil forms. While keeping living and fossil nomenclature separate at genus and species level is the most practical approach33, especially from the paleontological perspective, classification of dinoflagellates at suprageneric level is now well integrated1, and advances in molecular tools in combination with the fossil record is helping to reveal phylogenies33–35.
- Lentin and Williams Index of fossil dinoflagellates