Dinoflagellates are a group of chiefly planktonic one-celled organisms abundant in modern seas and lakes. Their fossil organic shells range from 15 to 150 microns or more and are commonly found in Jurassic and younger marine sediments by the same techniques as those currently more widely used for spores and pollen. Commonly they occur in the same samples. Being surface-dwellers, dinoflagellates are relatively independent of the type of bottom sediment, although most abundantly found in marine shales; fresh-water fossil types are very rare. Rapid evolutionary changes, combined with wide geographic distribution of many species, make them excellent fossils for zonation and correlation. This is exemplified by successions of distinctive assemblages in the late Mesozoic and Cen zoic of Australia, by two assemblages containing many identical species in the Upper Jurassic of Utah and France, and by the worldwide distribution of a particularly distinctive form in the Upper Cretaceous. The value of dinoflagellates for environmental interpretation is as yet largely unexplored.

Two major types of dinoflagellate fossils occur. One is the resistant shell, or theca, of the free-swimming organisms. This is commonly divided into polygonal plates and may contain a thick-walled and much ornamented protective structure, the cyst. The second type consists of isolated cysts, freed of their surrounding thecae. The latter type includes many of the minute spiny objects that have been called hystrichospheres. In fact, the majority of (but by no means all) post-Paleozoic hystrichospheres appear to be dinoflagellate cysts. Important criteria for distinguishing dinoflagellate genera and species include: the over-all shape, the number and arrangement of plates or of spine-like projections, the type of cyst, and the character of a special opening, the archeopyle, by which the rotoplasm left the theca or cyst.

The literature on fossil dinoflagellates and the number of described genera and species are still small, but now explosively expanding as interest in the group increases. Although fossil dinoflagellates are already useful tools of the applied paleontologist, our understanding of them and the full development of their potentialities for applied paleontology are in early stages.

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