Fossil dinoflagellates, the most important marine component of many Mesozoic-Tertiary palynological preparations, are remains of chiefly planktonic organisms. Commonly abundant and well preserved, they can be highly useful in problems of local, regional, and interregional dating and correlation. They have been used less than their potential warrants, at least partly because the unalerted eye, bewildered by the variety of their more spectacular structures, readily overlooks others of greater diagnostic value. This paper draws attention to one morphological feature of fossil dinoflagellates, the archeopyle, a distinctive opening in the test which is usually easy to see and is a significant aid in distinguishing among stratigraphically important genera and species.

The archeopyle is formed through release of an operculum along the primary archeopyle suture. The operculum is simple when it consists of a single piece and compound when it is divided into two or more parts by secondary archeopyle sutures; it is free when the primary archeopyle suture completely surrounds it and attached when that suture does not close on itself. Archeopyle shape and position relate to the basic pattern of plate arrangement, or tabulation, which is one of the striking features of dinoflagellates. Accordingly, an archeopyle may be apical, intercalary, precingular, or epithecal, depending upon the part of the test involved in its formation. In all, about 10 distinctive archeopyle types have been recognized. Some fossil dinoflagellates lack an archeopyle entirely and a ew have openings of combined types or of types that do not fit readily into a simple classification.

Careful observation of the archeopyle is prerequisite for precise and consistent identification of fossil dinoflagellates. Besides being a character of taxonomic value in its own right, the archeopyle often helps when attempting to determine specimen orientation, girdle and sulcus location, and tabulation. It is especially useful in studying those dinoflagellates with spherical bodies, long processes, and obscure tabulation, called hystrichospheres.

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