ABSTRACT

Several fossil dinoflagellates assignable to Microdinium, Histiocysta and Cladopyxidium are small, yet distinctive subspheroidal to ellipsoidal forms that share several discriminative morphological features. The epicyst is smaller than the hypocyst, and the cingulum, which is only slightly offset ventrally, is exceptionally wide. The specimens all have apical archeopyles and possess sutural features indicating a gonyaulacalean tabulation of 4′, 3–5a, 7″, 6c, 6‴, 1p, 1″″, and 5–6s. However, the proportions and shapes of some plates and their relative positions differ significantly from plate relationships on other gonyaulacalean genera having similar tabulation formulas. The differences pertain mainly to the hypocyst, but on some forms modification of certain plates on the epicyst is also involved. Point-by-point comparison of comparable features emphasises the morphological similarities within this group, accentuates dissimilarities between them, and highlights the major differences between them and other fossil cysts having a similar tabulation formula. Extant cysts with a comparable tabulation are unknown.

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