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.
Acknowledgements
Discussions and time at the microscope with Bill Evitt and Lew Stover during the course of many years contributed significantly to the interpretations in this paper. Robin Helby photographed the specimens of Microdinium ornatum, and provided the photomicrographs included on .
Disclosure statement
No potential conflict of interest was reported by the author.
PLATES
Illustrated specimens were collected from several localities. Type material of Microdinium ornatum previously illustrated by Cookson & Eisenack (Citation1960, Citation1971) and shown in herein are stored in the Museum of Natural History, Victoria, Australia (, figures 1–5) and in the Palynological Collection of the Geological Survey of Western Australia (, figures 6–12). Figured specimens of Histiocysta and Cladopyxidium were collected from six localities: (1) along the Albert Canal, and (2) in the Curfs Quarry and (3) ENCI Quarry, the Netherlands; (4) in a quarry near Boom, Belgium; (5) in outcrops along the Potomac River at Popes Creek, Maryland (described in Goodman Citation1979); and (6) in outcrops at Atlantic Highlands, New Jersey (described in May Citation1980).
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David K. Goodman
DAVID GOODMAN is president of the Irf Group, Inc., an international stratigraphic consultancy based in Anchorage, Alaska, that he established in 1999 (Gordon Wood was a senior scientist in the group from its start). Dave earned his BS and MS degrees in geology from Virginia Polytechnic Institute and State University, and his PhD from Stanford University working under Bill Evitt. His primary research interests are Upper Cretaceous and Paleogene dinoflagellates and their application to sequence biostratigraphy. He was a stratigrapher at Exxon Production Research Company in Houston from 1978 to 1983, and at several Arco affiliates in Dallas, Midland (Texas) and Anchorage from 1983 until he retired from Arco in 1999. Dave was a long-time Managing Editor for The Palynological Society, and editor of the journal Palynology from 1987 to 2000. He lives in Alaska and fishes throughout North America. On a clear day from his office (750 ft elevation) he can see 140 miles north to Denali (20,310 ft), and 130 miles southwest to the Iliamna Volcano (10,016 ft)…when not asleep at the scope, tying steelhead flies, or playing with sharp knives.