Contributions Series Nr. 7

Description

Abstract

The life cycle of some extant dinoflagellates includes two cyst stages, the resting cyst and the hypnozygote. The widely held belief that all fossil dinoflagellates are resting cysts is now being challenged by the hypothesis that some or all may be hypnozygotes, so that only dinoflagellates capable of sexual reproduction will have been preserved in the fossil record. What bearing does this have on our knowledge of species occurrence? Studies of distribution patterns in modern dinoflagellates are based primarily on the vegetative or motile stage; these reveal distinctive patterns which can vary monthly, seasonally or annually. The few surveys of modern cyst distribution patterns suggest that cysts, whether resting or hypnozygotes, may be more predictable in species diversity and concentrations, with climate, major current distribution and water depth being the major controlling factors. Thus predictions on ecological niches of certain cyst taxa can be made with some degree of accuracy in modern assemblages.

Detailed studies of fossil assemblages are also revealing distinctive distribution patterns, as in the Late Cretaceous peridiniaceans. The peridiniacean dinoflagellates are those taxa with a tabulation pattern similar to Peridinium, that is four apicals, three anterior intercalaries, seven precingulars, five postcingulars and two antapicals. In the Campanian three diagnostic peridiniacean assemblages, each with its own characteristic species, seem to reflect regional differentiation rather than local paleoecological control. These assemblages have been respectively named the Malloy suite, the McIntyre suite and the Williams suite. The Malloy suite, characterised by species of Andalusiella, Ceratiopsis, Lejeunia and Senegalinium, is restricted to tropical to subtropical paleolatitudes. The Williams suite, predominated by species of Alterbia, Chatangiella, Isabelidinium, Spinidinium and Trithyrodinium, appears to be a warm temperate assemblage. The McIntyre suite with species of Chatangiella and Laciniadinium being the dominant peridiniacean dinoflagellates, is interpreted as a boreal assemblage. Cataloguing of other fossil dinoflagellate assemblages will hopefully confirm the existence of similar distribution patterns both in the peridiniaceans and other groups.