Gradual morphological changes in some dinoflagellate cysts from the eocene (lower tertiary) of the north sea basin

Abstract

Two examples of morphological change in fossil dinoflagellate cysts through successive strata are described and discussed. Firstly, the morphological trend among specimens from the lower part of the stratigraphic range of Eatonicysta ursulae is examined. The oldest known specimens are atypical; however, gradual morphological changes over a relatively short period of time led to typical morphologies.

Secondly, a transitional morphological series from Rottnestia borussica to Hystrichosphaeropsis sp. is described. The morphological changes appear to coincide with environmental changes, such that the observed morphological trends may represent phe‐notypic adaptations within single species. In addition, the transformation between Rottnestia and Hystrichosphaeropsis seems to have occurred in both directions, and may have occurred twice in geological history.

Conversely, changes in cyst shape may represent speciation events. Such may be the case with Eatonicysta ursulae, where the observed morphological trend was both unidirectional and unique.