Morphological and abundance variations in Homotryblium-cyst assemblages related to depositional environments¤ uppermost Oligocene-Lower Miocene, Jylland, Denmark

Abundant occurrences of Homotryblium dinoflagellate cysts have been interpreted as reflecting deposition in near-shore, marginal marine areas, either in hypersaline or low-salinity environments. It has also been suggested that the process length of Homotryblium cysts is related to proximity to the coast.

The present study provides new insights in the ecology of Homotryblium and adds important information about the use of the morphological variations of Homotryblium cysts for environmental reconstructions. It presents an example where Homotryblium cysts show high relative abundances in a low-salinity, partly restricted marine depositional environment. Four different species of Homotryblium were recorded. Homotryblium? additense is proposed as a new species. The regional palaeogeographic distribution of the four species: H.? additense, Homotryblium vallum, Homotryblium plectilum and Homotryblium tenuispinosum, shows a distinct depositional proximal–distal distribution pattern. H.? additense occurs only in a narrow stratigraphic interval in the most proximal part of the study area. H. vallum only occurs sporadically, mainly in the proximal parts of the study area. H. plectilum dominates the proximal areas while H. tenuispinosum dominates the more distal areas. Variations in abundance and cyst-type also seem to respond to systems tracts, sequence boundaries and flooding surfaces.

The observed distributional patterns of Homotryblium species strongly indicate a response to the salinity of the depositional environment. It is further suggested that at least some of the recorded species of Homotryblium originated from the same motile dinoflagellate species, producing cysts of different morphology in response to variations in salinity, a phenomenon known from extant dinoflagellates.