Changes in the morphology of Late Ordovician ostracods along the shelf-to-basin transect of the Baltic Palaeobasin

The present work analyses the numerical distribution data of ostracods in the Late Ordovician (Katian) deposits of Estonia for establishing trends in carapace morphology along a facies profile from a shallow shelf to a deep basin. We present a quantitative analysis of facies-related trends in carapace size, shape and ornamental features. In addition, changes in diversity as well as major changes in the taxonomic composition of the assemblages are addressed, based on the number of individuals. The quantitative analysis of ostracod distribution data reveals distinct trends in the offshore direction: carapaces become larger and shorter, the dominance of smooth valves decreases and the share of ornamented valves increases. Tuberculate valves are most common in open-shelf environments. Species diversity decreases in the offshore direction. The taxonomic composition changes from a clear majority of podocopes to an association of podocopes and palaeocopes, and further to a majority of binodicopes. The fact that these patterns are persistent throughout the Late Ordovician (within three chronostratigraphic units) suggests that they represent environmental control. Some of the results are not in full agreement with previous observations.