AbstractThe diversity of several groups of Late Ordovician fossils in the Baltic is discussed and corresponding curves are presented. The curves are correlated with the latest δ13C trend and probable sea surface temperatures of the equatorial ocean in order to link biodiversity and environmental changes. Most groups of macrofossils and acritarchs showed a high‐level biodiversity in the Sandbian and early Katian during a sea level high stand and optimal temperatures. Conodont diversity peaked in the mid‐Katian, scolecodonts in the late Katian and rugose corals even in the Hirnantian, indicating their different ecological demands and the evolutionary status of each group. The diversity peak close to the Guttenberg δ13C excursion marks a turning point in environmental evolution when a new global cooling began but tropical conditions persisted in Baltica due to its northward drift. Biodiversity fluctuated during the Katian. Two diversity peaks noted in the Katian are followed by low‐diversity episodes showing a general step by step decrease in diversity, and a correlation with variable sea level and episodes of climatic cooling. It suggests that the Hirnantian mass extinction was preceded by a rather prolonged period of biodiversity change here called the Katian prelude. Katian biodiversity curves and their links with different facies parameters show that environmental conditions within the ecosystem were the main drivers of change, not solely temperature or more generally climate, but also sedimentary environments including water dynamics together with depth and substrate conditions. The biotic component is less well known, but also important. Copyright © 2011 John Wiley & Sons, Ltd.