The Paleogene greenhouse world comprises variable paleoclimate conditions providing an indispensable deep-time perspective for the possible effects of human-induced climate change. In this paper, paleotemperature data of the early Eocene climatic optimum (EECO) from the mid-latitude marginal marine Belgian Basin are discussed. They are derived from fish otolith δ18O compositions of four non-migratory species belonging to the families Congridae and Ophidiidae. Otoliths from several levels and localities within the middle to late Ypresian were selected. After manual polishing, bulk and incremental microsamples were drilled and analyzed by a mass spectrometer. A cross-plot of bulk otolith δ18O vs. δ13C results shows a discrepancy between both families used. Ophidiid data probably represent true bottom water temperatures of the Belgian Basin. The mean annual temperature (MAT) of the EECO is calculated at 27.5
°C, which is in line with other proxy results. However, variations in MAT up to 6 °C occur, suggesting a pronounced expression of climate variability in mid-latitude marginal basins. Incremental analyses revealed a ~9.5 °C mean annual range of temperatures, similar to modern seasonality. These results show that marginal marine environments such as the Belgian Basin are well suited to infer high-resolution paleoclimate variability.