The Ordovician world was long considered to be in a super-greenhouse state during almost its entire duration, marked only by intense glaciations at the end of the period. This end-Ordovician Hirnantian glaciation is famous for causing one of the three great Phanerozoic mass extinction events. However, emerging evidence now suggests that cooling towards the Hirnantian glacial maximum, and thus the onset of the Early Palaeozoic Ice Age (EPI) started a lot earlier than previously assumed, during the Early to Mid Ordovician. This suggested timing has fundamental implications for our understanding of what caused the ice age as well what its impact may have been. An Early-Mid Ordovician cooling would be concomitant with the onset of major changes in global biodiversity during the Great Ordovician Biodiversification Event (GOBE). Marine biodiversity then reached levels approaching the modern ones, for the first time in the history of the planet. However, none of the evidence for an early onset of cooling is entirely conclusive, while views of a perennial Early and Middle Ordovician greenhouse world continue to exist. In this project, we aim to better understand how the climate changed through the Early- to Mid-Ordovician, and to establish if any links exist between cooling and biodiversity pulses. To this purpose, graptolite and chitinozoan zooplankton maps will be reconstructed and used to ground-truth General Climate Models.