Although belemnites form a major clade of extinct cephalopods, the early stage of their diversification remains poorly known in time and space. Here we investigate the first diversification episodes of belemnites (order Belemnitida) using a new species-level database encompassing the Hettangian-Aalenian interval (Early Jurassic-earliest Middle Jurassic) and covering the Western Tethys. Rarefied richness shows a four-fold increase from the Planorbis chronozone to the Ibex chronozone, a strong decrease between the Margaritatus and Spinatum chronozones, followed...