Ups and downs of belemnite diversity in the Early Jurassic of Western Tethys

Abstract

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 by a drop in the Spinatum–Serpentinum interval that is coeval with the second-order Toarcian biological crisis. The Bifrons chronozone records a high richness that departs significantly from those of the surrounding chronozones. A last richness peak in the Dispansum chronozone precedes a decrease towards the Aalenian. Biogeographical analyses do not reveal any clear large-scale provincialism for belemnites, in sharp contrast with ammonoids. Such a long-term homogeneous spatial distribution of belemnites is probably due to: (1) the relatively poorly-documented fossil record of belemnites, especially in Mediterranean localities; and (2) contrasted dispersal abilities of belemnites compared to ammonoids over the studied time interval.