The Rapoport effect and the climatic variability hypothesis in Early Jurassic ammonites

The increase in species range size towards high latitudes, known as Rapoport's rule, remains one of the most debated and poorly understood macroecological patterns. Numerous studies have challenged both its universality and the main mechanism originally proposed to explain it: the climatic variability hypothesis. Here, we study this pattern using a group of fossil marine organisms: the early Pliensbachian ammonites of the western Tethys. We further take into account the influence of the marked provincialism prevailing at that time, with a Mediterranean province (MED) and a north-west European province (NWE) located on each side of a latitudinally oriented palaeobiogeographical barrier. We find that only species from the NWE province display a Rapoport effect, whereas species from the more tropical MED province show a boundary effect and have larger range sizes on average. This dual pattern can be explained by an alternative climatic variability hypothesis that better captures latitudinal seasonal variations and outlines the influence of the intertropical zone, characterized by stable and homogeneous climate that allows species to disperse over very large areas, regardless of their thermal tolerance. Accordingly, the NWE province probably displayed a gradient of seasonal climatic variations which caused the emergence of a Rapoport effect, whereas the MED province was probably located in the intertropical zone where no gradient in species range size is expected. Our multiscale approach further shows that the Rapoport effect is scale-dependent and may be labile through time. This probably explains the conflicting results of previous studies carried out at various spatiotemporal scales.