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

The increase of species range size towards high latitudes, known as the
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,
i.e. the climatic variability hypothesis. Here we study this pattern on 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 Northwest European province (NWE) located on each side of a
latitudinally-oriented palaeobiogeographic 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 in 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 multi-scale 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
spatio-temporal scales.