Abstract
Biodiversity currently peaks at the equator, decreasing towards the poles. Growing fossil evidence suggest that this hump-shaped latitudinal diversity gradient (LDG) has not been persistent through time, with similar species diversity across latitudes flattening out the LDG during past “greenhouse” periods. This provides a new starting point for LDG research. Most studies assume the processes shaping the LDG have acted constantly through time and seek to understand why diversity accumulated in the Holarctic at lower levels than at the equator, e.g. as the result of limited dispersal, or higher turnover in Holarctic regions. However, fossil investigations suggest that we need to explain when and why diversity was lost at high latitudes to generate the LDG. Unfortunately, diversity lost scenarios in the Holarctic have been repeatedly proposed but not yet clearly demonstrated. Here, we outline the ‘asymmetric gradient of extinction’ (AGE) framework, which contextualize previous ideas behind the LDG in the frame of a time-variable scenario. We suggest the current steep LDG may be explained by the extinction of clades adapted to warmer conditions from the new temperate regions formed in the Neogene, together with the equator-ward dispersal of organisms tracking their own climatic preferences, when tropical biomes became restricted to the equator. Conversely, high rates of speciation and pole-ward dispersal can account for the formation of an ancient flat LDG during the Cretaceous–Paleogene greenhouse period. Phylogenies and fossils of the Testudines, Crocodilia and Lepidosauria support the AGE scenario and showed the LDG to have varied over time, with high latitudes serving as a source of tropical diversity but suffering disproportionate extinction during transitional periods to cold climate. Our results demonstrate that the inclusion of fossils in macroevolutionary studies allows detecting extinction events less detectable in analyses restricted to present-day data only.
Footnotes
Data accessibility statement All the data used in this manuscript are presented in the manuscript and its supplementary material or have been published or archived elsewhere.