Ancient tropical extinctions at high latitudes contributed to the latitudinal diversity gradient

Global biodiversity currently peaks at the equator and decreases toward
the poles. Growing fossil evidence suggest this hump-shaped latitudinal
diversity gradient (LDG) has not been persistent through time, with
similar diversity across latitudes flattening out the LDG during past
greenhouse periods. However, when and how diversity declined at high
latitudes to generate the modern LDG remains an open question. Although
diversity-loss scenarios have been proposed, they remain mostly
undemonstrated. We outline the ‘asymmetric gradient of extinction and
dispersal’ framework that contextualizes previous ideas behind the LDG
under a time-variable scenario. Using phylogenies and fossils of
Testudines, Crocodilia and Lepidosauria, we find that the hump-shaped LDG
could be explained by (1) disproportionate extinctions of high-latitude
tropical-adapted clades when climate transitioned from greenhouse to
icehouse, and (2) equator-ward biotic dispersals tracking their climatic
preferences when tropical biomes became restricted to the equator.
Conversely, equivalent diversification rates across latitudes can account
for the formation of an ancient flat LDG. The inclusion of fossils in
macroevolutionary studies allows revealing time-dependent extinction rates
hardly detectable from phylogenies only. This study underscores that the
prevailing evolutionary processes generating the LDG during greenhouses
differed from those operating during icehouses.