Abstract
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.
Highlights
Using phylogenies and fossils of Testudines, Crocodilia, and Lepidosauria, we find that the hump-shaped latitudinal diversity gradient (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
The evolutionary mechanisms required to explain the formation of the current steep LDG are radically different whether we consider or not that high diversity levels existed previously in the Northern Hemisphere: if the pattern transitioned from flatten to steep, as suggested by fossils, one hypothesis can argue for diversity loss in the Northern Hemisphere to explain the current LDG, via extinction or range contractions (Hawkins et al 2006)
Speciation, extinction, and dispersal shape the LDG, but the contribution of these processes remains a debated topic in evolutionary ecology. Our results indicate these processes operated at different rates over time and space as climate changed
Summary
Ancient tropical extinctions at high latitudes contributed to the latitudinal diversity gradient*. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. When and how diversity declined at high latitudes to generate the modern LDG remains an open question. 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. This study underscores that the prevailing evolutionary processes generating the LDG during greenhouses differed from those operating during icehouses
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