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

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.

Global drivers of obligate mycorrhizal symbionts diversification

“…Indeed, in the last 100 Myr, many plant lineages evolved root symbioses with other fungal or bacterial groups or became non-symbiotic 30,31,44 , to the point that approximately 20% of the extant plants are not interacting with AMF anymore 3 . Additionally, the cooling of the Earth during the Cenozoic resulted in a reduction of the surface of tropical regions 45,46 , which are a reservoir of ecological niches for AMF 18,30,47 . The difficulty of reconstructing past symbiotic associations prevents direct testing of the hypothesis that the emergence of new root symbioses in plants led to a diversification slowdown in AMF.…”

Paleoclimate and current climate collectively shape the phylogenetic and functional diversity of trees worldwide

“…Moreover, the relatively stable environment, compared to high latitudes, may also resulted in low extinction rates, making the tropics both “cradles (species diversifying)” and “museums (species persistence)” of species diversity 49,66 . In addition, long speciation history and lower extinction rates in the tropics could result in both higher phylogenetic diversity and functional diversity 38,48,49,51, but see 35 .…”

Digest: The contribution of historical climate events in shaping the modern latitudinal diversity gradient of ancient reptiles

“…Meseguer and Condamine (2020), therefore, present a time‐variable framework using fossil and phylogenetic analyses to investigate the two proposed hypotheses for the formation of the LDG. They conducted studies across the temperate and tropical biomes, focusing on ancient species of Crocodilia (crocodiles), Lepidosauria (lepidosaurs), and Testudines (turtles).…”

“…Simplified model of processes driving LDG shape. Illustration of the time‐variable framework of Holarctic diversity loss proposed by Meseguer and Condamine (2020). Climate states “icehouse” and “greenhouse” are drivers of evolutionary processes in ancient species, including dispersal, speciation, and extinction.…”

“…This research by Meseguer and Condamine (2020) presses the scientific community to consider a new hypothesis, the “asymmetric gradient of extinction and dispersal” (AGED), which allowed for the formation of a dramatic LDG. The hypothesis suggests diversification and dispersion are temporally and spatially asymmetric, with cooling events decreasing diversity at high latitudes and allowing dispersion toward the equator.…”