Evolutionary time but not speciation rates explain the latitudinal diversity gradient in sharks and rays

Unraveling the mechanisms behind the latitudinal diversity gradient (LDG) remains a key challenge in ecology and evolution, yet it has been little explored in highly threatened rays and sharks. Using structural equation models and a time-calibrated phylogeny of elasmobranchs, we investigated the direct effects of current and past environmental conditions on their species richness, as well as the indirect effects mediated through evolutionary time and recent speciation rates. We complemented these analyses with macroevolutionary models, with a particular emphasis on the two main orders of rays. Speciation rates showed an inverse latitudinal gradient in Rajiformes, consistent with patterns in ray-finned fishes, whereas Myliobatiformes and sharks showed no such trend. Consequently, the link between recent speciation rates and species richness was weak, except in coastal Rajiformes, where climatically unstable high-latitude regions acted as cradles of diversity due to frequent turnover caused by limited recolonization after extinction events. Overall, evolutionary time, colonization history and past climatic stability emerged as the main drivers of the LDG in elasmobranchs, together with thermal constraints. This highlights the importance, for future studies predicting the impacts of climate change on elasmobranchs, of considering not only current temperature conditions but also the contrasting evolutionary histories among them.