Lower speciation, not higher extinction, drove African megaherbivore diversity collapse

Today's ecosystems are severely depleted in megaherbivores (≥ 1000 kg) and lack many of the ecological functions once provided by these landscape engineers. Whether attributed to humans or climate, prevailing views of megaherbivore decline have focused on size-biased extinction (i.e., higher extinction rates among the largest species) while neglecting the role of speciation rates. To fully contextualize the decline of African megaherbivores, we examine 23 million years (Myrs) of African ungulate diversification using neural network models. Starting around 7.2 million years ago (Ma), African ungulate faunas witnessed overall accelerations in speciation and extinction rates. While speciation plateaued after 3.6 Ma, extinction rates increased sharply at the onset of the Pleistocene (2.58 Ma), leading to widespread diversity losses across herbivore guilds. Yet, we find that extinction rates are intrinsically lower in large-size lineages and in larger species within families. High-crowned dentitions, which last longer under hard, abrasive diets, are also associated with a suppression of extinction rates. Importantly, speciation rates in the largest herbivores are intrinsically low, and in some lineages were suppressed in response to increasing aridification during the last 5 Myrs. Our findings highlight the macroevolutionary complexity behind megaherbivore decline, advancing understanding of biotic crises involving large-bodied taxa.