Brontotheres, or “thunder beasts” – an extinct lineage of herbivorous rhinoceros-like mammals (though more closely related to horses) – evolved their massive multi-ton body size as a way to survive in crowded and competitive ecological niches, according to a new study, revealing a macroevolutionary pathway to the early and rapid evolution of massive herbivorous mammals. After the Cretaceous-Tertiary extinction event, mammals were small and rarely exceeded 10 kilograms (kg) in mass. However, only 20 million years later, megaherbivores with bodies weighing more than a metric ton were abundant and profoundly influenced Cenozoic terrestrial landscapes. This rapid diversification of body size is often considered one of the most extraordinary feats in mammalian evolution. Although several hypotheses explaining this expansion have been proposed, the evolutionary processes underpinning the increase in mammal size diversity remain poorly understood. Oscar Sanisidro and colleagues test several competing theories by evaluating the well-known fossil record of the brontotheres, a group that underwent one of the most extreme size increases seen among mammals and one of the first mammal radiations that consistently evolved multi-ton sizes. While the first known brontotheres were only about 18 kg in size, most brontothere species during their time on Earth were estimated to have grown to be well over 1000 kg. Using phylogeny-based trait evolution modeling and diversification analysis, Sanisidro et al. discovered that brontothere body-mass evolution mainly occurred during speciation and found no evidence of directional selection or the gradual accumulation of microevolutionary changes, as predicted by the popular theory “Cope’s rule.” According to the authors, the findings suggest that smaller brotothere species who lived in highly competitive ecological settings experienced higher extinction risk, which drove biased diversification towards larger animal sizes, revealing a complex, long-term macroevolutionary pattern of increased survival of larger species due to reduced competition with other herbivores in crowded ecological niches. s
Science
A macroevolutionary pathway to megaherbivory
12-May-2023