A macroevolutionary pathway to megaherbivory

Several scenarios have been proposed to explain rapid net size increases in some early Cenozoic mammalian lineages: sustained and gradual directional change, successive occupation of adaptive zones associated with progressively larger body sizes, and nondirectional evolution associated with branching events in combination with higher diversification potential of the larger lineages. We test these hypotheses in brontotheres, which are among the first radiations of mammals that consistently evolved multitonne sizes. Body-mass evolution in brontotheres mainly occurred during speciation and had no preferential direction. Long-term directional change stemmed from the higher survival of larger lineages in less-saturated herbivore guilds. Our study emphasizes the role of differential species proliferation in explaining the long-term phenotypic trends observed in the fossil record, which are more than an accumulation of steady microevolutionary changes.

Automated summary

Several hypotheses have been proposed to explain the rapid increase in size in early Cenozoic mammalian lineages, including sustained and gradual directional change, successive occupation of adaptive zones associated with larger body sizes, and non-directional evolution associated with branching events and higher diversification potential. This study examines these hypotheses in brontotheres, one of the first mammalian radiations to consistently evolve large sizes. The results show that body mass evolution in brontotheres primarily occurred during speciation and did not exhibit a preferred direction. Long-term directional change was a result of the higher survival rate of larger lineages in less competitive herbivore communities. This study highlights the importance of differential species proliferation in explaining long-term phenotypic trends observed in the fossil record.