in Sanisidro, Mihlbachler & Cantalapiedra, 2023.
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.
After the extinction of large dinosaurs at the end of the Cretaceous, many mammals underwent a rapid increase in size. Several hypotheses for this change have been put forward, with much debate about the drivers. Sanisidro et al. looked at the record of body size in brontotheres, large herbivorous mammals from the Eocene that experienced orders of magnitude changes in size. The authors saw no evidence for directional selection, but instead found a pattern of increased survival of larger species caused by reduced competition with other herbivores in their niches. Thus, over time, the increase in body size in these mammals was caused by the macroevolutionary process of species sorting. —Sacha Vignieri
Oscar Sanisidro, Matthew C Mihlbachler and Juan L Cantalapiedra. 2023. A Macroevolutionary Pathway to Megaherbivory. SCIENCE. 380, 6645; 616-618. DOI: 10.1126/science.ade1833