After the extinction of non-avian dinosaurs at the end of the Cretaceous period, many mammals underwent a rapid increase in size. Several hypotheses for this change have been put forward, with much debate about the drivers. In new research, a team of scientists in Spain looked at the record of body size in brontotheres (thunder beasts), an extinct lineage of large herbivorous rhinoceros-like mammals (though more closely related to horses) from the Eocene epoch that experienced orders of magnitude changes in size.
Life restoration of a North American brontothere species called Telmatherium and other animals of the Eocene Clarno Nut Beds, Oregon, the United States. Image credit: Larry Felder, National Park Service.
After the end-Cretaceous mass extinction, a chain of events that led to the extinction of non-avian dinosaurs and 75% of life on our planet 66 million years ago, mammals were small and rarely exceeded 10 kg in mass.
However, only 20 million years later, megaherbivores with bodies weighing more than a ton were abundant and profoundly influenced 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.
In the new study, Universidad de Alcalá paleontologist Oscar Sanisidro and his colleagues tested 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 1,000 kg.
Using phylogeny-based trait evolution modeling and diversification analysis, the study authors discovered that brontothere body-mass evolution mainly occurred during speciation.
They also found no evidence of directional selection or the gradual accumulation of microevolutionary changes, as predicted by the popular Cope’s rule.
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.
“Body-mass evolution in brontotheres mainly occurred during speciation and had no preferential direction,” the researchers said.
“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.”
The results appear this week in the journal Science.
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Oscar Sanisidro et al. 2023. A macroevolutionary pathway to megaherbivory. Science 380 (6645): 616-618; doi: 10.1126/science.ade1833
