According to a study, the evolution of larger brontotheres was probably aided by differential species proliferation.

Working with a colleague from the New York Institute of Technology, two evolutionary ecologists from the Universidad de Alcalá report that the evolutionary increase in brontotheres’ size was likely caused by differential species proliferation. Oscar Sanisidro, Juan Cantalapiedra, and Matthew Mihlbachler studied the fossil record of the massive animals and developed computer simulations to demonstrate how they probably evolved in their study, which was published in the journal Science.

Edward Drinker Cope, a zoologist, devised a rule toward the end of the 1800s to explain how animals evolve to become larger, as larger species typically enjoy more advantages. That rule, which is now known as Cope’s rule, seems to apply to a lot of species. However, there are times when other factors may play a role that get around the rule and encourage growth for other reasons. In this new study, the researchers discovered that the evolution of brontotheres—a family of extinct mammals, at least some of which were known for their enormous size—may have been influenced by additional factors. What’s more, that distinction persuaded the analysts that Adapt’s standard didn’t have any significant bearing.

Dinosaurs were the only large land-dwelling animals before the Chicxulub impact. After their destruction, warm-blooded creatures started filling their niche, with brontotheres leading the way. Some evolved from small dogs to rhinoceros-like monsters that weighed more than a metric ton in just 16 million years. Sanisidro, Cantalapiedra, and Mihlbachler wanted to know more about the things that helped them grow so quickly in this new endeavor.

Based on their analysis of 276 brontothere fossils, the researchers developed simulations. They assumed that one of three scenarios governed brontothereal growth: Cope’s rule, adaptations to shifting adaptive zones, or speciational evolution occurring without preferential direction as the third possibility.

The researchers came to the conclusion that the third option was the most likely one after analyzing their models and data. They provide evidence that, over time, some species of brontotheres decreased in size, remained roughly the same, or increased in size. Up until the end of the Eocene epoch, when they went extinct, most likely due to a shift in the climate, the larger species enjoyed greater success and continued to grow in size.

More information: Oscar Sanisidro et al, A macroevolutionary pathway to megaherbivory, Science (2023). DOI: 10.1126/science.ade1833

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