The Brontotheres (Brontotheriidae, greek ‘thunder beast’) are a family of mammals related to horses, rhinoceros and tapirs that underwent an incredible change in body size throughout the course of their evolution.  The first Brontotheres appeared in both Asia and North America at the start of the Eocene, 53 million years ago. The oldest species were the size of a coyote and didn’t have horns. However, the latest representatives of this lineage looked similar to a rhinoceros and had imposing bony appendages above the nose. Their bodies became more robust, nearly reaching the size of an elephant, which made them one of the first mammals to weigh several tonnes.  Surprisingly, despite all of these evolutionary transformations, they only lasted some odd 16 million years.

According to the research team, “we can can learn a lot about a species from its size. Body size correlates with metabolism, energy expenditure, thermoregulation and the area of its territory, which makes it one of the most important parameters of species biology.   The evolution of body size caught the interest of the very first palaeontologists and the Brontotheres was the centre of the first debates on the theory of evolution in the era before Darwinian natural selection was recognised as the best explanation of evolution. Thanks to new advances in the simulation of evolutionary processes and modelling technology, we can reconstruct the process behind this rapid growth in body size. With the help of these cutting-edge techniques, palaeontologists detected two evolutionary mechanisms working on two different levels. On the one hand, we have a generation of species characterized by massive jumps, both small and large. On the other hand, there was strong selection pressure centred on the largest species, leaving the others behind.  These two processes explain how these beasts managed to achieve record size in a world, the Eocene, where immense animals had yet to exist.

Reference: Sanisidro O, Mihlbachler MC, Cantalapiedra JL (2023) A macroevolutionary pathway to megaherbivory. Science. 2023. doi: 10.1126/science.ade1833.

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