![]() The evolutionary pattern could reveal potential causes, such as getting larger and larger to ward off predators or accessing untapped food sources in the ancient environment.īrontotheres were not simply getting bigger with time, the team found. They were trying to determine whether the mammals underwent steady increases in size through time, evolved larger sizes in a small set of ecological niches, or diversified in a variety of ways without apparent direction. “This group was reaching remarkable size increases in a relatively short period of time,” says University of Alcalá paleontologist Oscar Sanisidro who led the new research.īy tracking brontothere body size and the appearance of new species through time, Sanisidro and his colleagues looked for any patterns in the fossil record. Of 57 known brontothere species, more than half weighed over a ton. Over time, however, brontothere species kept producing bigger descendants. The first brontotheres were relatively small, only about 40 pounds, and evolved about 54 million years ago in what’s now North America. ![]() A new study in the journal Science sheds light on this prehistoric growth spurt by revealing the backstory of the thunder beasts. The ones that survived the extinction weren’t much bigger than terriers, but by 20 million years later mammals weighing more than a ton had become commonplace. Mammals began to grow big only after the age of dinosaurs came to an end 66 million years ago. The secret to their impressive size, scientists have found, partly stems from competition at the prehistoric salad bar. The biggest of these rhino-like creatures, called brontotheres by experts, stood taller than eight feet at the shoulder and weighed more than three tons. ![]() Thunder beasts were among the first mammals to truly live large.
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