Remarkable Evolution: Tracing the Astonishing Journey of the ‘Thunder Beast’ from 20 Kilos to a Colossal Five Tons Over 50 Million Years

Brontotheres were among the first mammals to evolve large bodies after dinosaurs went extinct. Now we finally know the secret to their impressive size.

Thunder beasts were among the first mammals to truly live large. 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. The secret to their impressive size, scientists have found, partly stems from competition at the prehistoric salad bar.

Mammals began to grow big only after the age of dinosaurs came to an end 66 million years ago. 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. A new study in the journal Science sheds light on this prehistoric growth spurt by revealing the backstory of the thunder beasts.

The first brontotheres were relatively small, only about 40 pounds, and evolved about 54 million years ago in what’s now North America. Over time, however, brontothere species kept producing bigger descendants. Of 57 known brontothere species, more than half weighed over a ton.

“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.

By tracking brontothere body size and the appearance of new species through time, Sanisidro and his colleagues looked for any patterns in the fossil record. 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. 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.

Brontotheres were not simply getting bigger with time, the team found. Instead, both large and small members of this group continued to evolve throughout their history. “Different brontothere lineages produced both small and large-sized offshoots,” Sanisidro says. But even though brontotheres were not just evolving in one, ever-larger direction, the larger species tended to fare better.

Big brontotheres could reach food above the heads of their competitors, avoid being pounced on by carnivores that preyed on smaller species, and travel farther to greener patches of forest more easily than little herbivores. In a world where most mammals tried to carve out a living in the underbrush, brontotheres stepped out of the rat race.

Scientists have been pondering the evolutionary patterns of brontotheres for more than a century, says paleontologist Gemma Benevento who was not involved in the new research.

“Among mammals as a whole,” Benevento says, “some groups evolved larger sizes, some maintained a range of body sizes, and others were almost exclusively small-bodied.” Mammals were filling up the world in different ways after the decimation of the dinosaurs, and brontotheres responded with a wide range of sizes—but the bigger ones appear to have evolved more often and stuck around longer than their smaller counterparts.

Part of the reason why can be found in how Earth’s ecosystems were shaken up after the asteroid-induced mass extinction at the end of the Cretaceous. The world did not have any truly big land-dwelling animals right after the event.

“One of the biggest reasons brontotheres, and some other mammals, appear to have done well at larger sizes … was simply niche availability.” Plants that could only be reached by bigger animals, or that had leaves only larger mouths could chew, for example, were up for grabs during this time. Mammals that evolved bigger bodies could gain access to more resources with less competition.

The history of smaller brontotheres underscores the point. When Sanisidro and his colleagues looked at how long different species of these horned mammals survived and the communities they lived within, the team found that the smaller-sized brontotheres struggled to survive alongside plant-eaters that were dining on the same sorts of vegetation.

“The larger ones survived longer and were better at avoiding competition for food in a world full of small herbivores with similar browsing habits,” Sanisidro says.

The new research also outlines what might have driven brontotheres to extinction. The thunder beasts often munched on thick, leafy vegetation in humid forests, rather than grazing on tougher vegetation closer to ground level.

“When the climate became drier,” Sanisidro says, “later brontotheres got restricted to humid and riverside habitats.” As the global climate changed, and grasslands spread where lush forests once grew, thunder beasts were backed into an evolutionary corner. The environment changed faster than they could keep up, opening up the world for new mammalian giants to take the evolutionary stage.

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