Rhino-like ‘thunder beasts’ grew massive in the evolutionary blink of an eye after dinosaurs died off

Only 16 million years after the dinosaur-killing asteroid struck, ancient mammals known as 'thunder beasts' grew 1,000 times bigger.

The extinction of dinosaurs was a catastrophic event that is still shrouded in mystery. But what’s even more fascinating is what happened after the extinction. It turns out that the mammals that survived the impact thrived in the aftermath, especially a group of rhino-like horse relatives.

Rhino-like 'thunder beasts' grew massive in the evolutionary blink of an eye after dinosaurs died off 1
The rhino-like species existed until the end of the Eocene period, around 35 million years ago. © Oscar Sanisidro / Fair Use

They quickly grew to massive sizes, becoming known as “thunder beasts”. How did this happen so quickly? The answer lies in an evolutionary lightning strike that took place in the animal kingdom after the asteroid impact, according to a new study published on May 11 in the journal Science.

The findings suggest that large body size provided at least some mammals with an evolutionary advantage after the dinosaurs went extinct.

Mammals generally scurried at the feet of considerably larger dinosaurs during the Cretaceous epoch (145 million to 66 million years ago). Many were under 22 pounds (10 kilograms).

However, as the dinosaurs became extinct, mammals seized a key opportunity to thrive. Few accomplished it as well as brontotheres, an extinct mammal lineage that weighed 40 pounds (18 kg) at birth and is most closely linked to current horses.

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North American brontothere from the Eocene. © Wikimedia Commons / Fair Use

According to the study’s first author Oscar Sanisidro, a researcher with the Global Change Ecology and Evolution Research Group at the University of Alcalá in Spain, other mammalian groups attained large sizes before they did, brontotheres were the first animals to consistently reach large sizes.

Not only that, they reached maximum weights of 4-5 tons (3.6 to 4.5 metric tons) in just 16 million years, a short period of time from a geological perspective.

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Brontotherium hatcheri fossil at the National Museum of Natural History, Washington, D.C. © Wikimedia Commons / Fair Use

Brontotheres’ fossils have been found in what is now North America, and they earned the “Thunder Beast” moniker from members of the Sioux nation, who believed the fossils came from giant “Thunder Horses,” that would roam the plains during thunderstorms.

Paleontologists previously recognized that brontotheres grew quite swiftly. The trouble is that they had no credible explanation for how until today.

The group may have taken one of three different paths. One theory, known as Cope’s rule, proposes that the entire group gradually grew in size through time, much like riding an escalator from tiny to large.

Another theory proposes that instead of a constant increase over time, there were moments of fast increase that would plateau periodically, similar to running up a flight of stairs but stopping to regain your breath on the landings.

The third theory was that there was no consistent growth across all species; some went up, some went down, but on average, more ended up huge rather than little. Sanisidro and colleagues chose the most likely scenario by analyzing a family tree encompassing 276 known brontothere individuals.

They discovered that the third hypothesis best fits the data: instead of gradually growing larger over time or swelling and plateauing, individual brontothere species would either grow larger or shrink as they expanded into new ecological niches.

It didn’t take long for a new species to arise in the fossil record. However, larger species survived while smaller ones went extinct, increasing the group’s average size over time.

According to Sanisidro, the most plausible answer is competitiveness. Because mammals were small during the period, there was a lot of competition among smaller herbivores. Larger ones had less competition for the food sources they sought, giving them a higher chance of survival.

Bruce Lieberman, a paleontologist with the University of Kansas who was not affiliated with the study, told Live Science that he was impressed by the study’s sophistication.

The complexity of the analysis struck Bruce Lieberman, a paleontologist at the University of Kansas who was not involved in the research.

Sanisidro points out that this study only explains how rhino-like creatures became giants, but he plans to test his model’s validity on additional huge mammal species in the future.

“Also, we would like to explore how changes in brontothere body size could have influenced other characteristics of these animals, like skull proportions, the presence of bony appendages,” such as horns, Sanisidro said.

It is amazing to think about the rapid changes that occurred in the animal kingdom in the aftermath of such catastrophic events. The evolution of these species is a reminder of the incredible adaptability of life on Earth and how drastically the world can change in just a few moments.

The study was originally published in the journal Science on May 11, 2023.