At the end of the Triassic period, small dinosaurs roamed the Earth along with other arcosaur groups, until a mass extinction allowed the dinosaurs to greatly increase in size. Then, after the extinction of the dinosaurs (an asteroid-sized curtain known as the KT extinction), it was mammals’ turn to become the world’s new terrestrial giants.

In the age of the dinosaurs, the existence of mammals was a prey – tiny whitish creatures scurrying between the feet of clumsy giants. After surviving in their burrows at the fiery end of the Cretaceous, mammals quickly inherited a (non-avian) dinosaur-free planet known as the Cenozoic, and scientists say they wasted no time taking advantage.

One of the first animals to grow slightly larger was the rhinoceros, the ancestor of the horse known as brontotheres, which lived until the end of the Eocene (34 million years ago), about 56 million years ago. Starting at around 40 pounds, scientists estimate this “thunder beast” has grown to four or five. ton in just 16 million years – the blink of an eye for a planet roughly 4.6 billion years old. More than half of the approximately 60 known species of brontotheria weighed more than a metric ton.

Thanks to fossils found in North America, these ancient mammals got their impressive nickname from the Sioux Nation legend. Legend speaks of thunder horses smashing the plains during a storm.

There are three theories as to how these creatures reached such large sizes. Either they gradually increased in size through natural selection and grew with subsequent attacks of highland periods, or different species grew to different sizes and larger ones outstripped their smaller relatives.

Researchers from the Global Change, Ecology and Evolution Research Group at the University of Alcalá in Spain, after examining 276 individuals from the brontotherium family to determine the most likely scenario, concluded that the latter explanation most likely contributed to the explosive growth of brontotheriums. This means that brontothere species stay the same size until they branch out into different species—some larger, some smaller. However, over time, larger species survived where smaller species did not.

“This is a more complex evolutionary world than pure Darwinism suggests,” he said. Science News Juan Cantalapiedra is study co-author and paleobiologist at the University of Alcalá in Madrid. “It’s not this organized, predictable world where progress is natural and the fittest always survive.”

The study’s lead author, Oscar Sanisidro, expects that as these “thunder beasts” get larger in size, they will face less competition for food as there are still many smaller mammal species on Earth at the time. Thus, the smaller species competed for the same resources, while the larger species faced almost no competition. While this model only applies to the ancient ancestors of North American horses, the researchers hope to use similar techniques to reveal the giant size of other mammalian lineages.

The rules of life on Earth are constantly changing, but during the first act of the Cenozoic the mammal game plan for the “thunder beast” was simple: go crazy or go home.