How did massive prehistoric ‘thunder beasts’ get super big, super fast?

Small beginnings are common for very great things. For the gigantic, rhino-like herbivorous creatures known as brontotheres, which lumbered over North America and Asia during the Eocene Epoch, such was undoubtedly the case. The majority of brontothere species grew to become nearly as huge as elephants after beginning as dogs-size creatures, and they did so rather swiftly because smaller species were outcompeted into extinction, according to current research.

In reality, it seemed likely that brontotheres still had more room to grow. If not for environmental changes, they may have given rise to even more enormous species, according to research published on Thursday in the journal Science.

Brontotheres are related to contemporary horses, tapirs, and rhinos. According to the National Park Service, the largest brontothere species, which were approximately 8 feet (2.4 meters) tall and 16 feet (4.9 meters) long and had enormous Y-shaped horns on their noses, resided in the South Dakota Badlands. Most brontothere species weighed more than 2,200 pounds (1,000 kilograms).

According to the park service, Lakota oral legends about huge thunderstorms accompanied by giants served as inspiration for the name Brontotherium, or "thunder beast," which was given to the fossil in the 19th century by naturalist Othniel Charles Marsh.

According to the researchers, the oldest known brontotheres first appeared approximately 53 million years ago. They were hornless, about the size of a coyote, and weighed about 40 pounds (18 kilograms). At the period, animals were commonly that small in size. Mammals living under the shadow of dinosaurs in the past, during the Mesozoic era (252 million to 66 million years ago), were often little larger than a badger. When an asteroid strike caused a global extinction that killed 75% of all species on Earth, the dinosaurs' period came to an end. The mammals that survived the slaughter were often rat-sized.

But that would quickly alter. Mammals started to fill certain ecological niches as huge dinosaurs disappeared, and brontotheres were particularly effective at rapidly developing to be gigantic. As early as 16 million years after the emergence of the first brontotheres, "the last members of this group were multi-ton behemoths with extravagant bony protrusions over the head," lead study author Oscar Sanisidro stated in an email.

"What makes this group even more interesting is that it is the first in mammalian history to be consistently big," said Sanisidro, who conducted the study while at the University of Alcalá in Spain and is currently a postdoctoral researcher at the University of Valencia.

Other extinct animal groups likewise gradually increased in size through time, according to the fossil record, a trend known as "Cope's Rule" after the 19th-century scientist Edward Drinker Cope. Pasquale Raia, a paleontologist and professor at the University of Naples Federico II in Italy, noted that many early scientists felt that brontotheres grew larger as a consequence of "an inner motor pushing evolution towards attaining the largest and most specialized forms." In other words, regardless of external conditions, development toward enormous size is inevitable in animals given enough time.

Raia, who was not involved in the research, said that other scientists later proposed that size increases were instead shaped as species adapted to environmental pressures like food availability, competition for resources, and the presence of predators. However, they struggled to define what might lead to rapid and extreme growth.

The scientists used data from the voluminous fossil record of the group, which accounts for the majority of its evolutionary history, to investigate the development of brontothere size. In order to follow the specifics of how genetic features in brontothere species altered as the group developed, the researchers also used computer models. The scientists may then evaluate how such alterations might be connected to increases in body size using phylogenetic analysis, which assesses the evolutionary paths through which new species take shape.

In the patterns of species extinction, they discovered a crucial hint. Their findings demonstrated that brontotheres' body sizes developed in both directions; new species would occasionally be smaller and occasionally larger. However, smaller species were more vulnerable to extinction than their bigger relatives, and a pattern where larger species survived longer than smaller ones did started to develop.

All of the surviving species became giants by the late Eocene, according to Sanisidro. The scientists speculated that smaller brontothere species may have been more susceptible to competition from other plant-eating species and predation by predators as a result of this trend, which suggested that megaherbivory—becoming huge herbivores—benefited brontotheres.

"We can, for the first time, explain the evolution of brontothere size from an evolutionary perspective and propose a 'pathway' to reach megaherbivory that needs to be tested in other mammalian groups," Sanisidro added.

The work "makes an exceptional move" toward figuring out the environmental factors that caused brontotheres to grow enormous, according to Raia, who added in an email that it offers "a fresh new look to an old and incredibly attractive question: what drives the evolution of body size."

When the humid greenhouse conditions of the Eocene began to diminish, these herbivorous titans, however, lost their advantage for survival. Previously lush environments became less conducive for the thunder creatures as the climate got increasingly drier, ultimately leading to their demise.

It would be clearer, the researchers added, "how environmental change led to the demise of the brontotheres" if more study was done modeling ecological aspects, such as the rate of ancient climatic variations and how it influenced the amount of edible flora.