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Very big things often have small beginnings. This was certainly true of brontotheres, the huge rhinoceros-like herbivorous mammals that were breeding across North America and Asia during the Eocene epoch. Researchers recently discovered that Brontotheres started out as dog-sized animals, but then most species evolved to be roughly the size of elephants, and they did so relatively quickly because smaller species were vying for extinction.

In fact, brontotheres likely never reached the limits of the size they could reach. Scientists reported Thursday in the journal that they may have produced species that were even more massive, had they not all gone extinct due to environmental changes. Sciences.

Brontotheres are relatives of modern rhinos, horses, and tapirs. Most brontothere weigh more than 2,200 pounds (1,000 kilograms), and the largest live in the South Dakota Badlands, standing about 8 feet (2.4 meters) high and 16 feet (4.9 meters) long, with giant Y-shaped horns on their noses, according to the National Park Service.

The park service says the name Brontotherium—”thunder beast” coined by 19th-century paleontologist Othniel Charles Marsh—was inspired by Lakota oral history of violent thunderstorms accompanied by giants.

the Earliest known brontotheres appeared about 53 million years ago; Scientists report that they were hornless and about the size of a coyote, weighing about 40 pounds (18 kilograms). This modest size was not unusual for a mammal of the time. Previously, during the Mesozoic Era (from 252 million to 66 million years ago), mammals living in the shadow of dinosaurs were no larger than badgers. When the age of the dinosaurs ended An asteroid impact caused a mass extinction Which wiped out 75% of life on Earth, and the mammals that escaped the massacre were the size of rats on average.

But that will soon change. Once the big dinosaurs were out of the picture, mammals began to fill those ecological niches, and other animals in particular were successful in rapidly evolving into massive ones. Just 16 million years after the first brontotheres appeared, “the last member of this group was a multi-ton giant with extravagant bony protrusions atop the head,” said Oscar Sanisidro, senior author of the study, in an email.

“What makes this group even more interesting is that it’s the first in the history of mammals that is consistently large,” said Sanisidro, who conducted the research while at the University of Alcalá in Spain and is now a postdoctoral researcher at the University of Valencia.

The fossil record shows that other extinct animal groups steadily gained in size over time, an evolutionary phenomenon called the “Cope rule” after the 19th century paleontologist Edward Drinker Cope. Many early scientists argued that bronts became larger as a result of “the internal drive of evolution pushing toward the achievement of the largest and most specialized forms,” ​​said Pasquale Raya, a paleontologist and professor at the University of Naples Federico II in Italy. In other words: give the animals enough time, and evolution toward large size is inevitable, regardless of environmental factors.

Other scientists later suggested that the increases in size formed instead as species adapting to environmental pressures such as the availability of food, competition for resources and the presence of predators, but they have struggled to determine what might lead to rapid and aggressive growth, said Ria, who was not involved. . in search.

For their investigation of the evolution of brontothere size, the authors examined evidence from the group’s rich fossil record, which accounts for most of its evolutionary history. The researchers also produced computer models to track the details of how genetic traits change in brontothere species as the population evolves. By analyzing phylogenetics—evaluating the evolutionary trajectories of how new species formed—scientists can then determine how these changes might be linked to an increase in body size.

They find significant evidence in patterns of extinction across species. Their data showed that body size evolved in both directions in brontotheres — sometimes the new species being smaller, sometimes larger. But smaller species were more likely to go extinct than their larger cousins, and a trend emerged whereby larger species increasingly lasted longer than smaller ones.

“By the late Eocene, all of the remaining species were giants,” Sanisedro said. This pattern hinted that the massive herbivores—who became large herbivores—became benefited from the herbivores; Smaller species may have been more vulnerable to competition from fellow plant-eaters and to predation from carnivores, the authors reported.

“We can, for the first time, explain the evolution of bronthor size from an evolutionary perspective and suggest a ‘pathway’ to megafauna that needs testing in other mammalian groups,” Sanisidro said.

The study provides “new insight into an ancient and incredibly attractive question: what drives the evolution of body size,” and “makes an extraordinary step” toward identifying the external conditions that drove blondes to be bulky, Raya said in an email.

However, these herbivorous giants lost their ability to survive when the moist greenhouse conditions of the Eocene began to fade away. As the climate became progressively drier, the previously lush ecosystems became less suitable for the Thunder Beasts, which eventually led to their extinction.

The scientists said further research that models environmental factors, such as the rate of paleoclimate changes and how this affected the abundance of edible vegetation, “would clarify how environmental change led to the shrub’s demise.”