As a result of the asteroid impact kills dinosaursa second explosion rocked the animal kingdom.
This time, it was the mammals that exploded. Rhino-like relatives of horses that had lived in the shadow of dinosaurs became gigantic “thunder beasts” as suddenly as an evolutionary lightning bolt, according to new research published Thursday (May 11) in the journal . Science (opens in a new tab)shows.
The findings suggest that large body size may have provided at least some mammals with a clear evolutionary advantage after the dinosaurs died out.
During the cretaceous period (145 million to 66 million years ago), mammals mostly ran at the feet of much larger dinosaurs. Many weighed no more than 22 pounds (10 kilograms). But when the dinosaurs went extinct, the mammals seized a critical opportunity to succeed. Few did so impressively as the brontotheres, a lineage of extinct mammals that began at 40 pounds (18 kg) and is most closely related to modern horses.
“Although other groups of mammals reached large sizes before [they did]brontotheres were the first animals to consistently reach large sizes,” said the study’s first author. Oscar Sanisidro (opens in a new tab), researcher at the Ecology and Evolution of Global Change Research Group at the University of Alcalá in Spain. “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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Fossils of Brontotheres have been found in what is now North America, and earned the nickname “Thunder Beast” from members of the Sioux Nation, who believed the fossils came from the giant “Thunder Horses”, who roamed the plains during thunderstorms.
Paleontologists previously knew that brontotheres grew incredibly fast in size. The problem is, until now, they didn’t have a solid explanation of how.
There are three possible trajectories that the group could have taken. One, known as Cope’s rule, suggests that the entire group steadily increased in size over time, like riding an escalator from small to large. Another hypothesis suggests that instead of a steady increase over time, there were periods of rapid increase that periodically leveled off, such as running up a flight of stairs but hanging out on breaks to catch your breath. The third explanation was that there was not a constant increase in all species; some went up and some went down, but overall, more ended up being big rather than small.
Sanisidro and his colleagues analyzed a family tree containing 276 known brontothere individuals to choose the most likely scenario.
They found that the third explanation fit the data better: Rather than grow slowly over time or swell and level off, individual brontotheres species would grow or shrink in size as they expanded into new ecological niches.
Once a new species appeared in the fossil record, not much changed. However, the larger species survived while the smaller ones quickly became extinct, increasing the average size of the group over time.
The most likely explanation for this, Sanisidro told Live Science, is competition. Because mammals at the time tended to be small, there was a lot of competition between the smaller herbivores. The larger ones had fewer competitors for the food sources they sought, and therefore better prospects for survival.
bruce liebermann (opens in a new tab)a paleontologist at the University of Kansas who was not affiliated with the study told Live Science that he was impressed by the sophistication of the study.
Sanisidro notes that this study only explains how rhino-like mammals may have become giants, but he hopes to test the validity of his model in other large mammal species in the future.
“In addition, we would like to explore how changes in the brontotherial body size might have influenced other characteristics of these animals, such as skull proportions, the presence of bony appendages,” such as horns, Sanisidro said.