Brontotheres, the horse’s ancient North American ancestors, is an anomaly in giantism, expanding from roughly 40 pounds to four to five tons, in 16 million years. Small-sized dinosaurs roamed Earth with other kinds of archosaurs during the end of the Triassic period—that is until a worldwide extinction event opened the way for dinosaurs to grow to enormous proportions. Then, following the dinosaurs’ extinction (the asteroid-sized curtain call known as the K-T Extinction), it was the turn of mammals to become the world’s new terrestrial giants. The fossil record is replete with land animals that grew to impossible proportions. Researchers studied 276 individuals in the brontotheres and discovered that decreased competition among larger mammals resulted in the creation of enormous “thunder beasts.”
Mammals existed as prey during the Age of Dinosaurs, as little, squirrel-like critters scurried among the feet of lumbering giants. Mammals inherited a (non-avian) dinosaur-free planet after surviving the cataclysmic end of the Cretaceous epoch in their burrows, and scientists suggest they wasted little time taking advantage.
One of the first creatures to grow in size was brontotheres, a rhino-like predecessor of the horse that existed from roughly 56 million years ago until the end of the Eocene (34 million years ago). Scientists estimate that this “thunder beast” expanded from 40 pounds to four or five tons in approximately 16 million years—a blink of an eye for a planet that is 4.6 billion years old. More than half of the almost 60 known species of brontothere weighed more than a metric ton.
The Evolutionary Puzzle: Brontotheres’ explosive growth and survival of the fittest
With bones discovered in North America, these prehistoric creatures derive their formidable name from Sioux mythology. Thunder horses stampeded across the plains during thunderstorms, according to folklore.
There are three possibilities as to how these creatures grew to such massive proportions. Either they gradually grew in size due to natural selection, grew in surges followed by plateau periods, or different species grew to different sizes, with the larger ones outliving their smaller counterparts.
Researchers from the Global Change Ecology and Evolution Research Group at the University of Alcalá in Spain concluded that the final explanation most likely contributed to the brontotheres’ explosive growth after studying 276 individuals in the brontothere family. This suggests that a brontothere species remained the same size until it branched into several species, some larger and some smaller. However, the larger species survived when their smaller brothers did not.
“It’s a more complex evolutionary world than what pure Darwinism would tell us,” Juan Cantalapiedra, a co-author on the study and paleobiologist of the University of Alcalá in Madrid told Science News. “It’s not this organized, predictable world where progress is a thing in nature and the better adapted always ended up surviving.”
Ancient horses grew to monstrous sizes to avoid competition
Oscar Sanisidro, the study’s primary author, believes that when these “thunder beasts” expanded in size, they faced less competition for food because the Earth was still teeming with lesser mammal species at the time. As a result, while lesser species compete for the same resources, larger species face little or no competition. While this model only applies to the ancient horse ancestors of North America, the researchers intend to utilize similar techniques to reveal the giant-sized heritage of other mammals.
The rules of life on Earth are always shifting, but the mammalian game plan for the “thunder beast” during the first act of the Cenozoic was simple—get large or go home.
