Some Dinosaurs Became Bipedal Because Standing Upright Helped Them Run Faster And Longer, Researchers Say

Why did some early dinosaurs learn to stand up and walk on two feet?

For the longest time, scientists believed that this happened because many proto-dinosaurs needed their forelimbs to be free in order to catch prey. As a result, the dinosaur ancestors that walked on two feet had a marked evolutionary advantage over those that didn’t, and thus the trait of bipedalism was passed down through generations.

However, in a study published in the latest edition of the Journal of Theoretical Biology, researchers from the University of Alberta in Canada claim that this theory doesn’t stand up to scrutiny.

“Those explanations don’t stand up,” study co-author Scott Persons, a postdoctoral fellow at the university, said in statement released Friday. “Many ancient bipedal dinosaurs were herbivores, and even early carnivorous dinosaurs evolved small forearms. Rather than using their hands to grapple with prey, it is more likely they seized their meals with their powerful jaws.”

Instead, Persons and his colleague Philip Currie — a renowned paleontologist and a professor at the University of Alberta — argue that dinosaurs became bipedal because it allowed them to run faster and for longer distances. This is the same reason why early carnivorous dinosaurs evolved small forearms — it reduced body weight and improved balance.

“The tails of proto-dinosaurs had big, leg-powering muscles,” Persons said. “Having this muscle mass provided the strength and power required for early dinosaurs to stand on and move with their two back feet. We see a similar effect in many modern lizards that rise up and run bipedally.”

According to the researchers, this also explains why some herbivorous dinosaurs later went back to becoming quadrupeds — their bigger guts, which evolved to break down cellulose, added more weight to the front half of the body.

“In the groups where speed was no longer a concern, they often went back to being quadrapedal,” Persons told CBC News.

The question, then, is, if bipedalism can help animals move faster, why didn’t mammals like horses and cheetahs evolve to stand upright?

According to the researchers, this was because in the Permian period, roughly 250 million years ago, mammalian ancestors were adapting to dig and to live in burrows — something that requires strong front limbs.

As a result, not only did these burrowing mammalian ancestors not become bipedal, they also lost the strong tail muscles that would have allowed them to stand upright.

“Looking across the fossil record, we can trace when our proto-mammal ancestors actually lost those muscles,” Persons said in the statement. “That’s why modern burrowers tend to have particularly short tails. Think rabbits, badgers, and moles.”