Paleontologists at the University of Alberta have a new theory explaining why some dinosaurs stood on two feet instead of four – their ancestors' need for speed.
Earlier creatures called proto-dinosaurs originally walked on all fours, but at one point evolved to stand upright. That trait was passed on to their much larger dinosaur descendants.
The going theory had been that proto-dinosaurs became bipedal so that their forelimbs were free to catch prey.
Scott Persons, lead author of a paper published this month in The Journal of Theoretical Biology, said that theory doesn't stand up.
"While that works for some of the very, very early dinosaurs, which were certainly carnivorous, you see a bunch of herbivorous dinosaurs evolve later on and a good many of those groups actually keep their bipedal stance, which is a little strange," Dr. Persons said.
The key, he said, were strong muscles at the base of the tail that helped power the hind legs of the proto-dinosaurs, which enabled them to run faster and further.
Back legs evolved to become longer, while front limbs became shorter to reduce body weight and improve balance. Some proto-dinosaurs gave up walking on all fours entirely.
Later on, some dinosaurs reverted back to a four-legged stance. These were primarily creatures with heavy horns and plates around their heads that would have made it tough to balance upright.
Herbivores evolved over time to have bigger guts to break down cellulose in the plants they ate.
"That means you're adding on weight to the front half of the animal and so that tends to tip you down. It's harder to balance on just your back legs," Dr. Persons said.
"In the groups where speed was no longer a concern, they often went back to being quadrupedal."
Researchers also looked at why today's fast-moving mammals, such as horses and cheetahs, don't stand upright.
In the Permian period, about 250 million years ago, it seems some animals started losing the lizards' leg-powering tail muscles. Around that time, many creatures were becoming burrowers, so they needed strong front limbs for digging.
Hefty back legs and a big tail would have made it tough to manoeuvre underground and easier for a predator to snatch them.
Researchers theorize that living underground may have helped those proto-mammals survive a mass extinction. Their descendants would have evolved to run fast, but without the tail muscles that would have caused them to stand upright.
While today's lizards tend to move around on all fours, vestiges of their ancient bipedal relatives remain in some species. The basilisk lizard that lives in Central and South American rain forests can run upright on water, earning it the nickname "Jesus Lizard."
"That's a really funny and strange adaptation. Why would you choose to use just one set of limbs to help you run away when you're most desperate? And the answer has to do with that great big tail muscle," said Persons.
"It effectively sort of overpowers the back legs relative to the front legs. What the lizards are effectively doing is popping a wheelie as they speed off."