Researchers have a new theory as to why some dinosaur species stood on two instead of four feet: their predecessors’ need to run faster and for longer distances.
Bipedalism was handed down from their ancestors, the much smaller proto-dinosaurs, according to paleontologists from the University of Alberta in Canada. These ancient creatures used to walk on all fours but evolved to stand upright, a trait passed on to their much bigger dino descendants.
The Trick Is In The Tail
According to ongoing theories, proto-dinosaurs turned bipedal so as to allow their forelimbs to freely catch prey. For lead study author and postdoctoral fellow Scott Persons, this theory does not hold water.
“Many ancient bipedal dinosaurs were herbivores, and even early carnivorous dinosaurs evolved small forearms,” said Persons in a statement. “Rather than using their hands to grapple with prey, it is more likely they seized their meals with their powerful jaws.”
The key to this evolution are the tails of the proto-dinosaurs that are characterized by huge, leg-powering muscles. This muscle mass, Persons explained, offered the power and strength required for the ancient dinosaurs to stand on and move with their hind legs — a similar effect seen in many modern lizards running bipedally.
Proto-dinosaurs eventually learned to run faster and for greater distances, with their elongated hind limbs responding to their need for speed while their smaller forelimbs helping reduce body weight and enhance balance. This way, some of them entirely gave up moving on all four feet.
What About Dinos With Four-Legged Stance And Mammals?
Eventually some dinosaurs returned to a four-legged stance, and they are mostly those with heavy horns and plates circling their heads that would have made it difficult to balance upright.
Herbivores also evolved over time to maintain bigger guts in order to break down cellulose in their plant diets. That would mean a heavier weight on the front half of the animal’s body, making it harder to balance on just the hind legs, Persons added.
Scientists have also studied why some of the modern fast-running animals, including horses and cheetahs, aren’t bipedal. Around 252 million years earlier during the Permian period, it appeared that some creatures started to lose their leg-powering tail muscles, when many have become burrowers and needed strong front limbs to dig.
Living underground likely helped those mammals survive mass extinction events, as humungous back legs as well as a large tail would have probably made it difficult to move underground and evade predators.
When proto-mammals came out of burrowing and some of them evolved to become fast runners, they lacked the tail muscles that would have dictated a bipedal nature.
The findings have been discussed in the Journal of Theoretical Biology.
Earlier this month, a new method that harnesses high-powered lasers probed dinosaur fossils and helped uncover the ancient animal’s transition from being a small feathered dinosaur into a flying bird.
Using the technique on the four-winged, feather Anchiornis dinosaur, the team found that the prehistoric creature had drumstick-shaped legs, bird-resembling arms, and a long, slender tail. The Jurassic creature has not been classified as a bird but kept some characteristics found in bird and lived around the time birds moved away from their similar-looking dinosaur ancestors.
