Paleontologists at the University of Alberta, Canada, have developed a new theory to explain why the ancient ancestors of dinosaurs stopped moving about on all fours and rose up on just their two hind legs. Bipedalism — which refers to locomoting on two legs — in dinosaurs, was inherited from ancient and much smaller proto-dinosaurs. The trick to this evolution is in their tails, explains Dr. Scott W. Persons, postdoctoral fellow and lead author on the paper.
“The tails of proto-dinosaurs had big, leg-powering muscles,” he says. “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.” Over time, proto-dinosaurs evolved to run faster and for longer distances. Adaptations like hind limb elongation allowed ancient dinosaurs to run faster, while smaller forelimbs helped to reduce body weight and improve balance. Eventually, some proto-dinosaurs gave up quadrupedal walking altogether.