Dinosaurs once crawled on all fours like human toddlers before switching to walk on two feet as they reached adolescence, a first-of-its-kind study has found.
New research suggests that Psittacosaurus, the 'parrot dinosaur' walked on four feet and then two feet some 100 million years ago in what is now China.
It would have grown up much like the modern humans, at first exploring its world on all fours, like a toddler, and then graduating to upright motion, researchers said.
Using a combination of bio-mechanical analysis and bone histology, palaeontologists from Beijing, Bristol, and Bonn have shown how one of the best-known dinosaurs switched from four feet to two as it grew.
Psittacosaurus, the 'parrot dinosaur' is known from more than 1000 specimens from the Cretaceous, 100 million years ago, of China and other parts of east Asia.
As part of his PhD thesis at the University of Bristol, Qi Zhao, now on the staff of the Institute for Vertebrate Paleontology in Beijing, carried out the intricate study on bones of babies, juveniles and adults.
"Some of the bones from baby Psittacosaurus were only a few millimetres across, so I had to handle them extremely carefully to be able to make useful bone sections. I also had to be sure to cause as little damage to these valuable specimens as possible," Zhao said.
Zhao sectioned two arm and two leg bones from 16 individual dinosaurs, ranging in age from less than one year to 10 years old, or fully-grown.
The one-year-olds had long arms and short legs, and scuttled about on all fours soon after hatching. The bone sections showed that the arm bones were growing fastest when the animals were aged one to three years.
Then, from four to six years, arm growth slowed down, and the leg bones showed a massive growth spurt, meaning they ended up twice as long as the arms, necessary for an animal that stood up on its hind legs as an adult.
"This remarkable study, the first of its kind, shows how much information is locked in the bones of dinosaurs. We are delighted the study worked so well, and see many ways to use the new methods to understand even more about the astonishing lives of the dinosaurs," Professor Xing Xu of the Beijing Institute, one of Zhao's thesis supervisors, said.
"These kinds of studies can also throw light on the evolution of a dinosaur like Psittacosaurus. Having four-legged babies and juveniles suggests that at some time in their ancestry, both juveniles and adults were also four-legged, and Psittacosaurus and dinosaurs in general became secondarily bipedal," Professor Mike Benton of the University of Bristol, Zhao's other PhD supervisor, said.
The study was published in Nature Communications.