Only a few dinosaurs have switched from walking on all fours to two. Recently, a team of paleontologists have discovered that the “parrot dinosaur” did just that.

Officially known as psittacosaurus,  the 100-million-year-old dinosaur of China, was a small and fast herbivore that had a horny beak with no teeth. Using a combination of biomechanical analysis and bone histology, paleontologists from Beijing; Bristol, UK; and Bonn, Germany; discovered that the species, which is usually known as a bipedal animal, was a quadruped during its early stages of life.  

"This remarkable study, the first of its kind, shows how much information is locked in the bones of dinosaurs,” Professor Xing Xu of the Beijing Institute said in a statement.

Qi Zhao, from the Institute for Vertebrate Paleontology in Beijing, said the team studied sections of bones from baby, juvenile and adult parrot dinosaurs and analyzed them in a palaeohistology laboratory in Bonn. The team sectioned two arms and two leg bones from 16 individual dinosaurs that ranged from less than 1 year to 10 years old, according to Science World Report.

"Some of the bones from the baby Psittacosaurus were only a few millimeters across, so I had to handle them extremely carefully to make useful bone sections," Zhao said. "I also had to be sure to cause as little damage to these valuable specimens as possible."

The team found the 1-year-old dinosaurs had long arms, short legs and walked on all fours soon after birth. Between 4- and 6-years-old arm growth slowed down, while leg growth grew in a massive spurt. Their short legs and long arms made it necessary for them to stand on their hind legs as an adult.

Zhao says the study also points to the fact that the species may have evolved from being quadrupeds to bipeds.

"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," Zhao told the Australian Associated Press.

The study published in the journal Nature Communications on Friday says the shift from walking on all fours to just their two hind legs became widespread in dinosaurs and results show that the shift may be an “ancestral condition for the entire group.”