Scientists have made a rather remarkable discovery based on 19 dinosaur skeletons 160 million years old found in the Gobi desert in China. Basically, they found that a species of dinosaur called Limusaurus inextricabilis lost its teeth in adolescence and never grew another set, which could explain why birds have beaks but do not have teeth.

The findings, published in the journal Current Biology, are based on skeletons from dinosaurs that had perished after getting stuck in mud in the Xinjiang Province of China, according to a George Washington University statement. They found that the baby skeletons had small, sharp teeth but the adults didn’t have any teeth.

It’s a big find because it’s uncommon to find both babies and adult dinosaurs buried in the same place allowing researchers to compare things like their teeth. It also indicates that these baby dinosaurs had very different diets in their youth compared to adulthood, and thus may have transitioned from carnivore to herbivore.

“This discovery is important for two reasons,” said James Clark, a co-author on the paper and the Ronald Weintraub Professor of Biology at the George Washington University’s Columbian College of Arts and Sciences. “First, it’s very rare to find a growth series from baby to adult dinosaurs. Second, this unusually dramatic change in anatomy suggests there was a big shift in Limusaurus’ diet from adolescence to adulthood.”