Scientists have made an astonishing new discovery in China after finding skeletons of dinosaurs that appeared to eat meat as a baby, and then turn into plant-eaters as an adult – and it could shed light on how birds evolved. Scientists based the discovery on 19 dinosaur skeletons that were about 160 million years old and found in China’s Gobi desert, all of which showed a strange pattern when observing the change between adolescence and adulthood.

In a paper published in the journal Current Biology, researchers say the dinosaurs probably died by getting stuck in mud in the desert, which provided scientists a rare opportunity to compare baby dinosaurs to the adults. They noticed the young dinosaurs had teeth, but the older Limusaurus inextricabilis lost its teeth at some point in its youth and never grew them back.

This finding could help us understand why birds have beaks but do not have teeth. It also suggests that these particular dinosaurs had a diet that changed drastically as they got older, going from meat-eaters to herbivores.

“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.”

“For most dinosaur species we have few specimens and a very incomplete understanding of their developmental biology,” said Josef Stiegler, a graduate student at George Washington University and co-author. “The large sample size of Limusaurus allowed us to use several lines of evidence including the morphology, microstructure and stable isotopic composition of the fossil bones to understand developmental and dietary changes in this animal.”