Scientists in China have identified the first known dinosaur species that grew teeth as juveniles then lost them as adults, a finding that may explain why birds have beaks, said a study Thursday.

The research is based on fossils of a small and slender dinosaur known as Limusaurus inextricabilis, part of the theropod group of dinosaurs which were the ancestors of modern birds.

It likely ate meat as a youngster but transformed into a beaked-adult that probably subsisted on plants, said the study in Current Biology.

"We found a very rare, very interesting phenomenon," said lead author Shuo Wang of Capital Normal University in Beijing, China.

"Toothed jaws in juvenile individuals transition to a completely toothless beaked jaw in more mature individuals during development."

The results are based on an analysis of the fossilized remains of 13 ceratosaurian theropod dinosaurs, collected from the Upper Jurassic Shishugou Formation of northwestern China.

These remains allowed researchers to reconstruct the dinosaur's growth from a hatchling to age 10.

The first scientific paper on Limusaurus was published in 2001, when researchers had just one fossilized juvenile.

More specimens were unearthed in the following years.

"Initially, we believed that we found two different ceratosaurian dinosaurs from the Wucaiwan Area, one toothed and the other toothless, and we even started to describe them separately," said Wang.

But then researchers realized the fossils looked quite similar, except for the teeth. Eventually they concluded that the specimens were the same species, some were just younger and had teeth.

"This discovery is important for two reasons," said co-author James Clark, a professor of biology at the George Washington University.

"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."

The theory of a change in diet is supported by the chemical makeup of the fossilized bones, said the study.

This process could help explain "how theropods such as birds lost their teeth, initially through changes during their development from babies to adults," it added.

Among contemporary fish and amphibians, such tooth loss is commonly seen. The platypus, a beaked mammal, loses its teeth, too.

Researchers said the discovery of tooth loss in the Limusaurus marks the first in the fossil record and the first among reptiles.