Story highlights Discovery may shed light on why bird have beaks, not teeth

They died in mud pit that may have been created by a huge dinosaur footprint

(CNN) A group of 13 dinosaurs that died in a mud pit in China has yielded an unprecedented discovery.

The 154 million-year-old limusaurus had tiny, sharp teeth as a hatchling that it gradually lost as it grew up, according to new research published in the journal Current Biology on Thursday.

The finding is a first for the fossil record and may shed light on why birds have beaks and not teeth.

The group ranged from babies to adults and showed a pattern of tooth loss over time. The fossilized skeletons were found in mysterious death pits in Xinjiang in the far west of China.

"At first we thought they were different dinosaurs -- one with teeth and one without and we started to study them separately," said Wang Shuo, a co-author of the study and an evolutionary biologist at Capital Normal University in Beijing.