Image of Limusaurus. Photo: Yu Chen

Limusaurus inextricabilis, a small bipedal dinosaur that roamed the Earth 160 million years ago underwent an unusual evolution as it aged – instead of growing more teeth as it grew, the reptile lost all its chompers by adulthood.

Researchers noted in the study, published in Current Biology that the transformation was a “gradual, complex process.”

The unique transition, known as ontogenetic edentulism, has been observed in modern fish and amphibian species, but this discovery marks a first for any known reptile species, alive or extinct.

The first Limusaurus specimen was found in 2001, but the team of paleontologists, led by Shuo Wang, of Capital Normal University in Beijing, China, were unable to categorize the species. After several years of digging, the team found a total of 19 specimens representing six different life stages of the same animal.

The findings showed that Limusaurus was about 5 feet from the snout to the end of its tail, and weighed slightly more than 50 pounds.

"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.

The fossilized hatchlings showed that youngsters had dozens of rooted teeth, but by the time they reached full maturity, all teeth were absent, and a toothless, beaked jaw replaced them.

“Our discovery has significant implications for understanding the evolution of the beak, an important feeding structure present in several tetrapod clades, including modern birds,” the authors wrote.

The progression from a mouthful of teeth to a toothless beak suggests that the reptile experienced a drastic diet change, from omnivorous to herbivorous, according to the study authors.

Although the loss of teeth may have been the most unexpected discovery, the researchers documented more than 75 other anatomical changes over the dinosaur’s lifetime, including changes in skull size and altered upper jawbone shape.

The team will continue to conduct detailed analyses of the species in hopes of determining what exactly led to these transformative changes.