A group of international scientists has discovered fossils of a dinosaur species that grew teeth as juveniles, but lost them in adulthood. Fossil evidence suggests that the juveniles ate meat with their teeth, but they transformed into beaked adults that would munch on plants. The dinosaur species, identified as Limusaurus inextricabilis, lived in China more than 150 million years ago.

Limusaurus inextricabilis Dinosaur
Image Credit: vlaaitje / Pixabay

They switched from one feeding type to another

Findings of the study were published Thursday in the journal Current Biology. The discovery could help understand why birds have beaks. Limusaurus inextricabilis belongs to theropods, which were ancestors of modern birds. Shuo Wang of Capital Normal University in Beijing, the lead author of the study, said it was a “very rare, very interesting phenomenon.” The dinosaurs were switching completely from one type of feeding that required teeth to another in which teeth had no advantage.

Researchers analyzed fossilized remains of 13 ceratosaurian theropod dinosaurs that were unearthed from the Gobi desert in China. The fossils were about 160 million years old. Some dinosaurs had teeth, others had just shallow depressions of tooth sockets, yet others had no evidence of teeth. The mature specimens not only had a beak, but also had rocks in their gullet, which modern birds use for mechanical digestion.

The oldest of them was ten years old

The fossils helped scientists reconstruct the Limusaurus‘ growth from a hatchling to age 10. Among the 13 specimens scientists recovered, the oldest one was 10 years old at the time of its death while the youngest ones were less than three years old. These animals matured when they were six years old, but they would lose teeth by the time they turned one.

Initially, scientists thought that they had found two different dinosaurs from the Wucaiwan Area, one toothless and the other toothed. But when researchers began to describe them separately, they realized that the fossils were quite similar, except for the teeth. The unusually dramatic shift in the anatomy of Limusaurus indicates there was a big change in their diet from adolescence to adulthood.

The baby Limusaurus ate insects

James Clark of the George Washington University, co-author of the study, said the babies probably ate small insects. But the beaked, toothless jaws of adults were suited for a herbivore diet, and the gastroliths helped them digest grains. The change in diet allowed the juveniles and adults to “live together without competing for food.” Though most reptiles lose and regenerate teeth throughout life, replacement teeth never emerged in these dinosaurs.

Scientists said it was the first time the loss of teeth was discovered in fossil records. Limusaurus belongs to the same group as Velociraptor and T. rex.