Researchers have discovered that a species of dinosaur, Limusaurus inextricabilis, lost its teeth in adolescence and did not grow another set as adults. The finding, published today in Current Biology, is a radical change in anatomy during a lifespan and may help to explain why birds have beaks but no teeth.

The research team studied 19 Limusaurus skeletons, discovered in "death traps," where they became mired in mud, got stuck and died, in the Xinjiang Province of China. The dinosaurs ranged in age from baby to adult, showing the pattern of tooth loss over time. The baby skeleton had small, sharp teeth, and the adult skeletons were consistently toothless.

"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."

Limusaurus is part of the theropod group of dinosaurs, the evolutionary ancestors of birds. Dr. Clark's team's earlier research of Limusaurus described the species' hand development and notes that the dinosaur's reduced first finger may have been transitional and that later theropods lost the first and fifth fingers. Similarly, bird hands consist of the equivalent of a human's second, third and fourth fingers.

These fossils indicate that baby Limusaurus could have been carnivores or omnivores while the adults were herbivores, as they would have needed teeth to chew meat but not plants. Chemical makeup in the fossils' bones supports the theory of a change in diet between babies and adults. The fossils also could help to show how theropods such as birds lost their teeth, initially through changes during their development from babies to adults.

"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."