New York: Researchers have discovered a dinosaur species that shed its "milk teeth" but did not replace them, a finding that not only changes its anatomy but also explains why birds have beaks and not teeth.
Limusaurus inextricabilis is part of the theropod group of dinosaurs, the evolutionary ancestors of birds.
The study is based on an analysis of 19 Jurassic ceratosaurian theropod Limusaurus inextricabilis skeletons, discovered in "death traps" where they became mired in mud, got stuck and died.
Their fossils were recovered from Xinjiang in China.
The skeletons 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, Professor at the George Washington University, US.
"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."
In addition, 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, the researchers noted.
The study was published in the journal Current Biology.