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The dentary bones of hadrosaurs are very variable between one individual and another.

Qilinyu had three bones, the dentary, maxilla and premaxilla, that characterize the modern vertebrate jaw seen in bony fish, amphibians, reptiles, birds and mammals, though they are absent in the cartilaginous sharks and rays.

The dentary is a bone of the lower jaw.

"In us, the lower jaw is made entirely from the dentary. Most of the upper jaw is composed from the maxilla, but the bit that carries the incisor teeth is the premaxilla," Ahlberg said.

The fossil find in China's Xiaoxiang Reservoir, reported by the journal "Nature" on Thursday, is the most primitive vertebrate discovered with a modern jaw, including a dentary bone found in humans. " finally solves an age-old problem about the origin of modern fishes," said John Long, a professor in palaeontology at Flinders University in Adelaide.