Scientists have grown dinosaur-like snouts on chicken embryos, revealing how the bird beak may have evolved.

Birds are thought to have evolved from dinosaurs, but have very different jaws. Bird-like dinosaurs such as the velociraptor have two bones at the tip of their upper jaws. In birds, those bones are fused to form a beak.

By blocking two proteins that are activated when chicken embryos grow their beaks, U.S. researchers caused their jaws to "revert" to a velociraptor-like snout. The changes were observed in chick embryos that developed until they were close to hatching.

To their surprise, the birds' palates, on the roof of their mouths, also became dinosaur-like.

"This was unexpected and demonstrates the way in which a single, simple developmental mechanism can have wide-ranging and unexpected effects," said Bhart-Anjan Bhullar, lead author of the study, in a statement from Yale University. The study was co-led by Arhat Abzhanov, Bhullar's doctoral supervisor at Harvard University.

The researchers published their results in the journal Evolution.

In order to figure out what might be responsible for beak development, Bhullar and his colleagues compared the activation of genes in birds to that in reptiles such as crocodiles, turtles and lizards as their jaws formed during embryonic development.

They found two proteins that were only activated in birds, and those were the ones they blocked during their experiment.

The result suggests the kinds of evolutionary changes that led to the development of the beak. It also helps predict what undiscovered "missing link" species between dinosaurs and birds may have looked like, the researchers said.

It's not the first time chicken embryos have been reverted to produce a dinosaur-like trait — in 2006, German scientists identified a genetic mutation that causes chicken embryos to grow teeth.