American and Chinese paleontologists have found that a bird-like dinosaur probably used venom to hunt its prey, the first time venom has been found in the ancestors of birds.

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This fossil of sinornithosaurus shows the raptor's long, grooved fangs. It was found with a wide array of other animals, including other dinosaurs and early birds. ((David Burnham/University of Kansas Biodiversity Institute))

The researchers found depressions and grooves on the sides of the skull of sinornithosaurus, a small, feathered dinosaur that lived 129 million years ago in what is now northeastern China.

The structures are similar to those seen on the skulls of some venomous reptiles, such as the beaded lizard and "rear-fanged" snakes.

"This thing is a venomous bird, for all intents and purposes," said Larry Martin of the University of Kansas Natural History Museum and Biodiversity Institute.

In a study published in this week's issue of the Proceedings of the National Academy of Sciences, the researchers say sinornithosaurus was about the size of a turkey and almost certainly had feathers, being a close relative of microraptor, a dinosaur whose fossils include impressions of feathers and wings.

The scientists think the depressions on the sides of the skull correspond to poison glands and the venom was delivered along a long groove to the upper teeth.

The structures are consistent with a primitive system to deliver venom, one that seeps the poison into a bite wound rather than injecting it at high pressure.

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A drawing of a reconstructed sinornithosaurus skull shows the structures researchers believe housed a primitive venom delivery system. ((National Academy of Sciences))

Like the beaded lizard's venom, sinornithosaurus's poison probably wasn't lethal, but sent its prey into shock, making it harder for it to escape or retaliate against the predator.

"You wouldn't have seen it coming," David Burnham of the University of Kansas said.

"It would have swooped down behind you from a low-hanging tree branch and attacked from the back. It wanted to get its jaws around you. Once the teeth were embedded in your skin, the venom could seep into the wound. The prey would rapidly go into shock, but it would still be living, and it might have seen itself being slowly devoured by this raptor."

The researchers say the dinosaur's long rear fangs suggest that it could penetrate the thick layer of feathers of its preferred prey: birds and bird-like dinosaurs.