A fierce, feathered raptor might have been terrifying enough to small dinosaurs, lizards, birds and mammals living 128 million years ago, but add venom to its arsenal and the threat would be paralyzing—literally.

First described a decade ago, Sinornithosaurus had peculiar dental and facial features—including some long, grooved teeth and indentations in its face—that initially escaped explanation. But a new paper, published online Monday in the Proceedings of the National Academy of Sciences, proposes that this Lower Cretaceous raptor used those adaptations to deliver prey-stunning poison that aided in killing.

"When we were looking at Sinornithosaurus, we realized that its teeth were unusual," Larry Martin, a professor and curator of vertebrate paleontology at the Natural History Museum and Biodiversity Institute at the University of Kansas in Lawrence and paper co-author, said in a prepared statement. "Then we began to look at the whole structure of the teeth and jaw, and at that point, we realized it was similar to modern-day snakes." Like many rear-fanged snakes, these dinosaurs might have used grooved teeth—rather than hollow ones—to deliver venom into their victims' wounds.

Examining specimens from the two known Sinornithosaurus species, millenii and haoiana, the researchers, based at Northeastern University in Liaoning, China, and the University of Kansas in Lawrence, found the creatures had features reminiscent of today's rear-fanged snakes and helodermatid lizards. 

Its especially long, grooved teeth "may result from the need to penetrate a thick layer of feathers," the authors wrote, noting that small early birds might have been popular targets. "Once through the feathers, the grooved fang would penetrate 4 to 6 millimeters into the skin. This would be sufficient to cut into the subdermal tissue and allow poison to enter the bloodstream but would be too shallow to cause death or immobilization through trauma alone."

This diabolical dromaeosaur was closely related to the gliding Microraptor gui, and the paper authors highlight its avian qualities: "This thing is a venomous bird for all intents and purposes," Martin said. The Sinornithosaurus was about the size of a turkey but had a taste for meat and perhaps even a stealthy hunting strategy. 

"You wouldn't have seen it coming," David Burnham, also of the KU Natural History Museum and Biodiversity Institute, said in a prepared statement. "It would have swooped down behind you from a low-hanging tree branch and attacked from the back…Once the teeth were embedded in your skin, the venom could seep into the wound."

The venom, however, likely wasn't used to kill the prey, but rather to subdue it. "The prey would go rapidly into shock, but it would still be living," Burnham said. "It might have seen itself being slowly devoured by this raptor."

Image of fossilized Sinornithosaurus teeth courtesy of David Burnham, University of Kansas Biodiversity Institute; drawing of reconstructed Sinornithosaurus skull courtesy of National Academy of Sciences