© Wikimedia CommonsA fossil cast of what's believed to be a Sinornithosaurus is seen at the American Natural History museum in New York in this file photo.
Sinornithosaurus was petite as dinosaurs go - think of a turkey with teeth. It ran with a tough crowd, though; it was cousin to the oh-so-scary velociraptor of Jurassic Park fame.
Paleontologist David Burnham from the University of Kansas and a Chinese colleague were puzzled by a 125-million-year-old Sinornithosaurus fossil in a museum - specifically, by its upper teeth.
"We finally realized that we're looking at the outside of these teeth and they're grooved," Burnham recalls. "And we both looked at each other and thought,
'What? Why would an animal have grooved teeth?' "
It turns out some venomous snakes and lizards use grooved teeth to deliver poison. Burnham thinks Sinornithosaurus did, too. Besides the teeth, another clue was a cavity in the jawbone where, he suggests, a venom gland could have been. He also found a channel leading from the cavity to ducts near some of the teeth.
These teeth are about 3 inches long. Unlike the hypodermic-type teeth of a viper, they're farther back, where our pre-molars would be. In much the same spot as those of rear-fanged snakes and lizards living now.
Burnham says the teeth and jaw of the animal were too frail for a "grab and gulp" strategy like big carnivores. Instead, it might have employed a "grab and hold" technique, something like the Gila monster of the American Southwest.
"They grab onto you and they don't let go," Burnham says. "So they just get a vise grip on your hand or whatever they can grab from you, and you know, that's enough to get the venom into your tissues." The animal essentially chews the venom into the wound.
Burnham describes this hypothesis about the Sinornithosaurus in a paper in the Proceedings of the National Academy of Sciences published Monday.
He can't tell what kind of venom the creature might have had. But if it were like rear-fanged venomous animals alive today, he thinks it would immobilize rather than kill - so its prey could be eaten more leisurely.
"They ... probably lived in trees, or hunted from trees anyways," Burnham says, "and so anything that they could get a hold of, perhaps a lizard or small mammal as well, would be just fine."
The announcement of a new kind of dinosaur, especially one so radically endowed, usually brings out some skeptics. Paleontologist Tom Holtz of the University of Maryland says he's not convinced yet. "They give a number of different physical features that they interpret as signs of poison or poison delivery systems," says Holtz, who is an expert in carnivorous dinosaurs, "but which, in my opinion, are more easily interpreted in other types of biological contexts."
For example, Holtz says many dinosaurs have a cavity in their jawbone, but it's thought to have held an air sac for cooling. He says the grooves could be something else, maybe wear and tear. Holtz adds that he wouldn't be terribly surprised if it turned out that Sinornithosaurus was venomous; he just doesn't think the Burnham paper provides enough evidence yet.
But Burnham says he has found more fossils in China with grooved teeth. And when he goes back to China, he says, he'll be looking at dinosaur teeth a lot more closely than he used to.
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