Though its dinosaurs looked pretty good,
Sinornithosaurus was one of the first feathered dinosaurs to be discovered. Covered in at least two types of feathers, it was a small dromaeosaur, or a relative of larger predators such as Velociraptor and Deinonychus. According to the new study by Enpu Gong, Larry Martin, David Burnhamb and Amanda Falk, however, Sinornithosaurus differed from its more famous cousins in that it had space for a venom gland and grooved teeth capable of delivering venom into the body of its prey.
When looking at the skull of Sinornithosaurus, the researchers believed they found traits commonly associated with venomous reptiles (namely lizards and snakes) such as a space in the skull for a venom gland, a hole in the jaw that would allow the venom to ooze out, and grooved teeth would channel the venom directly into the wounds of the dinosaur's prey. If all this is accurate, the authors speculate, then Sinornithosaurus was probably armed with a kind of venom that would have immobilized its prey and allowed it to eat it at its leisure.
These are some pretty fantastic claims, but do they hold up to scrutiny? Dinosaurs belong to a wider group of reptiles called archosaurs which also includes crocodiles and birds. To date no evidence has ever been found of a venomous archosaur. But in the introductory portion of the paper, Sinornithosaurus is described as an "avian dromaeosaur" that was part of the "early avian radiation." That means the authors are suggesting that Sinornithosaurus was not a dinosaur at all but a true bird that became secondarily flightless. Despite the overwhelming evidence that birds are dinosaurs, some scientists disagree, and the only way some of these critics (such as Larry Martin, one of the authors of the new paper) can make sense of feathered dinosaurs it to claim that they really were birds which were unrelated to dinosaurs.
The significance of this alternate view is that if Sinornithosaurus was a venomous bird derived from a more ancient stock of lizard-like reptiles (and hence unrelated to dinosaurs) it would have been evolutionarily closer to lizards and snakes, two groups that we know have venomous members. This association would not be proof positive that Sinornithosaurus had a venomous bite, but it would make it seem more likely that Sinornithosaurus was also venomous. Unfortunately for the authors, though, Sinornithosaurus was a feathered dinosaur that was only a cousin of some of the earliest avians (which were flying around during the time it lived). The existence of a venomous archosaur is still an extraordinary claim that requires extraordinary evidence.
As things presently stand that evidence has not been provided. The authors did not compare the skull of Sinornithosaurus with those of other predatory dinosaurs to see if the same traits occurred among other dinosaurs in differing combinations (perhaps having grooved teeth, for example, but not a "venom gland").
There could still be alternate explanations for some of the traits they found. Other theropod dinosaurs have grooves in their teeth which appear to have reduced mechanical stress as they bit into prey. The tips of the teeth would penetrate into the prey but the grooves would provide a space for air so that the amount of suction on the tooth would be reduced as it was being removed. Likewise, the "venom gland" could just be an extension of an opening in the skull seen in many theropod dinosaurs and does not necessarily require a new explanation. The authors of the new paper did not discuss alternative hypotheses, and most of the structures they interpret as being indicative of a venomous bite can be otherwise explained.
The situation is made all the worse by a confused press release about the research generated by the University of Kansas. Even though the team did not actually discover Sinornithosaurus (it was named in 1999), the press release proclaims "Venomous prehistoric 'raptor' discovered by research team from KU and China." And, as expected, Sinornithosaurus is not presented as a dinosaur but was instead described as "a venomous bird for all intents and purposes" by Larry Martin.
Are venomous dinosaurs a possibility? Absolutely, but in order to confirm their existence, particularly strong evidence is required. The new study, while interesting, does not include compelling evidence that Sinornithosaurus or any other dinosaur was venomous. For another take on this story, see Ed Yong's piece at Not Exactly Rocket Science.