Scott D. Sampson, curator of vertebrate paleontology at the Utah Museum of Natural History and assistant professor of geology and geophysics at the University of Utah, provides the following answer:
There is no conclusive evidence that any dinosaur possessed poisonous saliva. The poison-spitting dinosaur reconstructed in Jurassic Park is Dilophosaurus. At the time the movie was produced, there was no evidence that this or any other dinosaur spat poison or had poisonous saliva of any kind. In 2000, however, Mexican paleontologist Rub¿n A. Rodr¿guez de la Rosa of the Museum of the Desert in Saltillo presented suggestive evidence at the annual meeting of the Society of Vertebrate Paleontology. He has recovered a single odd tooth from an unknown carnivorous dinosaur that possesses a groove reminiscent of those seen in snakes for transmitting poison. I have examined this tooth firsthand and, at this point, remain unconvinced. More examples need to be recovered before this hypothesis can be evaluated.
That said, it is well known that the largest lizard predator today, the Komodo dragon, does carry a potentially lethal bacterial load in its saliva that is used to poison prey. Not surprisingly, several people have suggested that some theropod (carnivorous) dinosaurs may have exhibited similar and equally unsavory behaviors, biting prey and letting the poison do the rest of the work. This might have proved an especially useful tactic for Jurassic predators such as Allosaurus, which most likely tackled prey animals many times its body size: the gargantuan, long-necked sauropods. Nevertheless, it is important to note that all of the above remains firmly embedded in speculation.
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Answer originally posted September 4, 2001.