|A diagram of the Sinornithosaurus skull showing |
grooved teeth (labelled vg) and a sinus above the
palate(labelled sff for subfenestral fossa), both of which
suggest a venomous bite.
Credit: Gong et al., 2009
Sinornithosaurus has long, spiky fangs on its upper jaw, which resemble the fangs of "rear-fanged" snakes. These fangs additionally have grooves, and the space above the palate appears to have room for a venom gland. One important detail about these long, narrow teeth is that the creature's bite could not have been very forceful lest the teeth break (and in fact a the tip of a raptor tooth has been recovered in the fossilized wing bone of a Pterosaur). Thus this skull looks like it belonged to a creature with a bite-and-hold predation style.
This idea has remained somewhat controversial. Grooves occur in teeth of non-venomous animals, where they can can be involved in sucking or grooming behaviors, and certainly the possibility of selective tooth wear during fossilization has to be considered. Regardless, even close relatives to Sinornithosaurus do not have these tooth grooves. So it may be that this venomous lineage was an evolutionary dead end.