A Blog by Ed Yong

Groovy teeth, but was Sinornithosaurus a venomous dinosaur?

It’s a dinosaur tooth, and clearly one that belonged to a predator – sharp and backwards-pointing. But this particularly tooth, belonging to a small raptor called Sinornithosaurus, has a special feature that’s courting a lot controversy. It has a thin groove running down its length, from the root to the very tip. According to a new paper from Enpu Gong of the Chinese Academy of Sciences, it was a channel for venom.

Thanks to a certain film that shall remain nameless, a lot of people probably think that we already know that some dinosaurs are venomous. But the idea that Dilophosaurus was armed with poison, much less spat its toxins at its prey, is non-existent. Some scientists had speculated that they were venomous based on their bizarrely notched and allegedly weak jaws. But these notches have since been found in many other species and no one has ever actually measured the strength of Dilophosaurus‘s jaws.

The best sign that a dinosaur was venomous would be the presence of grooved or hollow teeth. With some notable exceptions, most animals with poison bites use grooves like these to channel their toxins from glands in their mouth to whatever they bite. And grooves are exactly what Gong and his colleagues found in Sinornithosaurus‘s well-preserved skull. Bryan Fry, who discovered venom glands in Komodo dragons earlier this year, says, “It is an absolutely fantastic piece of work. I actually got goose-bumps reading it! Other studies have suggested dinosaurs may be venomous but this is the most solid piece of evidence.”

Sinornithosaurus (meaning “Chinese bird-lizard”) is a small feathered dromaeosaurid (or, more commonly, ‘raptor’) and an early distant cousin of the birds. Its teeth are unusually large and Gong says that those in the upper jaw are “so long and fang-like that the animal appears to be saber-toothed”. They’re very similar to the fangs of back-fanged snakes like boomslangs and vine snakes.

Gong says that other aspects of the skull in support of his venom hypothesis. His team noticed that Sinornithosaurus has a small hollow on the side of its jawbone that could have housed a venom gland. They also found a thin groove running along the animal’s jaw, with small pits at the top of each tooth. They interpret this canal as a “collecting duct” that channelled venom from the gland to the teeth, and each pit could have acted as small, local venom reservoirs. David Burnham, a co-author on the paper, says, “Other fossil animals (dinosaurs, lizards, mammals) have been suggested to be venomous simply on the presence grooved teeth but out work found multiple lines of evidence.”

If the team are right about Sinornithosaurus‘s poisonous bite, there could be implications for the evolutionary origins of venom. “It is interesting that we find this in the lineage directly leading to birds,” says Burnham. “It could be that venom really is primitive for archosaurs [the group that includes dinosaurs, birds, pterosaurs and crocodiles] and lizards. Venom could be fundamentally primitive but no one has checked this out.”

But Gong’s take on Sinornithosaurus‘s skull isn’t clear-cut. Fry describes it as “quite incontrovertible”, but others disagree. Brian Switek of Laelaps says, “Since venomous dinosaurs are entirely unknown, I think much more evidence is needed. The new paper makes an observation that the dinosaur had teeth similar to those of some venomous reptiles but how can we determine whether those teeth were for delivering venom or for something else?” Thomas Holtz Jr, who specialises in predatory dinosaurs, is also unconvinced. “I find the evidence fairly weak. Not impossible by any means, but weak.”

Holtz sees Sinornithosaurus‘s skull in a different light. He views the alleged venom grooves as more pronounced versions of the depressions that line the teeth of most theropods (predatory dinosaurs). The animal’s large “fangs” are more likely to be ordinary teeth that have slipped out of their sockets. The same hollow where Gong envisages a venom gland, Holtz sees an extension of an air sac that all theropods have in front of their eyes. The canal where the venom duct supposedly sat may not even be a real structure. That region is damaged in one specimen of Sinornithosaurus and in another, it might just be part of the skull’s contours.

Philip Currie, who has done a lot of work on feathered dinosaurs, is more positive about Gong’s interpretation, saying that the combination of long, grooved teeth and pits on the upper jaw does “suggest the possibility of a poison delivery system, but it may be very difficult to prove unequivocally in the absence of soft anatomy”.

Currie also notes that scientists have noticed grooves on theropod teeth before. “It has always been assumed that they functioned in much the same manner as grooves on a bayonet, which supposedly break surface tension for easier extraction.” But Fry says that grooved teeth are only very rarely found in non-venomous animals. Mandrills, for example, have grooved canines for the reason that Currie described: to make it easier to slip the tooth in and out without creating suction.” However the positions of Sinornithosaurus‘s grooves are important. “Rear-toothed grooving has never been found in non-venomous animals,” claims Fry.

Despite the scepticism, no one has outrightly dismissed the idea of a venomous dinosaur. Fry says, “Venom is an early evolving, key evolutionary event in almost every [living] lineage.  It was quite logical that dinosaurs evolved venom one at least one occasion but we have not had the crucial proof.” But does Gong’s paper provide that proof? “I don’t think that they have made their case,” says Holtz. However, he acknowledges that if a venomous dinosaur existed, it would probably have been one of the smaller hunters. “A venomous dromaeosaurid is more likely than a venomous tyrannosaurid.”

Currie hadn’t even considered the possibility of venomous theropods before, pointing out that they seem “well adapted to kill without the assistance of venom”. But he adds that it was also surprising to learn that Komodo dragons use venom, even though most of their attacks don’t need it and their teeth are even more shallowly grooved than those of Sinornithosaurus. “Even though this is a living animal it has taken close to two centuries to learn of their venom.”

Gong says that the dinosaur’s teeth are so long that its bite force would be low (and the narrow snout wouldn’t have helped matters either). If the teeth were inserted deeply into flesh, they would probably have become quickly damaged. So sabre-toothed appearance aside, it’s unlikely that Sinornithosaurus wielded its fangs as stabbing weapons. Gong thinks that this wasn’t a predator that used a simple “grab-and-gulp” strategy; Gong thinks that the unusual maw belonged to a highly specialised hunter.

His hypothesis is that Sinornithosaurus was a specialist bird-hunter, using its fangs to puncture through thick layers of feathers. A quick injection of venom might have sent the victim into rapid shock, much like back-fanged snakes and some venomous lizards do today. Sinornithosaurus also had teeth at the very front of its snout that were angled slightly forward, and Gong thinks that the predator could have used these teeth to pluck its victims.

Burnham confirms that this is a guess, but an educated one. “Basically, it’s an idea based on what was available as prey items [in the local] forests.  I’m sure it would eat lizards or mammals, too.  But the long teeth were helpful to penetrate bird plumage.” Again, Holtz is sceptical. To him, the simplest explanation for the dinosaur’s weak bite is “that it ate small prey”, much like the modern secretarybird does.

A survey of other species might provide more evidence. Switek says, “If I were in a museum right now, I would be scheduling CT scans of theropod skulls to look for similar traits in other species.” That’s exactly what the team is doing. Burnham tells me that they are actively searching for the same structures in other small dromaeosaurs, like Microraptor. This debate is not over by a long shot.

Reference: PNAS doi:10.1073/pnas.0912360107

Images: Drawing by de Palma

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18 thoughts on “Groovy teeth, but was Sinornithosaurus a venomous dinosaur?

  1. Yep. See the bit above about mandrills. Fry’s quote in full:

    Grooved teeth are only very rarely found in non-venomous animals, Mandrills are an example where their very long canines are grooved. In this case it facilitates the entry and exit, so that the tooth removal is not impeded by suction. Much as many spear tips or other blades (particularly long ones) have such grooving. However, rear-toothed grooving has never been found in non-venomous animals. The differential sizing is also a key piece of evidence.

  2. Just to confuse the issue even more, long blades were grooved to reduce weight w/o losing much strength or rigidity, re comment #2

  3. Many reptiles are purported to have “filthy mouths” that infect prey with pathogenic organisms after a little nip. Perhaps most notable was the Komodo Dragon. So it is thrilling to learn that it is indeed venomous and perhaps not so vile of breath as previously thought. Although would not groovy teeth facilitate the injection of septic fluids as well? Perhaps Sinornithosaurus was just incubating gram negatives for the moment of truth.
    Also, I think venomous snakes have very well developed Jacobson organs to aid in following the scent trail of the victim. If this were found to be enlarged in Sinthingy I would argue that it supports venomy.
    Fantastic reading, thanks very much!

  4. *7…hence the selective pressure for enhanced Organs of Jacobson. I suggest you read http://en.wikipedia.org/wiki/Komodo_dragon#saliva
    if you’re a predator and you are able to slow your prey down, even if it takes days to complete the kill… that capability confers survival advantage. Presumably, virulent septic bacteria in the blood would tend to impede the prey animal. Not exactly rocket science.
    Dr. Procyan

  5. Er, you’ve just referred Bryan Fry to a Wikipedia link that discusses, among other things, his own research. I think Bryan’s fully aware of the current evidence around Komodo mouth bacteria! Check out the end of this article for some discussion about the bacterial hypothesis.

  6. The problem with the blade analogy is that the whole anti-suction explanation for the fuller (groove) is a myth. It’s really there to lighten and stiffen the blade without sacrificing strength. So unless that’s also an issue for animal teeth (unlikely, since teeth are not anywhere near as dense and heavy as metal), the grooves are not analogous. And unless teeth DO create suction in flesh, unlike blades (this seems highly improbable to me, but I’m not a biophysicist), the suction explanation seems goofy.

  7. Interesting idea, but I’m siding with Holtz on this one (for now). A radical theory requires pretty radical evidence, and I don’t think that’s on the table yet. That pictures makes Sinornithosaurus the new Ornitholestes. LOL

  8. “But the idea that Dilophosaurus was armed with poison, much less spat its toxins at its prey, is non-existent.”
    The idea isn’t non-existent – just the supporting evidence, Ed!

  9. if you’re a predator and you are able to slow your prey down, even if it takes days to complete the kill… that capability confers survival advantage.

    It seems to me a large lizard that can go a long time between meals and preys on large animals so a single meal can do the trick is not a very good model for a small theropod predator eating birds. Going days between meals seems unlikely for a small predator likely to have had a metabolism more like that of a bird than that of a large lizard.
    There’s also that the komodo faces only other komodos as competitors to scarf down its slowly poisoned prey, whereas every carnivorous animal around would be able to snap up a poisoned bird over the days it would take to die. And then, a bird might simply fly away if the poison isn’t quick acting and that will make it much harder to track than if it is bound to the ground as is the komodo’s prey.

  10. From the paper:

    Interestingly, much of the effective erupted length of the teeth is composed of the tooth root. The erupted portion of the largest maxillary tooth in the type specimen of S. millenii (IVPP V12811) measures 12 mm long and occupies the seventh alveolus. There is a distinct groove on the labial side running from the base of the root to the tip. The tooth crown is not really as elongated as it appears because of a hyper-erupted tooth root, and the tooth sockets are not especially deep.

    This tends to support Dr Holtz’s suggestion that the teeth have slipped partly out of their alveoli; the authors’ claim that “the tooth sockets are not especially deep” requires quantification before the unusual condition of deeply exposed roots (in life) should be inferred.
    The roots of teeth in archosaurs are hollow, and the cavity tapers upward inside the crown to some extent, varying through the cycle of tooth replacement as new mineral layers are deposited on the inner surface. Since the Sinornithosaurus fossils are strongly compressed on a bedding plane of fine-grained sediment, individual bones have undergone plastic deformation. This lateral compression of hollow teeth can be expected to produce lateral grooves; I’d predict them in any small archosaur with this kind of preservation, as an artifact. However, such grooves would not be expected to reach the tip of the tooth, and despite the statement in the paper they actually don’t seem to do so in this fossil. Also not mentioned is that grooves are also present on the lingual surface (anterior dentary teeth in fig. 2), equally compatible with compression artifact but not supporting the idea that the lateral groove is specialized for venom conduction. The grooves look broad and shallow enough to be artifacts, rather than having distinct margins formed by enamel ridges.
    Also, there’s nothing in lizards or snakes analogous to the subfenestral fossa of Sinornithosaurus; it looks more like evidence of pneumatization, or possibly a salt gland (or maybe a specialized senseory structure like the pit organ of crotalid snakes), than a venom gland. The posterior orientation of the fossa canal (supposedly carrying the venom duct to the tooth bases) is exactly in the wrong direction for that function, so it would need to make a hairpin bend and then run forward for the full length of the fossa. We can’t expect perfect design (cf. recurrent laryngeal nerve), but the venom duct hypothesis doesn’t predict or explain this feature very well.
    Snakes that prey on birds tend to have very long, slender and weakly recurved anterior teeth on both the maxilla and dentary; the dentary teeth here are practically isodont, while the largest uppers are in the middle of the jaw, not the front. (The authors refer to the upper dentition as ‘heterodont’, but that’s the wrong term because they vary in size but hardly in morphology; they should have written ‘anisodont’.) Even if the teeth really were as long in life as they appear in the fossil, it’s typical dromaeosaur dentition and no analogy suggests a specialized diet compared to other small theropods.
    So I guess I’m not convinced by the paper (even that it should have been published at this stage without further tests), but there are lots of testable ideas in there.

  11. This is shaking my religious faith in David Attenborough. I recall seeing, on one of his progs, a Komodo dragon kill, and the prey was indeed stalked for days. This link is not conclusive, but I don’t have time for a deeper search just now.
    Also, I was always under the impression that venomous snakes have their grooves at the rear of the tooth, not the side. See this site for more on snake anatomy.

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