Published online 17 November 2010 | Nature | doi:10.1038/news.2010.617

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When snake fangs moved out of the groove

Ancient reptile hints at how venom injection might have evolved in modern snakes.

snake fangsThe oldest snakes known in the fossil record already had hollow fangs for injecting venom.B. Mansell/naturepl.com

Fossilized teeth from an ancient reptile are revealing the way the venom injection system in modern snakes first evolved.

The research, reported in the journal Naturwissenschaften1, focused on Uatchitodon, a reptile of the late Triassic period, around 200 million years ago, and known only from its teeth. Although not closely related to snakes, Uatchitodon's hollow fangs suggest it was venomous, and it has given researchers a glimpse into how syringe-like teeth can arise.

Modern venomous snakes have tube-like fangs that inject poison directly into their victims, helping them to kill prey without a struggle. But how this system evolved has been something of a mystery. The oldest venomous snakes in the fossil record, from the Miocene epoch, around 5 million-20 million years ago, already had hollow teeth perfectly adapted to inject venom.

One possible explanation is that hollow fangs evolved from grooved teeth like those of today's Gila monsters, which use them to mix painful venom into the flesh of their victims as they chew. But fossil evidence for this has been scant.

Developmental biologists analysing tooth formation in snakes have generally agreed with this idea. As fangs grow in a snake's mouth, they change shape. Before they erupt they have an open groove, but emerge from the gum line as a sealed tube. Yet whether this developmental path reflects venomous snakes' evolution as been a matter of speculation.

Jonathan Mitchell at the University of Chicago in Illinois, the lead author of the new study, realized that Uatchitodon might provide a glimpse into the evolution of venom injection. The roots of its teeth suggest that the animal is more closely related to dinosaurs and alligators than to modern snakes, but the 'hypodermic needle' structures found in many specimens are remarkably similar to snake fangs and probably followed a similar evolutionary path.

Story in stone

Uatchitodon fossils are found at three major locations: Tomahawk in Virginia, Moncure in North Carolina and the Placerias Quarry in Arizona. All of the sites date from the late Triassic, with the Tomahawk specimens older than the other two. Mitchell and his colleagues analysed Uatchitodon teeth from the different sites and found that the fossils from Tomahawk had grooves instead of fully developed tubes.

“A syringe without a plunger is pretty useless. I'd love to get a look at the jaws.”

Wolfgang Wüster
Bangor University, UK

In the oldest fossils from Tomahawk, the venom canal was a shallow groove, extending from the base of the tooth less than a quarter of the way to the tip. Later teeth from the site had a longer and deeper groove. In specimens from Moncure and the Placerias Quarry, the groove had become all but sealed in, with just a hair-like seam to mark it, creating a canal that could convey venom once the tooth had pierced the skin.

The researchers interpret these changes as a demonstration of evolution in action. "The tubes in the teeth of Uatchitodon look to have evolved from grooves," says Mitchell.

In all, the researchers found 14 Uatchitodon specimens from Tomahawk with grooved teeth, and 26 from Moncure and Placerias with fully developed tubes. They argue that these actually represent two separate species of Uatchitodon, an older one that chewed venom into its prey like a Gila monster and a later one that injected venom with hollow teeth.

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The progression seen in the Uatchitodon teeth is very similar to the development of snake replacement fangs, in which early-stage fangs form a groove, and late-stage fangs are tubular. "This fossil really suggests that you can't get hollow fangs any other way," says herpetologist Wolfgang Wüster at Bangor University, UK.

This progression makes sense, he says, because Gila-style grooved teeth would benefit animals even in the initial evolutionary stages. For alternative possible methods of producing a fang, such as boring a tube straight through a tooth, this would not be the case.

Yet questions still remain, because researchers can only glean so much information from the teeth alone. For example, Wüster says that finding Uatchitodon jaws would be important, to check whether specimens with hollow fangs — but not those with grooved teeth — had compressor muscles that could squirt venom into prey. "A syringe without a plunger is pretty useless, so I'd love to get a look at the jaws," says Wüster. 

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