Extinct Mammal Had Venomous Bite, Fossils Suggest

John Roach
for National Geographic News

June 22, 2005

About 60 million years ago, a small shrew-like mammal captured its prey by stabbing it with dagger-like teeth that delivered a nasty dose of venom, paleontologists reported today.

"Nothing like that has ever been described before," said Richard Fox, a vertebrate paleontologist at the University of Alberta in Edmonton, Canada.

Fox and his colleague Craig Scott found fossilized teeth at two sites in central Alberta. The remains are the first evidence to suggest that extinct mammals used venom to either capture prey or fend off predators.

Venoms are common in snakes and spiders, but only four living mammal species use venom today. According to Fox, evolutionary biologists have long wondered why venom is so rare in modern mammals.

The newly discovered tooth fossils—some belonging to the extinct shrew-like Bisonalveus browni and others to an unidentified creature—indicate the toxic strategy was perhaps more widespread among early mammals, Fox said.

Fox and Scott detail their findings in tomorrow's issue of the science journal Nature.

Mark Dufton, a chemist and venom expert at the University of Strathclyde in Glasgow, Scotland, said the discovery is a fascinating contribution to the understanding of early mammals.

"If a venomous capability was a frequent feature of these creatures, it means their power-to-weight ratio was generally higher than thought previously," Dufton wrote in an e-mail to National Geographic News. "They could have posed a much more serious threat towards competing vertebrates within their niches."

Dagger Teeth

B. browni fossils were first discovered in 1956. They consisted of lower jawbone fragments containing molar teeth, and were dated to about 60 million years ago.

Analysis of these fossils showed the ancient creature to be a small mammal, possibly distantly related to the modern scaly anteater known as the pangolin.

The newly discovered B. browni fossils are the best preserved remains of the extinct mammal. They include never-before-seen front teeth and a skull fragment containing the dagger-like grooved tooth that the paleontologists say is the tell-tale signature of a venomous bite.

"There's no corresponding surface [on the lower jaw] that would fit the grooved tooth," Fox explained. "It's not part of the chewing apparatus."

According to Fox, B. browni's venom delivery system most likely resembled that of the solenodon, a poisonous shrew-like mammal living today on the Caribbean islands of Cuba and Hispaniola.

The solenodon has a grooved dagger-like tooth on its lower jaw that it uses to inject venom that paralyzes its insect prey. The mammal stores its victims—still alive, but immobile—in a cache that it can devour at leisure, Fox said.

"We don't know if [B. browni] did the same thing, but the canine is a stabbing tooth and indicates [the animal] was predating on something, presumably small invertebrate insects like beetles," he said.

Dufton, the University of Strathclyde venom expert, said Fox and Scott make a strong case that B. browni had teeth specially adapted to introduce venom—actually poisonous saliva—into its prey.

"The likelihood that the saliva was toxic and was required to subdue active prey is high," he said. "But one must also consider that if the animal was a highly active forager … introduction of saliva for digestive reasons could also be important."

Evolutionary Enigma

In addition to the solenodon, the only mammals that use venom today are the North American short-tailed shrew, the Eurasian water shrew, and the Australian duck-billed platypus. This rarity of venom among living mammals has long baffled biologists, Fox said.

"Why hasn't it taken off? We don't have a clue. It would seem that it would be [an] effective [strategy]," he said.

The discovery that B. browni and, in all likelihood, a few other extinct mammals used venom to secure prey suggests that venomous mammals were more widespread in the past.

As the fossil record of mammals from B. browni's era improves, Fox said, he expects even more venomous mammals will be discovered.

According to Dufton at the University of Strathclyde, venom may be scarce among mammals today because predatory mammals use surprise, speed, and strength so efficiently in their attacks, and can inflict lethal damage with teeth and claws.

"The kill can be immediate," he said, "whereas a venom, however sophisticated, takes time."

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