Extinct Mammal Had Venomous Bite, Fossils Suggest

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"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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