Killer bite: Ancient, tiny mammal probably used venom.

Recently excavated fossils of a mammal species originally described decades ago suggest that the mouse-size creature had a venomous bite, a trait previously unreported in ancient mammals.

Paleontologists first unearthed remains of Bisonalveus browni in Wyoming more than 50 years ago. However, those fossils included only a few rear teeth and fragments of skulls and lower jaws, says Richard C. Fox. a paleontologist at the University of Alberta in Edmonton. The newer fossils that Fox and his university colleague Craig S. Scott discovered in 60-million-year-old rocks at two sites in central Alberta include several creatures' snouts. Most notably. these fragments feature a long, pointed canine tooth that has the shape of known venom-delivery structures.

The tooth has a distinctive groove that's wide and semicircular near the base of the tooth and that narrows into a V shape near the tip. Because the channel is lined with enamel, it doesn't seem to be an imperfection caused by cracking during the animal's life or by poor preservation after its death. Also, the groove didn't result from tooth wear because none of the animal's lower teeth would have fit in that space when the creature shut its mouth. Fox and Scott speculate in the June 23 Nature that the tooth groove delivered venom.

The researchers note that the shape of the groove in B. browni echoes that of channels in the fangs of some venomous snakes. In those reptiles, biting down on prey exerts pressure on glands at the base of the fangs, squeezing the venom from those tissues and forcing it through the tooth groove into the victim.

While many reptiles and insects are renowned for their venoms, few modern-day mammals use such toxins. Male platypuses have venomous spurs on their hind feet but produce venom only during breeding season. Several species of shrews have highly toxic saliva but don't have grooves on their teeth for injecting it into their victims. Only the solenodons--two rare species of rat-size mammals that live today in Haiti and Cuba--have grooves in their teeth that deliver venom.

In Blarina brevicauda, the short-tailed shrew of eastern North America, the benefits of venom are well documented, says Joseph Merritt of the University of Pittsburgh's ecological-research station in Linesville, Pa. That species uses its venomous saliva to immobilize its prey, which includes adult and larval beetles, then stores them in underground tunnels. B. brevicauda, which doesn't hibernate, is one of the few species of shrew in which members maintain their weight during the winter, thanks in large part to its continuous supply of stored food, says Merritt.

The dearth of venomous mammals has posed an enigma for evolutionary biologists. After all, says Mark J. Dufton, a biochemist at the University of Strathclyde in Glasgow, Scotland, many early mammals may have been limited in their hunting by their small size and having a slower metabolism than that of their modern kin. Oral venom, which evolved from the chemical cocktail of saliva, would have enabled the creatures to tackle prey their own size or even larger. Once mammals evolved a more energetic metabolism or larger body size, the incremental benefit of venom may have no longer made evolutionary sense.