Wyoming wonder: tiniest mammal ever?

Paleontologists have discovered the fossilized jaw of a mammal so tiny that the entire animal would have tipped the balance at only 1.3 grams. "That's about the weight of a dollar bill," says Jonathan I. Bloch of the University of Michigan in Ann Arbor. "This represents the smallest known mammal."

Bloch and his colleagues discovered the Lilliputian lightweight, named Batodonoides vanhouteni, in 53-million-year-old limestone nodules from Wyoming. Distantly related to shrews, B. vanhouteni had teeth only a fraction of a millimeter wide, the researchers reported at an October meeting of the Society of Vertebrate Paleontology in Snowbird, Utah. The paleontologists estimated the weight of the animal from the size of its lower first molar, which typically gives a good indication of total body size, says Bloch.

The discovery challenges scientists' ideas about the lower limit on the size of mammals. The smallest known living mammal is the 2.0-gram bumblebee bat. Researchers had estimated that mammals could not get any smaller because they would lose too much body heat. The planet was warmer, however, during the Eocene epoch, which may explain how smaller animals could have flourished then, says Bloch.

John J. Flynn of the Field Museum of Natural History in Chicago says there is some uncertainty in estimating body weight, but he agrees that the B. vanhouteni jawbone is smaller than that of any other mammal known.