Researchers made the assessment based on a set of jaws and a few teeth so small that their distinguishing details can be viewed only with a powerful scanning electron microscope. The find was announced yesterday at the annual meeting of the society for vertebrate paleontology.

The bones of batodonoides, coloured jet black and in pristine condition, were found in a large tropical tree trunk entombed in limestone sediments, some of which dated to 65 million years ago.

Paleontologists pried the petrified stump from a tree-less badlands section of north-central Wyoming. The fossil of the ancient mammal, believed to weigh as little as 1.3 gm, was discovered in a University of Michigan laboratory by researchers using acid to dissolve the surroundinglimestone.

``At first I thought I was looking at a fish jaw,'' said Jonathan Bloch. ``Under the microscope, I realised I was looking at the smallest mammal teeth I had ever seen. It's very primitive.''

Other researchers at the meeting said Bloch's discovery was likely, but they had not yet examined his data to confirm the finding. The teeth of the batodonoides measure a millimetre or less in height. That and other features suggest that it was significantly smaller than the two smallest living mammals -- the least shrew at 2.5 gm and the bumblebee bat at 2 gm.

The discovery challenges long-held views about the smallest body a warm-blooded creature can support. The fossil's modern cousins have the largest heart size-to-body ratio of any mammal to support bodies that are comparatively heavier. Their hearts pump furiously and they have to eat voraciously to support a rapid metabolism and generate enough energy.

The shape of the batodonoides' teeth is similar to a family of ancient mammals distantlyrelated to shrews. They scurried in the underbrush of tropical forests where dinosaurs roamed until the giant reptiles died 65 million years ago, possibly due to a colossal comet impact combined with climate change and disease.

These tiny mammals, perhaps no larger than a gumdrop, are among the few creatures in the fossil record that hung on, along with squirrel-sized primates and other adaptable creatures.

Other members of this shrew-like group have been found as far west as San Diego. They persisted far and wide for 45 million years after the dinosaurs vanished.

Like its cousins, the smallest mammal's teeth have high, sharply-pointed crowns that probably were used to puncture and shear insects.

Scientists can only speculate how the bones wound up in the tree stump. Shrew-like animals are hunted by birds, and the stump has also yielded a complete fossil egg and eggshell fragments. Fine-grained silt may have killed the tree and filled the stump, covering the bones and eventually hardening intolimestone.