Jaw is from smallest known land mammal Think of a mammal so tiny that its jaw is only a quarter of an inch long. Imagine holding one of these little animals in your palm and realizing it weighs only as much as a dollar bill.

It would be the smallest known land mammal, extinct or living. A rare Asian bat might have been smaller - but then, maybe not.Think of a creature that little and you'll have some idea of the 53-million-year-old fossil that Jonathan I. Bloch discovered in north-central Wyoming in 1994. It is an extinct distant relative to the shrews, with the scientific name Batodonoides vanhouteni.

Despite its minuscule size, when Bloch announced Batodonoides at Snowbird, it was one of the biggest sensations of the annual meeting of the Society of Vertebrate Paleontology.

A 28-year-old graduate student from the University of Michigan, Ann Arbor, Bloch held reporters spellbound Thursday as he described the find. He is the son of two Holladay residents, George and Sandy Bloch. His father is a psychology professor at Brigham Young University.

The young scientist could tell the remains he found belonged to a mammal because mammals have distinctive teeth.

Calculating the ancient animal's mass based on size of teeth, scientists figure it weighed less than one-twentieth of an ounce. At one time, researchers theorized land mammals had to be more than about one-tenth of an ounce.

"This leaves us with a question - what is the lower limit of mammalian size?" asks a report by Bloch and others in the scientific magazine Journal of Mammalogy.

The discovery is part of a collection of incredibly small fossils that Bloch and other experts have been finding in Wyoming recently. Most minute bones aren't preserved, but these were protected in an unusual matrix.

According to Bloch, they were found in unusual nodules of limestone picked up in the badlands of the Clarks Fork and Bighorn basins, Wyoming.

"The idea is that these (nodules) might be the remains of ancient tree trunks . . . hollow tree trunks," Bloch said.

Small animal skeletons could have been left in the trees by larger mammals or birds that preyed on them. Over the eons, the rotting tree might have triggered the deposit of material that hardened into limestone.

The limestone nodules range in size up to about four feet across. Some of them have well-preserved fossils of tiny animals inside.

"I dissolve them (the nodules). It's a very slow process. I use dilute formic acid," he said. A big chunk might take a year to dissolve. But when the limestone is gone, black, shiny, delicate fossil bones are left. The acid doesn't harm them.

"We're getting a whole diversity of really small things as a result of dissolving the limestone," he said. A few days ago, he came across a jaw of a marsupial almost as small as Batodonoides.

Why were they so tiny? Perhaps it was because the Eocene historic era, when they lived, was a mild period. That meant mammals didn't need as much mass to remain warm.

Also in the press conference, researcher Catherine Forster of the State University of New York described some of the many new dinosaurs found in 1998, including a meat-eater from Argentina, a duck-billed dinosaur from Antarctica, and some of the earliest armored dinosaurs from Southeast Asia.

Ohio State University paleontologist Lawrence Witmer said new studies indicate popular depictions of some dinosaur facial features is wrong. Tyrannosaurus rex probably did not have lips and Triceratops probably had no cheeks.

The conference has drawn hundreds of researchers from around the world. It concludes Saturday.