A nearly whole skeleton of a mammal believed to have been the ancestor of an entire order of mammals has surprised scientists because it is far more specialized and advanced than previously believed.

The skeleton may be the only one ever found of this pivotal creature, the Diacodexis. Until now, based on the evidence of a few ankle and leg bones, it was believed to have been the ancestor of two varieties of hoofed animals, including pigs and hippos on one side and cattle, goats and gazelles on the other.

Scientists thought that Diacodexis was a relatively unspecialized animal, and that only later in evolutionary history did the specialized forms of bones for running and jumping develop.

But the reconstructed skeleton of the 53-million-year-old Diacodexis shows that it was a highly developed animal, already a fine runner and jumper.

So its place as the sole ancestor of both jumpers and the more heavily built pigs and hippos is now in question.

"The important thing is that we found a nearly whole skeleton. The question of classification is another problem," said Kenneth D. Rose, an assistant professor of medicine at Johns Hopkins University, and author of the report on the discovery in the May 7 issue of the journal Science.

Rose and Thomas Bown of the U.S. Geological Survey led the expedition, which was supported by the National Geographic Society. Mary J. Kraus, a graduate student at the University of Colorado, found the skeleton on an expedition in 1979.