Fossil remains of the oldest known undisputed member of the primate order -- which includes monkeys, apes and human beings -- have been discovered in Wyoming, in a region that once harbored vast, alligator-infested subtropical forests.

The fossils, consisting of teeth and jaw fragments, are of a newly discovered squirrel-sized species that scampered among the treetops about 53 million years ago and which may have been distantly ancestral to the human lineage.

"You could say this is in some broad sense the oldest ancestor of humans," said Philip D. Gingerich, a University of Michigan paleontologist who reported his discovery in the current issue of Nature, the British scientific journal.

Gingerich said the species resembled the modern lemur, a monkeylike lower primate that is today confined to Madagascar but has features reminiscent of tarsiers, a smaller, bug-eyed form of lower primate found in Southeast Asia. Experts disagree on whether lemurs or tarsiers eventually gave rise to the higher primates.

Gingerich said the newly discovered species resembles both in different ways. Because it is older than either one, the resemblances suggest that it may have been their common ancestor or a close relative of the common ancestor.

"The form we found is nearly suitable for a common ancestor," Gingerich said, "but it has a couple of problems. It's too much on the lemur side, and it's in the wrong place."

Africa, not North America, is widely considered to have been the scene of many key events in primate evolution. It is there, Gingerich believes, that primitive primates, thought to resemble the modern tree shrew, evolved into larger forms.

No one knows exactly how this happened, but paleontologists have fossils from the Eocene (from 53 million to 37 million years ago) of two general types of primates -- ones that resemble the modern lemurs and others resembling modern tarsiers. About 40 million years ago the first monkeys arose, followed about 5 million years later by the earliest apes. The earliest hominids, primates that resemble humans more than they do apes, appeared only about 5 million years ago.

While most of these events are believed to have happened in Africa, that continent has yielded comparatively little fossil evidence on any step but the rise of early humans.

"We just don't have the fossils in Africa," Gingerich said, "but we do pick up these species after they have migrated out to other parts of the world. When we see them, they've probably changed a bit, so we have to infer back to what they might have been."

North America is one of the best places in the world for early primate fossils. Gingerich and his students have been searching for their remains for 11 years in Wyoming, in a badlands area near the Yellowstone River.

In the summer of 1984, Victor Torres, one of Gingerich's graduate students, discovered the fossils and added them to a collection of more than 15,000 specimens of many species accumulated over the years. It was months later before Gingerich examined them and recognized their significance.

Gingerich named the new species Cantius torresi, honoring his student but placing the species in the same genus as previously known, related fossil species.

Other fossils from the same deposits show that the extinct primate lived in a forested subtropical river valley inhabited by primitive horses, popularly known as Eohippus, early carnivores that resembled weasels and alligators.