A Harvard biologist yesterday announced that an expedition he led this summer unearthed remains of a mammal previously unknown in North America, and now believed to be the oldest mammal specimen on the continent.

Farish A. Jenkins Jr., professor of Biology and curator of vertebrate paleontology at the Museum of Comparative Zoology, said at a press conference in Washington, D.C., the discovery "shows us that mammals were more diverse than we had thought at the very beginning of mammal evolution."

"This is the most exciting find of my career because it stimulates our research of the earliest stages of mammalian history." Jenkins said last night of the one-cm.-long jaw bone found during a six-week dig in northeastern Arizona sponsored by the National Geographic Society.

Jenkins said that before the remains of the mouse-sized creature were found only two types of mammals were known to exist 180 million years ago. "Now we've found a third type," Jenkins said, "and this will cause us to modify our views of the earliest stages of mammalian evolution."

Molars

The jaw-bone fossil, which included four teeth, is unusual because of its completeness. Charles R. Schaff, an assistant to Jenkins who also worked on the Navajo Indian site in Arizona, said yesterday.

"A lot of time you're out there digging and you don't see anything," Schaff said. "But this specimen was so well preserved that you looked down and it was readily observable that it was a mammal."

The expedition also uncovered the skeleton of an advanced mammal-like reptile, the remains of several small dinosaurs about the size of chickens, a representative of the earliest known American turtle, and other lizards and reptiles not yet studied.

During a 1974 expedition to the Badlands of Montana, Jenkins discovered a nearly complete fossil skeleton then believed to be the oldest mammal specimen in North America.

About two tons of rock were excavated this summer from the 5000-ft.-high quarry, but so far only about 300 pounds have been studied.

Some of the fossil specimens will eventually be put on public display at Harvard while others will be returned to a Northern Arizona museum, Schaff said.

Jenkins said yesterday he plans to return to Arizona to continue excavation on the site, located about 75 miles Northeast of Flagstaff, Arizona.

Ancient

The first mammals lived almost 200 million years ago, but mammals did not proliferate until the extinction of the dinosaurs about 65 million years ago.