Discovery of the jawbone of a new form of mammal that is the oldest found in North America, and as old as any elsewhere in the world, was announced yesterday by paleontologist Farish A. Jenkins of Harvard University.

Until now, only two families of mammals were believed to be the ancestors of all warm-blooded creatures on Earth.

The discovery, in the Painted Desert of Arizona, may complicate greatly man's picture of the evolution of mammals, Jenkins said at a news conference at the National Geographic Society here. The society funded the four-year project.

"This significance of this is that we had a very simple story, when you had only two kinds of mammals at the earliest dawn of the mammals. . . . This shows us that the story was not simple and we better rethink our hypothesis about the relations of the earliest mammals," Jenkins said.

The jawbone found in Arizona was of a mammal about the size and shape of a shrew or mouse and was dated at about 180 million years ago when mammals and dinosaurs first developed on Earth. At the time, Arizona was an often-flooded flatland similar to the Mississippi delta.

Recent theory about mammals has held that one type of the tiny, shrew-like creatures began the evolutionary line that produced nearly all of Earth's mammals, including man, horse, whale, dog and others that give birth to live animals, rather than laying eggs. The other type, also 180 million years old, was believed to be the ancestor of all egg-laying mammals such as the platypus.

The unnamed Arizona animal may change this simple picture of mammalian ancestry because, although the new find is like the other two mammal ancestors in size and body shape, it apparently had features different from both and combining some characteristics of each.

The discovery came on the second-to-last day of digging, after four years of brushing, scraping and chipping sandy rock in the desert's 110-degree heat.

As Jenkins described it, several diggers were squatting in the dust, chipping out pieces of rock, looking for skull or jaw fragments. Among them was Kathleen Smith, an assistant professor of anatomy at Duke University.

"This kind of work is extremely boring and tedious. I hate it. We were all very tired, and we had seen so many things that looked like jaws but weren't," Jenkins said.

"She broke off a piece of rock, she saw the piece of bone and handed it to me saying, 'Here, Jenkins, here's your lousy jaw.' " Jenkins said that Smith was friendly but tired and sarcastic and that the adjective she used was not "lousy" but one unrepeatable in public.

An examination of the jawbone embedded in the rock "immediately set me going," Jenkins said. The features of the teeth were quite distinctive, and he knew immediately that the discovery was something new, he said.

Jenkins explained that one of the first mammals, Kuehneotheriid, has tall, pointed teeth, with cusps in a triangular array. The other type known until now, Morganucodontid, has low rounded points on the teeth, arranged in a straight line. The new jawbone's teeth are tall and pointed and have cusps in straight array.

At the same site, the Jenkins team found the oldest turtle skeleton ever found in North America.

Jenkins said William Downs of the Museum of Northern Arizona was first to identify the site as a good spot to dig for the earliest mammal bones when he found a Morganucodontid tooth earlier in the dig. Downs will sift through about two tons of rock chipped from the quarry but not examined in detail. Jenkins said he hopes to find further examples of the new mammal.