Who would have thought a therapsid could cause such a fuss?

Not only did it force down our throats the mad notion that the very continents of our earth are just a bunch of floating islands, but it caused a three-day Smithsonian conference. Now, that's a fuss.

Especially when you consider that the last therapsid lay down and died 200 million years ago, give or take a few weeks. Now, that's influence.

You want to know what a therapsid is. Yes. Well. You better find a comfortable chair. There are three main kinds, with innumberable variations appearing as it evolved. It can be any size from a rat to a rhino. It can be a light-footed meat-eater along the lines of a saber-toothed tiger, or it can be a grass-eating heavy-legged plodder. It can have a five-boned reptilian jaw or a single-boned mammaliam jaw. It might have fur. Or it might not.

The main thing is that the therapsid seems to be a link between reptiles and mammals. It was going pretty good there for awhile, dominating the world's animal scene almost 75 million years, getting to feel more and more like a mammal, before it went extinct. The dinosaur society came after it and doubtless helped it get extinct because the price of turning into a mammal was that you became smaller. And dinosaurs, as everyone knows, were big.

But dinosaurs, as far as we people are concerned, wound up in an evolutionary blind alley. That's why scientists love therapsids. They might tell us something about ourselves.

We take you now to Alfred Wegener, a German meteorologist who in 1912 began to insist that for eons the continents have been drifting. Get a globe and look how the east coast of South American fits into the west coast of Africa, how our own eastern seaboard could slide right up against the northwest African bulge. Wegener called it continental drift, and it led to a theory which has been seen as one of the three great explosions of earth science discovery in the last 500 years: the plate tectonic theory of geology. s(The others were from Copernicus and Darwin.)

The idea of the earth's surface being a series of shifting rock plates, like armor, was hard to take at first. One had to accept a whole new set of theoretical continents. Yet in the last decade or so, science has been turned around almost completely. New techniques for studying the ocean floor were a big help. But it was the therapsid that really did it. You can't argue with a fossil.

You are now ready for Dr. Edwin H. Colbert, a 75-year-old born scientist who had a fossil museum when he was 12 in Missouri, who has traveled across the world digging up things, who finds "magic in those skeletons and skulls," which evoke for him "visions of a world long vanished, when Nebraska was a land of lush savannahs inhabited by hosts of unfamiliar animals . . ." as he writes in his "A Fossil-Hunter's Notebook" (Dutton, 1980).

For 40 years Colbert worked at the American Museum of Natural History in New York, winding up as a department chairman. He has put together more skeletons than Lindbergh built model planes. He can tell you about the time the firemen rushed through the halls with their hoses knocking against the exhibits, and how he watched breathless while a rare horse-sized moropus he had just laboriously assembled teetered and teetered -- and decided to remain standing. He is full of stories about the dinosaurs of Ghost Ranch (he found the first bones actually on the surface of the New Mexico desert.)

But mostly he likes to talk about the lystrosaurus. What an invention. It's about as big as a sheep, but massive and lumbering, with the feet of an alligator, the chest of a gorilla, the tail of a pollywog and the head of a giant snake, with fangs and high-set eyes and nostrils that speak of its waterborne ancestry. Oh yes, it is a therapsid.

He had found the lystrosaurus from India to South Africa. He was coming to view it as an old pal. And then one day in 1968 someone brought him a bone he had spotted in Antarctica. Was it a fossil bone? the man asked. Colbert unwrapped the four-inch fragment from its cotton. And almost jumped out of his skin himself. It was a fossil bone.

Now it is the chill evening of Dec. 4, 1968, on the slopes of Coalsack Bluff, Antarctica. Colbert and his colleagues have found some fossil bones already, but mere bones are tricky to identify sometimes. He returns to camp, starts to brush and clean the specimens of the day. Suddenly he does a double take. There it is, right in his hand: the jawbone of a lystrosaurus. With a tusk still in place . . .

He wrote in his notebook, "There can be no doubt that Antarctica was once in contact with other continental blocks." Dr. Laurence M. Gould, dean of Antarctic scientists and a veteran of the 1929 Byrd expedition, said rather more. He called the discovery "not only the most important fossil ever found in Antarctica but one of the truly great fossil finds of all time."

Colbert, who now works out of Flagstaff, Ariz., spoke at the therapsid conference yesterday on plate tectonics. He was pretty calm about the whole Antarctica business. "We went down there and we found the thing and that was it," he said. So much for rewriting the map of the world.