Why did history's biggest, strongest and most aggressive creatures die out 65 million years ago?

Were dinosaurs' brains so small that they clung to a diet of poisonous plants that did them all in? Did they grow so big they were no longer able to migrate or even procreate? Were dinosaurs victims of a nearby supernova that flooded earth with so much radiation that the only creatures to survive were those agile enough to hide from it?

These have been the pet theories on dinosaurs extinction for the last 100 years . . . until now.Fresh scientific scrutiny and new fossil evidence from deep beneath the seas and from as far afield as Pataronia in Argentina and the Gobi Desert in Mongolia strongly suggest that dinosaurs suffered none of these fates.

"The single catastrophic event can no longer explain extinction," says Johns Hopkins University's Dr. Steven M. Stanley, one of a new breed of paleobiologists out to rewrite the history of life. Echoes 29-year-old Stephen Jay Gould of Harvard University: "These deys exmachina theories were cited out of utter frustration. They were used to explain something felt to be totally mysterious."

To hear these new natural historians tell it, there is less mystery about dinosaur extinction today than there has been at any time in the last 100 years.

There is less mystery today about any of the mass creature extinctions of the last 225 million years, only one of which included dinosaurs but all of which involved the largest animals then living on land and in the seas.

"The one thing the extinctions have in common is that the large animals were driven out preferentially," said John Hopkins. Dr. Robert T. Bakker, a young graduate of Yale and Harvard who has spent his last six years studying mass extinction. "Not the little ones and not the plants . . . the large animals were the onew who caught it."

Naturalists have documented seven extinctions, starting with the Permian period 225 million years ago and moving through the Triassic, Jurassic, Cretaceous, Paleocene, Eocene and Miocene less than 20 million years ago.

The most massive extinctions came at the end of the Permian and the end of the Cretaceous. Half the earth's marine life was gone by the end of the Permian. One fourth of the remaining marine species disappeared at the end of the Cretaceous 65 million years ago. So did at least one fourth of the land animals.

Gone were all the biggest creatures. The flying reptiles, the big lizards, the seagoing reptiles and the dinosaurs. When the last dinosaur disappeared, large mammals begans to appear, but by the end of the Eocene most of the large land animals were gone too. By the start of the Miocene the primitive whales were missing from the seas.

Why? In each of the extinctions the oceans around the world fell hundreds of feet, draining the shallow coastal seas that had been built up by the spreading and bulging of the ocean floor. The shallow sea covering the western half of North America dried up at the end of the Cretaceous, just as the dinosaurs disappeared.

From time to time, naturalists would tie sea withdrawal to marine extinction but never to land extinctions. Most times, they sought simpler explanations like nearby supernova explosions. They often invoked climate change, which they said dried up food and water supplies on land and made it harder for life to survive in the sea.

"I don't think climate has ever caused extinction," said Johns Hopkins' Bakker. "The climate has changed throughout the history of life and I can't believe that dinosaurs or big mammals who have the widest tolerance for climate have been killed off by shifts in the climate."

Today's naturalists believe all land and sea extinctions were triggered by declines in the sea and they have fresh evidence to back up their beliefs.

In the last three years, the research vessel Glomar Challenger has drilled countless holes at least two mies into the floors of the Pacific Ocean, the Gulf of Mexico, the Caribbean Sea and the Atlantic Ocean, lifting tons of sediment and fossils that go back 180 million years and cover five of the seven major extinctions.

The cores raised by the Glomar show that extinctions coincide with sharp decreases in fossil numbers for animals living in the shallow inland seas, meaning fewer and fewer animals survived while the shallow seas drained into the oceans. Ocean sediments linked to extinctions all turn remarkably black, mostly because of a lack of oxygen in the oceans during extinctions.

Whatever oxygen persists in the oceans during extinctions appears to move down from the upper oceans into the depths. The evidence for this is a sharp decrease in ocean fossils during extinctions, but only for the animals living in the upper oceans. Fossil numbers actually increase for animals who dewll on ocean floors.

Piecing it together, scientists think that during extinctions the oceans fall and drain the shallow seas, which cools the oceans nearest the coastal plains and the land left dry by the drainage. It also triggers change in the circulation of ocean currents, making the water colder in the polar regions and then carrying that colder water toward the Equator.

Records show that palor ocean temperatures were much warmer in those periods than they are today. Glomar sediments show that ocean temperatures everywhere fell an average of 20 degrees during extinctions.

"None of this tells us why the oceans shrink when they do," said Dr. Alfred G. Fischer of Princeton University, where many of the Glomar sediments have been analyzed. "But the periods of major crisis in animal life seem associated with sea-level drops."

Fresh evidence matches extinction of large land animals with the drainage of shallow seas. New fossil dinosaurs from the semi-arid plains of Argentina and Mongolia's Gobi Desert and new looks at old North American and European fossils all reveal that entire dinosaur families declined as the inland seas were drained.

Drainage of the shallow seas meant colder and blander landscapes. Gone were the lush subtropical forests as far north as Canada that kept dinosaurs in one place. Gone were barriers like the Bering Straits, which kept North American dinosaurs from mixing with Mongolian dinosaurs.

Open were new passages through Greenland from North America to Europe, which encouraged different dinosaurs to come together for the first time. Three things happened when dinosaurs began to mix. They competed for the same food. They fought and began to kill each other. They mated and bagan to reproduce the same animal.

The result of this stange mix of animal war and inbreeding was fewer species of dinosaur, who themselves became more vulnearble to mundane causes of extinction like disease. Parasites may have begun to wipe out entire dinosaur families, whose numbers were not being replaced by new speche textbook of the last 100 years.

There are other fresh findings to explain dinosaur extinction. The most significant is that a majority of today's naturalists believe dinosaurs were warm-blooded and more like mammals than the cold-blooded lizards they've been compared with in the textbook of the last 100 years.

Not that some cold-blooded lizards didn't survive the Cretaceous extinction. Crocodiles did. So did turtles and some snakes. A few prehistoric lizards still live on islands off New Zealand and South America. But animals the size of dinosaurs would have had far less a chance of surviving an extinction period if their blood had been warm instead of cold.

The evidence that dinosaurs were warm-blooded is "overwhelming," to hear John Hopkins paleobiologist Bakker tell it.

Bakker starts with dinosaur bones, extremely fine structures shot through with the kinds of canals and capillaries that would carry blood in a warm-blooded animal with a high metabolism. Bones from a dinosaur's leg are hard to tell apart under a microscope from the human femur, the long bone that extends from the hip to the knee.

Reptiles and lizards have bones that are coarse, almost solid. Few blood vessels are needed to carry blood through lizard bones because lizards burn such little energy. Lizards are never found in cold climates because they're unable to regulate body temperatures. The bones of dinosaurs have been found as far north as the Yukon, 500 miles from the Arctic Circle.

For the last six years, Bakker has made an exhaustive study of the North American fossil community to match the numbers of dinosaur predators against the numbers of animals they needed so much food to feed their thing for predator lizards and reptiles and has found that dinosaurs made up 3 per cent of the community they shared and that lizards were 60 per cent of their fossil community.

To Bakker, this means that dinosaurs were a minority group because they needs so much food to feed their burning metabolic engines, fully 10 times the amount of food the cold-blooded lizard needed. It means that dinosaurs killed and ate prey at as high a rate as birds and mammals, whose warm blood helped them survive change on earth and escape extinction

Then why did the warm-blooded dinosaur succumb to extinction? Dr. Loris Russell of the University of Toronto and Dr. John H. Ostrom of Yale University point out that mammals had fur and birds had feathers to insulate against the cold. There is no evidence that dinosaurs had anything like fur or feathers. It could be that an Ice Age was the crowning blow that did in the last dinosaurs.

Bakker believes that dinosaurs died out at least in part because they were warm blooded. He believes that the great size (up to 80 feet long) and traveled such great distances that they needed more and more food to feed their engines. Bakker does not believe they ran out of food. He believes they wandered so far afield that they stopped producing new species to replace those that died out naturally.

The last dinosaur was Triceratops, the rhinoceros-like animals with great long horns above the eyes, a single horn above the nose and a long bony shield over the neck. It was 20 feet long and may have weighed 10 tons. Bakker doesn't believe there was anything dramatic about its extinction, which could have occurred over a period of 2 million years.

"I've gone out into the field every summer and seen dinosaur skeletons that are 78 feet long and individual bones that are over six feet long," Bakker says in wonder and puzzlement. "What's unique about the last dinosaur is not that it died or why it died. What's unique is that nothing new moved in to take its place."