Cretaceous Fairy Tales

In one episode of the classic Walt Disney film Fantasia, the dinosaurs take over the earth to the impassioned strains of Stravinsky's Rite of Spring. An adorable clutch of baby tricerotopses hatches from eggs; sloe-eyed brontosauruses wade in marshes; a bony-backed stegosaurus struggles for its life against the meat-eating Tyrannosaurus rex. But as the years flash by, the world mysteriously grows hotter and more violent: swamps evaporate, earthquakes trigger giant tidal waves, and the princely reptiles crawl across an encroaching desert to meet their certain doom.

Disney's celluloid fantasy is scarcely more elaborate than some of the theories that researchers have concocted to explain the demise of the dinosaurs at the end of the Cretaceous period, 65 million years ago. Some of the more notable flights of fancy, while capturing the public's imagination, have strained scientists' credulity. Yet, complains Physicist Richard Muller, "these are the theories kids are taught by their elementary school teachers."

Perhaps the most popular of these Cretaceous creations deals with eggs. It holds that small mammals appearing during the first half of the dinosaurs' reign stole and ate all the reptiles' eggs; the dinosaurs could not fight back effectively because the warm-blooded thieves were too fast and could easily dash into crevices for protection. This theory might account for the fact that so few fossilized dinosaur eggs have been found, but it does not explain how the dinosaurs were then able to coexist with mammals for so long a time--more than 100 million years.

One hoary belief involves dinosaur stupidity: the hapless creatures died out because their bodies continued to grow bigger while their brains remained small. Indeed, cranium measurements seem to indicate that at least some species were not terribly cerebral: one type of brontosaurus, for example, weighed about 30 tons, and probably had only a half-pound brain. If the dinosaurs did indeed become progressively less intelligent, the theory goes, they would have lost the ability to adapt to changes in the environment.

Even those dinosaurs known to have proportionately larger brains like tyrannosaurus may have simply been too massive to survive on land. How could their bulk have been lethal? According to one suggestion that many weekend athletes can identify with, the dinosaurs suffered from slipped disks, which left them unable to forage for food. Great heft could even trigger infertility. In 1946 a paleontologist concluded that because large animals do not shed excess heat as efficiently as small animals do, a temperature increase of just 2 degrees F could have baked the considerable testicles of a ten-ton male dinosaur enough to kill his sperm.

If the size of some dinosaurs did not do the trick, maybe their culinary habits did. They could have been fussy eaters, for example. "If they ate mainly one plant, just as the koala bear lives on eucalyptus," says James Hopson, a dinosaur expert at the University of Chicago, "they would be in trouble if that plant were no longer available." Or maybe dinosaurs were not picky enough. Perhaps they died from indiscriminately eating poisonous plants. Ronald K. Siegel, a psychopharmacologist at the UCLA school of medicine, points out that alkaloidproducing angiosperms began evolving toward the end of the dinosaur era. These plants produce toxic chemicals, but they also taste so bitter that most modern creatures know to eschew rather than chew them. If the dinosaurs lacked a palate, however, some of them may have died on an overdose of the poisons. Others have suggested that because the angiosperms replaced ferns, a possible dinosaur dietary staple containing laxative oils, the reptiles may have been the first creatures to succumb to constipation.

Unlike theories about catastrophic impacts, or changing climates and sea levels, these fanciful concepts all fail to account for the hundreds of other species that perished at the end of the Cretaceous. Says Physicist Luis Alvarez of the University of California, Berkeley: "The problem is not what killed the dinosaurs but what killed almost all the life at the time." Muller believes that comets are a far more plausible explanation for the death of the dinosaurs, but he wryly allows one other possibility. Says he: "Maybe there just wasn't enough room for them on the ark."