The Dinosaur Riddle

What caused the mysterious extinction of the mighty dinosaur some 70 million years ago? Scientists have offered a lengthy catalogue of explanations, ranging from the inadequacy of the dinosaur's brainpower in the face of the earth's changing environment to a burst of lethal radiation from a nearby exploding star. Now a pair of experts have offered some other solutions to the old puzzle:

> Writing in Nature, Robert T. Bakker of Harvard's Museum of Comparative Zoology argues that unlike other reptiles, dinosaurs were probably warm-blooded creatures, like most birds and mammals. He cites, among other arguments, anatomical evidence that some dinosaurs may have been able to lope across the countryside at speeds of up to 50 m.p.h. The necessary energy, he says, could not have been mustered by cold-blooded animals; their metabolic systems do not work fast enough. With only their hairless skin to protect them, the warm-blooded dinosaurs were highly vulnerable to sharp drops in temperature. And according to some geological evidence, a global chill may have set in at the end of the Cretaceous period, when the dinosaurs were still flourishing. Too large to burrow into the earth and lacking other means of getting out of the cold, says Bakker, the big reptiles simply died of overexposure.

> Heinrich K. Erben of Bonn University's Institute of Paleontology bases his theory on a treasure trove of dinosaur eggs unearthed near Aix-en-Provence in Southern France. So many fossilized egg fragments were found there that Erben concluded that dinosaurs had used the site as a regular nesting place for thousands of years. Using a scanning electron microscope, he determined that the average thickness of the eggshells in the lower or older layers ranged from 1.7 to 2.6 mm., while the shells in the younger layers were only about half as thick. Such fragile eggs could easily become dehydrated and broken, killing the dinosaur embryo. Scientists now have evidence that DDT can interfere severely with the production of hormones involved in the making of eggshells, but such chemicals were obviously not around in the dinosaur's day. Erbea suggests that similar hormonal imbalances might have been caused by unusual stress. What stresses were there for dinosaurs in Late Cretaceous France?

Overcrowding, suggests the paleontologist. Geological signs, he says, show that Southern France was then becoming an increasingly desert-like region. Forced to take shelter in gradually shrinking oases, the big beasts were cramped for living space; such nerve-wracking conditions could have upset their hormone production until their eggs became too thin-shelled for their offspring to survive.

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