William Hammer of Augustana College in Rock Island, Illinois, digs at an elevation of 13,000 feet on the slope of Mount Kirkpatrick, about 400 miles from the South Pole. He has pried out the bones of Cryolophosaurus ellioti, a 22-foot-long meat-eater with a bony crest curving up from its forehead like a cowlick. He has also found fossil evidence of a prosauropod, an ancestor of enormous dinosaurs such as Brachiosaurus and Apatosaurus.
At the opposite end of the globe, on Alaska's North Slope, Anthony Fiorillo, a paleontologist from the Museum of Nature and Science in Dallas, has been exhuming bones from dinosaur graveyards along the Colville River. Although northern Alaska wasn't as cold 70 million years ago as it is today, winters would still have brought snow and ice. Back then, sharp-toothed relatives of Tyrannosaurus rex stalked the 35-foot-long, duck-billed dinosaur Edmontosaurus. The surprise from these finds, Fiorillo says, is that the same types of dinosaurs lived in Alaska as in toastier environments farther south, such as Montana and even Texas. So far, he hasn't unearthed any dinosaurs that appear to have lived only in frosty climes.
Dinosaurs had two choices when winter arrived—tough it out or try to escape. The question of how dinosaurs survived the polar cold has gotten entangled with the broader question of whether the ancient beasts were warmblooded (endothermic), like modern birds and mammals, or coldblooded (ectothermic), like modern reptiles. In a cold environment, endotherms keep their bodies warm enough for muscles to flex and nerves to fire by generating heat through their metabolism. Ectotherms, by contrast, warm their bodies by absorbing heat from their surroundings—think of a lizard basking on a rock. Endothermy isn't necessarily better, notes David Fastovsky of the University of Rhode Island. Endotherms have the edge in stamina, but ectotherms need much less food.
The prize discovery from Rich's Dinosaur Cove excavation suggests that Leaellynasaura stayed active during the long polar winters. A two-inch-long Leaellynasaura skull the color of milk chocolate is the closest to a complete dinosaur skull the team has found. The base remains partly embedded in a disk of gray rock scored by numerous grooves where Kool meticulously exposed the fossil with a fine needle. Enough of the bone is visible for Rich to analyze the size of the eye sockets. Hypsis generally had big eyes, but Leaellynasaura's are disproportionately large—perhaps so they could capture more light during the protracted murk of polar winters. Moreover, the back of the same skull has broken off to expose a mold of the brain, known as an endocast. Rich found that the dinosaur had bulging optic lobes, parts of the brain that process visual information. Leaellynasaura's optic lobes are larger than those from hypsis that lived in non-polar environments, suggesting that it had extra brainpower to analyze input from its big eyes.
Similarly, Fiorillo and Roland Gangloff, a retired paleontologist from the University of Alaska, have found that the small meat-eater Troodon was much more common on the North Slope of Alaska than farther south. Troodon might have gained an advantage over the other carnivorous dinosaurs in the north because it also had large eyes and a hefty brain, perhaps useful for hunting all winter long.
Other dinosaurs might have migrated south for the winter (or north, if they lived in the Southern Hemisphere). Rich says his dinosaurs would have made unlikely travelers. They were small, and an inland sea would have blocked their path to warmer climes. But Edmontosaurus, from Alaska's North Slope, is a better candidate for seasonal migration. Adults were about the size of elephants, so they would not have been able to crawl under rocks when temperatures fell. Rough calculations suggest that by ambling at about 1 mile per hour—"browsing speed" for animals of that size—herds of Edmontosaurus could have journeyed more than 1,000 miles south in three months, says paleobotanist Bob Spicer of the Open University in Milton Keynes, Britain. Such a migration would have taken them out of the "zone of darkness" and into areas where plants might have still been growing.
For his part, Fiorillo doubts it. He and Gangloff contend that juvenile Edmontosaurus grew too slowly to have tramped long distances. They couldn't have kept up with a herd, so the animals must have stayed put, regardless of temperatures. This kind of back-and-forth might be dizzying, but it's how science moves ahead, especially in paleontology, where researchers have to draw conclusions from small numbers of often-fragmentary fossils.
The dinosaurs had an impressive run. They settled every continent, grew bigger than any other land animals and lasted for more than 150 million years. And then they vanished. Their demise has spawned more than a little speculation about its cause. Scenarios range from disease or competition with mammals to the flyby of an as-yet-undetected companion to the sun, a kind of death star.
Most paleontologists have accepted another extraterrestrial killer, an asteroid more than six miles wide that socked Earth 65 million years ago. It gouged a crater more than 100 miles wide on what is now the Yucatán Peninsula in Mexico. According to the leading scenario, the impact threw huge amounts of dust and other debris into the atmosphere, blocking sunlight and sinking the Earth into darkness for weeks or even months. A global disaster certainly struck at the time, according to overwhelming fossil and geological evidence. As Fastovsky and Weishampel write in The Evolution and Extinction of the Dinosaurs, "the world's oceans were virtually 'dead'" as photosynthesis by plankton ceased and marine food webs unraveled. The dinosaurs died, while the ancestors of today's mammals, birds and reptiles hung on.
Paleontologists disagree about what the existence of polar dinosaurs says about the asteroid-winter scenario. Fiorillo says he is skeptical of it because "dinosaurs in Alaska were doing just fine in conditions just like that." He argues that climate changes caused by shifts in circulation of the atmosphere and oceans probably did in the dinosaurs.