Coldblooded or warmblooded? Paleontologists have tussled for more than 100 years over which camp dinosaurs belong in. The balance of evidence swings back and forth. "This is Friday, so I'll be on the side of endothermy [warmbloodedness]," says paleontologist David Weishampel of Johns Hopkins University. "But if you ask me on Tuesday, I won't be."
According to David Fastovsky of the University of Rhode Island, polar dinosaurs support the idea that dinosaur metabolism differed from that of modern reptiles. You just don't see reptiles in frigid climates today, he observes. Terrestrial reptiles reach massive dinosaur scale—the 25-foot anacondas and 20-foot crocs—only in the tropics. Alaska has, at most, garter snakes.
The evidence does favor warmbloodedness for some dinosaurs, says Museum Victoria's Tom Rich, who contributed to the debate by sawing pieces off two precious Australian specimens and sending them to the South African Museum in Cape Town. There, Anusuya Chinsamy-Turan scrutinized the Timimus and Leaellynasaura samples for lines of arrested growth, or LAGs, dark streaks visible when you look at a bone's cross-section under the microscope. Like tree rings, LAGs indicate that growth ceased temporarily. Modern reptiles that dwell in seasonal environments show LAGS, as do mammals that hibernate, but birds and other mammals typically don't.
Chinsamy-Turan found that Timimus had LAGS but Leaellynasaura didn't. Their absence doesn't prove that Leaellynasaura was warmblooded, and their presence in Timimus doesn't mark it as definitely coldblooded. But the disparity between the species indicates that they coped with cold in different ways, Rich notes. Timimus probably hibernated away the dark, chilly months, perhaps by taking refuge beneath vegetation or even underground—a strategy used by many coldblooded animals. (In Montana, paleontologists recently discovered the fossils of burrowing dinosaurs that perished in tunnels, giving credence to the hibernation notion.) In contrast, Rich speculates, Leaellynasaura remained active all winter, even if snow fell and ice sealed rivers and creeks; the animals could nibble leaves of the evergreens that predominated in the region, and they might have kept warm with a layer of fat.
There might be a happy medium, after all. Metabolically speaking, the animals might have fallen between today's lizards and mammals, says Fastovsky. If dinos weren't like today's ectotherms or endotherms, he says, that would explain why researchers have had such a hard time fitting them into either category.