東京外大 2015年前期 3

Dinosaurs weren't quite like cold-blooded reptiles, but【　①　】 . Instead, they fell right in the middle. Comparisons with modern animals reveal that dinosaurs' metabolisms probably resembled those of great white sharks, researchers report in the June 13 Science.

The findings offer new clues into how the animals lived and also rekindle a long-standing debate. 'This paper will make us go back to the drawing board," says paleobiologist Martin Sander of the University of Bonn in Germany.

For years, paleontologists assumed that 【　②　】and other cold-blooded creatures, or ectotherms: slow-growing, low-energy sluggards that bask in sunlight for heat and don't need much food. "When I was a kid, dinosaurs were just scaled-up, tail-dragging reptilian brutes," says Gregory Erickson, a paleobiologist at Florida State University in Tallahassee.

The field took a U-turn in the 1960s, he says, when【　③　】 . Over the next few decades, most paleontologists came to think of dinosaurs as more birdlike: warm-blooded animals, or endotherms, that grew quickly, expended lots of energy and regulated their body heat internally. That thinking inspired popular depictions such as the speedy beasts of Jurassic Park. But trying to fit dinosaurs into one of two categories might be too simplistic, says John Grady, a paleoecologist at the University of New Mexico in Albuquerque.

Previous work had hinted that 【　④　】. So Grady and colleagues designed a massive study to pinpoint dinosaurs' place on the spectrum of cold- and warm-blooded life.

His team tabulated the growth rates and energy use, or metabolism, of 353 modern animal species. The census included everything from slow- growing, low-metabolism crocodiles to fast-growing, high-metabolism ostriches. Then the researchers capitalized on other paleontologists' careful analyses of dinosaur bones to collect the growth rates of 21 dinosaurs, includingTyrannosaurus and Apatosaurus.

Grady and his team couldn't determine the metabolic rates of creatures that have been extinct for at least 65 million years, but【　⑤　】 . When Grady plotted the animals' growth rates against their metabolisms, he found a clear link: Those with high growth rates tended to have high metabolisms and vice versa. This strong correlation allowed him to chart the 21 dinosaurs on the same graph.

The animals fell right between cold-blooded animals and warm-blooded ones. "I was a little surprised to see dinosaurs in the middle," Grady says. "If they're not like reptiles and they're not like mammals, then what the heck are they?"

【　⑥　】.T-Rex and other dinosaurs may have had metabolisms similar to those of great white sharks, tuna and leatherback sea turtles, Grady says. These animals, called mesotherms, eat more than cold- blooded fish and reptiles do, but【　⑦　】.

Understanding dinosaurs' metabolic peculiarities could offer clues into other debated aspects of the animals' lives, such as【　⑧　】 Grady says.

ア dinosaurs most resembled modern reptiles

イ dinosaurs did match up with a few living animals

ウ how they hunted and why they grew so large

エ researchers found similarities between dinosaurs and white sharks

オresearchers started to find similarities between dinosaurs and modern birds

カ the animals might not sort so cleanly into either group

キ they could make estimates based on data from living animals

ク they don't stick tightly to a set body temperature like warm-blooded birds and mammals

ケ they weren't like warm-blooded birds either