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第1段

1 .Many scientists have problems with the arguments you read in the passage.

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2 .They don't think those arguments prove that dinosaurs were endotherms.

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3 .Take the polar dinosaur argument.

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4 .When dinosaurs lived, even the polar regions, where dinosaur fossils have been found, were much warmer than today, warm enough during part of the year for animals that were not endotherms to live.

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5 .And during the months when the polar regions were cold, the so-called polar dinosaurs could have migrated to warmer areas or hibernated like many modern reptiles do.

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6 .So the presence of dinosaur fossils in polar regions doesn't prove the dinosaurs were endotherms.

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7 .Well, what about the fact that dinosaurs have their legs placed under their bodies, not out to the side like crocodiles.

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8 .That doesn't necessarily mean dinosaurs were high-energy endotherms built for running.

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9 .There is another explanation for having legs under the body.

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10 .This body structure supports more weight, so with the legs under their bodies, dinosaurs can grow to a very large size.

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11 .Being large had advantages for dinosaurs, so we don't need the idea of endothermy and running to explain why dinosaurs evolved to have their legs under their bodies.

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12 .Ok, so how about bone structure?

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13 .Many dinosaur bones do have Haversian canals, that's true.

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14 .The dinosaur bones also have growth rings.

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15 .Growth rings are thickening of the bone that indicates periods of time when the dinosaurs weren't rapidly growing.

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16 .These growth rings are evidence that dinosaurs stopped growing or grew more slowly during cooler periods.

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17 .This pattern of periodic growth, you know, rapid growth followed by no growth or slow growth, and then rapid growth again, is characteristic of animals that are not endotherms.

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18 .Animals that maintain a constant body temperature year-round as true endotherms do grow rapidly even when the environment becomes cool.

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