When he started, he assumed that signs of the bends would be rarer in younger fossils, reflecting their gradual evolution of measures to deal with decompression.Instead，he was astonished to discover the opposite.More than 15% of Jurassic and Cretaceous ichthyosaurs had suffered the bends before they died，but not a single Triassic specimen(标本)showed evidence of that sort of injury.

　　If ichthyosaurs did evolve an anti－decompression means, they clearly did so quickly—and, most strangely, they lost it afterwards.But that is not what Dr Rothschild thinks happened.He suspects it was evolution in other animals that caused the change.

　　Whales that suffer the bends often do so because they have surfaced to escape a predator (捕食动物)such as a large shark.One of the features of Jurassic oceans was an abundance of large sharks and crocodiles，both of which were fond of ichthyosaur lunches.Triassic oceans，by contrast，were mercifully shark－ and crocodile－free.In the Triassic, then, ichthyosaurs were top of the food chain.In the Jurassic and Cretaceous，they were prey(猎物)as well as predator—and often had to make a speedy exit as a result.

　　61．Which of the following is a typical symptom of the bends?

　　A．A twisted body.

　　B．A gradual decrease in blood supply.

　　C．A sudden release of nitrogen in blood.

　　D．A drop in blood pressure.

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