Ifadiversurfaces(浮出水面)tooquicklyhemaysufferthebends（减压病）.Nitrogen(氮)dissolved(溶解)inhisbloodissuddenlyliberated（释放）bythereductionofpressure.Theconsequenceifthebubbles(气泡)accumul

Question

If a diver surfaces(浮出水面) too quickly he may suffer the bends（减压病）. Nitrogen (氮) dissolved (溶解) in his blood is suddenly liberated（释放）by the reduction of pressure. The consequence if the bubbles (气泡) accumulate in a joint is sharp pain and a bent body—thus the name. If the bubbles form in his lungs or his brain the consequence can be death.

Other air-breathing animals also suffer this decompression (减压) sickness if they surface too fast: whales for example. And so long ago did ichthyosaurs(鱼龙). That these ancient sea animals got the bends can be seen from their bones. If bubbles of nitrogen form inside the bone they can cut off its blood supply. This kills the cells in the bone and consequently weakens it sometimes to the point of collapse. Fossil (化石) bones that have caved in on themselves are thus a sign that the animal once had the bends.

Bruce Rothschild of the University of Kansas knew all this when he began a study of ichthyosaur bones to find out how widespread the problem was in the past. What he particularly wanted to investigate was how ichthyosaurs adapted to the problem of decompression over the 150 million years. To this end he and his colleagues traveled the world’s natural-history museums looking at hundreds of ichthyosaurs from the Triassic(三叠纪) period and from the later Jurassic(侏罗纪) and Cretaceous(白垩纪) periods.

When he started he assumed that signs of the bends would be rarer in younger fossils reflecting their gradual evolution（进化）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.

(　　)58. 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.

(　　)59. The purpose of Rothschild’s study is to see ______.

  A. how often ichthyosaurs caught the bends

  B. how ichthyosaurs adapted to decompression

  C. why ichthyosaurs bent their bodies

  D. when ichthyosaurs broke their bones

(　　)60. Rothschild’s finding stated in Paragraph 4 ______.

  A. confirmed his assumption           B. speeded up his research process

  C. disagreed with his assumption       D. changed his research objectives

(　　)61. Rothschild might have concluded that ichthyosaurs ______.

  A. failed to evolve an anti-decompression means

  B. gradually developed measures against the bends

  C. died out because of large sharks and crocodiles

  D. evolved an anti-decompression means but soon lost it

Answer 1

ABCA