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　　進入專題：2015年聽力風暴來襲，英語四級每日一聽！

　　Tens of millions of years ago, an asteroid hit what is now the Yucatan Peninsula of Mexico. The event led to a global mass extinction that has been linked to the end of the dinosaurs. New research shows what that great disaster led to: a modern age of fish.

　　Sixty-six million years ago, the ocean was a different place. Sharks and octopus-like creatures were powerful. But, scientist Elizabeth Sibert says there were other important species as well.

　　"There were also many marine reptiles that were swimming around. And of course there were fish. Absolutely there were ray-finned fish around. It's just that they weren't very dominant."

　　Ms. Sibert is a graduate student at the University of California, San Diego. She and her professor compared micro-fossil fish teeth and shark scales in sediment that dated before and after the mass death.

　　Fish teeth and shark scales from around the mass extinction event 66 million years ago. (Credit: E. Sibert on Hull lab imaging system, Yale University)
　　Fish teeth and shark scales from around the mass extinction event 66 million years ago. (Credit: E. Sibert on Hull lab imaging system, Yale University)
　　"Basically the deeper down you get, the older you get. So we looked at sediments from 75 million years ago to about 45 million years ago, looking every 200,000 years in some cases, looking at every 10,000 years in other cases."

　　Fossil evidence showed that the ratio of fish to shark changed sharply after the asteroid hit. Elizabeth Sibert explains.

　　"We see that instead of having about equal number of shark fossils and fish fossils, we see that the fish fossils more than double and that trend continues while the shark fossils stay approximately the same."

　　The study suggests that the mass extinction killed many animals at both the top and bottom of the ocean food chain. This permitted animals in the middle of the chain to increase their numbers. Those animals are the ray-finned fish that represent nearly all fish species today.

　　"And they radiated into all these newly vacated spots, and possibly some new things that wouldn't have existed at all if the extinction hadn't happened."

　　Elizabeth Sibert says her next step is to go back to the micro-fossil record. She will try to learn from it how the fish dealt with other stresses in the ocean, like global warming.

　　I'm Marsha James.

　　Rosanne Skirble reported this story from Washington. Marsha James adapted it for Learning English. Caty Weaver was the editor.

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