VOA慢速英语--研究称食草恐龙到达北半球的时间较晚

Study: Plant-eating Dinosaurs² Arrived Late to Northern Half of the World

Plant-eating dinosaurs probably arrived in the northern half of the world millions of years after meat-eating dinosaurs, a recent study found. Their late arrival was likely a result of climate changes that took place hundreds of millions of years ago.

The study used a new way to estimate the age of dinosaur¹ remains³ found in Greenland. Researchers found that the plant-eating dinosaurs were about 215 million years old. The mineralized remains of bones are called fossils⁴. Earlier, those fossils were thought to be as old as 228 million years.

The findings appeared in Proceedings⁵ of the National Academy of Sciences of the United States of America. They could change how scientists think about dinosaur movements, or migrations⁶.

The earliest dinosaurs all seemed to first develop about 230 million years ago or longer. They appeared in what is now South America. Then, the creatures moved north and spread all over the world. The new study suggests not all dinosaurs moved at the same time.

So far, scientists have not found any evidence of plant-eating dinosaurs living in the Northern Hemisphere⁷ before 215 million years ago. One of the best examples of these is the Plateosaurus, a two-legged, 7-meter-long plant-eater that weighed around 4,000 kilograms.

However, meat-eaters lived worldwide by at least 220 million years ago, said Randy Irmis of the University of Utah. Irmis was not part of the research.

Dennis Kent of Columbia University was the lead writer of the study. The plant-eaters "were late comers in the Northern Hemisphere," he said. Then he asked, "What took them so long?"

By studying the atmosphere and the climate of the time period, Kent believes he has found an answer. During the Triassic period, 230 million years ago, carbon dioxide levels were 10 times higher than now. The Earth was hotter with no ice sheets at the poles and two areas of extreme deserts north and south of the equator⁸, he said.

It was so dry in those areas that there were not enough plants for the plant-eaters to survive. But there were enough insects so that meat-eaters could, Kent said.

About 215 million years ago, carbon dioxide levels dropped. The drop resulted in more plant life in the deserts. The plant-eaters were then able to make the trip. Kent and other scientists said Triassic changes in carbon dioxide levels were from volcanoes and other natural forces. The changes were different from our time, when the burning of coal, oil and natural gas are considered the main causes of climate change.

Kent used changes in Earth's magnetism⁹ to find the age of fossils in Greenland. Several outside experts both in dinosaurs and ancient climate said this shows that there was a time difference between the migrations of the two kinds of dinosaurs.

Irmis described Kent's theory about how ancient climate affected¹⁰ dinosaurs as "super cool because it brings it back to contemporary issues." Contemporary is a term that means happening or beginning in recent times.

Hans-Otto Portner of the Alfred Wegener Institute in Germany also was not part of the study. Portner suggested that the study's findings fit with some animals behaviors today.

Paul Sereno of the University of Chicago said that the study makes sense. But, he added, it does have one possible weakness.

Just because researchers have not found evidence of fossils older than 215 million years does not mean there were no plant-eaters in the Northern Hemisphere. The fossils just may not have survived, he noted¹¹.

Words in This Story

hemisphere -- n. a half of the Earth

sheet -- n. a wide, flat surface or area of something

pole – n. either end of the imaginary line around which something (such as the earth) turns

magnetism –n. the property of attracting certain metals : the attracting property of a magnet

cool –adj. appealing, approved of

1 dinosaur   

n.恐龙

参考例句：

• Are you trying to tell me that David was attacked by a dinosaur?你是想要告诉我大卫被一支恐龙所攻击？
• He stared at the faithful miniature of the dinosaur.他凝视著精确的恐龙缩小模型。

2 dinosaurs   

n.恐龙( dinosaur的名词复数 )；守旧落伍的人，过时落后的东西

参考例句：

• The brontosaurus was one of the largest of all dinosaurs. 雷龙是所有恐龙中最大的一种。 来自《简明英汉词典》
• Dinosaurs have been extinct for millions of years. 恐龙绝种已有几百万年了。 来自《简明英汉词典》

3 remains   

n.剩余物，残留物；遗体，遗迹

参考例句：

• He ate the remains of food hungrily.他狼吞虎咽地吃剩余的食物。
• The remains of the meal were fed to the dog.残羹剩饭喂狗了。

4 fossils   

n.化石( fossil的名词复数 )；老顽固；食古不化的人；老古董（老人）

参考例句：

• fossils over two million years old 两百多万年的化石
• The geologist found many uncovered fossils in the valley. 在那山谷里，地质学家发现了许多裸露的化石。 来自《简明英汉词典》

5 proceedings   

n.进程，过程，议程；诉讼(程序)；公报

参考例句：

• He was released on bail pending committal proceedings. 他交保获释正在候审。
• to initiate legal proceedings against sb 对某人提起诉讼

6 migrations   

n.迁移，移居( migration的名词复数 )

参考例句：

• It foundered during the turmoils accompanying the Great Migrations. 它在随着民族大迁徙而出现的混乱中崩溃。 来自辞典例句
• Birds also have built-in timepieces which send them off on fall and spring migrations. 鸟类也有天生的时间感应器指导它们秋春迁移。 来自互联网

7 hemisphere   

n.半球，半球地图

参考例句：

• This animal is to be found only in the Southern Hemisphere.这种动物只有在南半球才能找到。
• In most people,the left hemisphere is bigger than the right.多数人的左脑比右脑大。

8 equator   

n.赤道，（平分球形物体的面的）圆

参考例句：

• Singapore is near the equator.新加坡位于赤道附近。
• The United States is north of the equator.美国位于赤道以北。

9 magnetism   

n.磁性，吸引力，磁学

参考例句：

• We know about magnetism by the way magnets act.我们通过磁铁的作用知道磁性是怎么一回事。
• His success showed his magnetism of courage and devotion.他的成功表现了他的胆量和热诚的魅力。

10 affected   

adj.不自然的，假装的

参考例句：

• She showed an affected interest in our subject.她假装对我们的课题感到兴趣。
• His manners are affected.他的态度不自然。

11 noted   

adj.著名的，知名的

参考例句：

• The local hotel is noted for its good table.当地的那家酒店以餐食精美而著称。
• Jim is noted for arriving late for work.吉姆上班迟到出了名。