VOA慢速英语:科学家计算出地球上共有过25亿只霸王龙

A new study suggests that up to 2.5 billion Tyrannosaurus Rex dinosaurs lived on Earth over a period of a few million years.

However, the research shows that not many of these powerful creatures lived in the same place at the same time.

Tyrannosaurus Rex, commonly known as T. rex, existed on Earth for millions of years. Scientists involved in the study estimate about 2.5 billion of them lived over a period of 2.4 million years. The dinosaurs mostly lived in the area of the world now known as North America.

A team at the University of California at Berkeley came up with a method to estimate how many T. rex, called "the dinosaur king," had ever lived. The findings were recently published in the journal Science.

The method aimed to establish a relationship between body size and population density. The scientists based their research on the theory that the larger an animal is, the smaller its population will be.

To come up with an overall number, the team considered the dinosaur's body size, sexual maturity and energy needs. Experts say it is the first time that such an estimate has been made.

Some scientists who study dinosaurs said they are very interested in the new findings.

Kristi Curry Rogers is a paleobiologist at Macalester College. She said she "did a double-take" when she saw the number.

Charles Marshall was the main writer of the study. He is the director of the University of California Museum of Paleontology.

"That's a lot of jaws," he said. "That's a lot of teeth. That's a lot of claws."

The researchers said a city the size of Washington D.C. would likely have had only two T. rex living in the area. They estimated that around 4,000 would have lived on a piece of land the size of a large state, like California.

The estimates are useful for scientists who study dinosaurs because it helps them understand the percentage of T. rex bones, or fossils, that have survived.

Marshall said there have only been about 100 identifiable T. rex fossils ever found. That means that if there had only been 2.5 million of the dinosaurs, humans might never have learned about them.

The scientists considered that a T. rex lived about 28 years and that it had food needs somewhere between a large lizard and a lion.

Since all the numbers are just estimates, the scientists say the results are not perfect. They think it is possible the actual number of T. rex was somewhere between 140 million and 42 billion. The middle number is about 2.4 billion.

Some people may question why scientists choose to research animals that lived so long ago. James Farlow, a geology professor from Purdue University offered his opinion. He said he believes the study is important, but also, "this kind of thing is very cool."

I'm Dan Friedell.

Seth Borenstein of The Associated Press wrote this story. Dan Friedell adapted it for Learning English. Bryan Lynn was the editor.

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