雅思课外读物--Missing link in tyrannosaur family tree discovered

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1.7亿年-1亿年前的暴龙(tyrannosaur)体型很小，奔跑在天地之缘。然而，到8000万年前，暴龙却成了体积最大、最恐怖的捕食者。那么，在这2000万年间，到底发生了什么?新发现试图填补暴龙家族史中这缺失的一环。

A new species(物种) of tyrannosaur(暴龙)adds to the fearsome group of

dinosaurs(恐龙)to which Tyrannosaurus rex belonged. Scientists found its

90-million-year-old bones in the harsh deserts of Uzbekistan.

The newly described species, Timurlengia euotica, is an oddball(古怪的人或物). The

horse-sized dinosaur resembles(与……相似)earlier,smaller tyrannosaurs. Yet its large

brain and advanced ears—their outlines preserved(保存)in a skull

fragment(碎片)—mirror those of later tyrannosaurs like T. rex.

“Timurlengia is truly a curveball(曲线球;难以把握的事物),”says Thomas Carr of Carthage

College, who wasn’t involved with the study. “I had to readjust the way I

thought about tyrannosaurs. It’s a weirdo(古怪的人或物).”

A Growing Mystery

The new species, described Monday in the Proceedings of the National Academy

of Sciences, could also help untangle(解开谜团)one of the most frustrating questions

in dinosaur evolution: How and why did tyrannosaurs get so big?

Tyrannosaurs first appear in the fossil record about 170 million years ago,

and until about 100 million years ago, they scampered(惊慌地奔跑)about on the

ecological margins(边缘), some no bigger than dogs.

But by the late Cretaceous(白垩的，白垩纪的)—about 80 million years ago—tyrannosaurs

such as T. rex had ballooned into some of the biggest, most intimidating(令人害怕的)

land predators(捕食者) ever to walk the planet. Head to tail, they could get as

long as a bus, and their enormous heads and powerful jaws could generate bites

with nearly 13,000 pounds of force.

“The funny thing is that T. rex is such a pop star,” says Lawrence Witmer of

Ohio University, who wasn’t involved with the study. “None of us can even

remember a time when we didn’t know about Tyrannosaurus. But the reality is that

how [it] evolved is a bit of a mystery.”

What’s more, T. rex wasn’t just big. It was gifted.

“Contrary to what people learned in the first Jurassic Park movie, T.rex had

acute senses,” says study co-author Hans-Dieter Sues of the Smithsonian National

Museum of Natural History. “It clearly had very keen eyesight, a spectacularly

keen sense of smell, [and] keen hearing.”

Its killer senses stemmed from(来源于……)its impressive brain: T. rex’s two

olfactory(嗅觉的)lobes—the brain regions devoted to smell—were each the size of a

grapefruit. And its long, looped ear canals helped it hear the low-frequency

sounds emitted(发出)by distant prey’s footsteps.

“It was sort of a superpredator,” says Sues.

But paleontologists have struggled to pin down how tyrannosaurs got brainier

and brawnier, largely because the fossil record between 100 and 80 million years

ago, when small tyrannosaurs would have evolved into giants, is so

sparse(稀缺的).

“At that particular point in time, sea levels were very high,” says Sues. “We

just didn’t have the record from the land animals.”

Digging for Answers

The fall of the Soviet Union, however, presented researchers with a new

opportunity: the Bissekty Formation, a site in what’s now Uzbekistan with

fossils between 92 and 90 million years old. Soviet paleontologist Lev Nesov had

investigated the site since the mid-1970s, but as the old regime dissolved(溶解),

Nesov,his Ph.D. student Alexander Averianov, and colleague J. David Archibald of

SanDiego State University planned a major excavation(挖掘;出土)of the site—one that

the National Geographic Society eventually would help fund in 1997 and 1998.

Bigger Than T. rex: Spinosaurus Fossils discovered in 2014 revealed that

Spinosaurus, bigger than T. rex, was an excellent swimmer, unlike any other

dinosaur. Found in the sands of Morocco,the bones suggest that, unlike its

land-dwelling cousins, this meat-eating creature likely fed on sharks and other

fish.

The team worked in difficult conditions.“The highest temperature I remember

was 43 degrees Celsius (109 degrees Fahrenheit) in the shadow,” wrote Averianov,

a co-author of the study. Sues recalls that it once got so hot inside his tent,

his tooth paste’s fluoride stripes separated out, liquified by the intense

heat.

In 2004 the team found an intriguing bone lump the size of a grapefruit. It

was part of a braincase, which would have anchored the dinosaur’s neck muscles

and protected its brain and ear canals.

The braincase remained with Averianov for ten years, stored in a cardboard

box in the Zoological Institute of the Russian Academy of Sciences, until

tyrannosaur expert Steve Brusatte saw it in 2014.

“When I looked at it, it struck me really quickly that this looked like a

tyrannosaur braincase,” says Brusatte, of the University of Edinburgh.“Not

exactly T. rex, much smaller; the same bones in a T. rex would be bigger than a

basketball.”

Detailed CT scans proved Brusatte right. For instance, they revealed that

Timurlengia, like later tyrannosaurs, had remarkably long inner ear canals,

which would have endowed(赋予)it with superb low-frequency hearing, hence its

species name euotica, Greek for “well-eared.”

However, the braincase was short—more in keeping with Xiongguanlong, an early

tyrannosaur—and lacked some of the deep recesses and knobs in later tyrannosaurs

like T. rex.

“The braincase was really the Rosetta stone,” says Sues, “because it showed

that this animal had the same basic brain structure of the later T. rex but had

primitive brain structure, as well.”

Other bone fragments all pointed the same direction: to a new tyrannosaur

about the size of a horse, seemingly caught between the early and late

tyrannosaurs in terms of age, size, and sensory ability.

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