Name: 霸王龙如何丢下手臂（How the T-Rex Lost Its Arms）
Uploaded: 2021-05-03T04:22:54.000Z
Duration: 7 min 41 s
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If there are three things any 6-year-old knows about dinosaurs, it's that: Tyrannosaurus

rex was big, Tyrannosaurus rex was vicious, and Tyrannosaurus rex had tiny arms.

T. rex dominated the Western United States during the Cretaceous Period, from 68 million

And in its day, was the largest carnivore on land.

But for such a fearsome predator, it certainly had its quirks – like those teeny tiny arms,

the source of so many jokes about how a tyrannosaurus couldn't do push ups, or give a decent high

Even from a biological perspective, they look pretty small.

The average T rex weighed about 10,000 kilograms, and stood between four and a half to 6 meters

But its two-fingered forelimbs were less than one eighth as long as its hindlimbs – probably

And the story of how T-Rex lost its arms is, itself, pretty simple.

But the story of why it kept those little limbs, and how it used them?

The short answer as to why T. rex had tiny arms was, well, they ran in the family.

Early ancestors of the genus Tyrannosaurus included the prosauropods, which first appeared

way back in the Triassic, and had forelimbs just as long as their hindlimbs.

But as time went on, more and more of these early dinosaurs became bipedal.

And once they were standing on their hind legs, their front limbs were suddenly freed

And many of the early carnivorous bipedal dinosaurs had sharp claws on their forelimbs,

which made them more useful for hunting with than for standing on.

But the group that wound up producing Tyrannosaurus rex took an evolutionary right turn somewhere

Instead of relying on their forelimbs to help them hunt, members of the superfamily

known as Tyrannosauroidea - which includes Tyrannosaurus and its relatives - started

One of the earliest Tyrannosaurs is Guanlong,

a small, 150-million-year-old dinosaur from China.

Guanlong had a tiny head, and long forelimbs - well, long for a Tyrannosaur anyway, about

But fast forward about 25 million years in China and you find Raptorex , which had the

same basic body plan, but with different proportions.

Raptorex was still only about 1 meter tall, but its head was about one-third larger than

Guanlong's, and its forelimbs were about half as long.

So, Raptorex looked a lot like a tiny, pocket-sized version of Tyrannosaurus rex – but it shows

up in the fossil record about 70 million years before T. rex did.

That means that the characteristics we think of as defining T-rex -- namely a huge head

and tiny forelimbs – were actually part of a 90-million-year trend in that direction.

Its enormous jaws gave T. rex a bite force up to 57,000 Newtons, which is enough power

With a bite that nasty, having long forelimbs really wasn't necessary.

But beyond that, long arms weren't just “not needed” – for some dinosaurs, they

That's because of how bipedal dinosaurs walked.

We swing our arms when we walk and run, to help us maintain balance to keep our upper

But dinosaurs like tyrannosaurs had long, rigid tails, which acted as stabilizers.

If anything, having big forelimbs would have just slowed them down.

From what we know about the mechanics of their bones and musculature, we know that the natural

body posture for many tyrannosaurs was with their forelimbs tucked close to their body.

Some specimens have even been found in this position as fossils!

This type of research, by the way, is also how we know that Tyrannosaurus and other carnivorous

dinosaurs held their forearms with their palms facing each other, and that they didn't

have the wrist mobility to hold them facing down.

So, when you're doing your T. rex impression, make sure you're like ahhhh

And if you want to be totally accurate about it -- and of course you would, because why

-- then you should only use your thumb and your forefinger, because those two digits

are the ones that T. rex retained. So like ahhhhhhh

So that's how Tyrannosaurus rex, or Tyrannosauroidea in general, lost their big arms: Over 90 million

years, their heads got bigger, as their jaws became their primary weapons.

And their forelimbs got smaller, because that probably made it easier for them to move around.

If they were such a liability, why didn't it just lose them entirely?

Initially, paleontologists thought that T. rex forelimbs were vestigial, just leftover

relics of true arms that were so small as to be useless.

But after lots of study of some well-preserved fossil finds, we've learned that their forelimbs

So, while they weren't very strong, their arms could move.

One of the most common theories is that T. rex used its forelimbs during mating.

Studies on the biomechanics of its arms have shown that they were pretty powerful at adduction,

which is the technical term for the movement you make when you hug something.

So, adduction could've been used either as part of courtship – cuz who doesn't

like a good cuddle?! -- or to help hold male Tyrannosaurus in place during mating.

However, other scientists think T. rex could have used its limbs not for loving but for

Its fingers, after all, were tipped with claws that were about a quarter the length of the

Still, that's less than half as large as one of their teeth.

And although the arms were muscular, they didn't measure up to force of its bone-crunching

So if these arms were used to slash at prey, it was only as a supplement for the killing

Others have suggested that T. rex used its hugging skills to kill, planting its forelimbs

into its prey while biting down, dealing a death blow and a death hug at the same time.

And of course, it's always possible that there wasn't actually any use for these

Maybe T. rex arms were what we call a spandrel – a byproduct of other evolutionary changes

that doesn't necessarily have a purpose or advantage of its own.

Over millions of years, Tyrannosauroidea followed an evolutionary course toward having smaller

So maybe, if they hadn't gone extinct, the descendants of T rex would have had gone one

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霸王龙如何丢下手臂（How the T-Rex Lost Its Arms）

perspective

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