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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

  • to 66 million years ago.

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

  • But for such a fearsome predator, it certainly had its quirkslike those teeny tiny arms,

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

  • five.

  • 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

  • tall.

  • But its two-fingered forelimbs were less than one eighth as long as its hindlimbsprobably

  • only a little bigger than your own arms.

  • 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?

  • that’s a little more complicated.

  • 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

  • up to be used in different ways.

  • 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

  • in the Jurassic, 150 million years ago.

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

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

  • to use their jaws

  • 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

  • half the length of its legs.

  • 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 rexbut 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 forelimbswere actually part of a 90-million-year trend in that direction.

  • And that big head wasn’t just for looks.

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

  • to pulverize bone.

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

  • But beyond that, long arms weren’t justnot needed” – for some dinosaurs, they

  • were kind of in the way.

  • 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

  • body stable.

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

  • So they didn’t need to swing their arms.

  • 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 youre doing your T. rex impression, make sure you're like ahhhh

  • Never like this. That's inaccurate!

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

  • wouldn’t you?

  • -- 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.

  • But, T-rex still had arms!

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

  • Well, that’s a little more complicated.

  • 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, weve learned that their forelimbs

  • were covered in muscle attachments.

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

  • So what could they have been used for?

  • 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 couldve been used either as part of courtshipcuz 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

  • fighting.

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

  • whole forelimb.

  • 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

  • bite.

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

  • power of its jaws.

  • 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

  • tiny forelimbs.

  • 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

  • forelimbs and bigger heads.

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

  • step further and lost their forelimbs entirely.

  • Truth be told, if were ever going to really know how Tyrannosaurus rex used its armsif

  • it did at allwere going to need to find more fossils.

  • For example, to figure out if T. rex used its forelimbs for slashing, paleontologists

  • would need to find a fossil that had slash marks that clearly came from T. rex claws.

  • And we haven’t found that yet.

  • And, even more challenging, to know whether Tyrannosaurus rex used its forelimbs during

  • mating, we’d have to find two T. rexes preservedin the act”, so to say.

  • And that would be a remarkable find

  • But that’s part of what makes paleontology so interestingthere are always more questions

  • to be answered.

  • To figure them out, we just have to dig up the right fossils.

  • Thanks for joining me today!

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  • to really thank our eontologists, Duncan Miller and David Rasmussen.

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If there are three things any 6-year-old knows about dinosaurs, it’s that: Tyrannosaurus

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

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    joey joey 發佈於 2021 年 05 月 03 日
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