How the T-Rex Lost Its Arms


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 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 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 hindlimbs – probably 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 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. 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 just
“not 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 you’re 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, we’ve 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 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 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 we’re ever going to really
know how Tyrannosaurus rex used its arms – if it did at all – we’re 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 preserved
“in the act”, so to say. And that would be a remarkable find But that’s part of what makes paleontology
so interesting – there 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! And in case you didn’t know, Eons is now
on Patreon! Patreon is a voluntary subscription service
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100 thoughts on “How the T-Rex Lost Its Arms

  1. Ha Ha! In which none of this is or ever will be, though they will make some fiction about It, i.e. Jurassic park! scientific method – systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses and maybe they will get their method right, in billions of years?

  2. Maybe they are vestigial organs, like in whales, oops, on the wrong end or maybe, like they transformed, into wings, then to birds, where the eater, now can be eaten by everybody, very nice adaption:)This Guy said it went extinct, before it evolved, yet they teach; Every bird living today is a distant relative of Tyrannosaurus rex, the fearsome "king of the dinosaurs"

  3. If I woke up one morning in the Cretaceous period and I saw a T-Rex, I wouldn’t be making jokes about his arms lol

  4. How is losing something supporting evolution? And how does a leg anatomically change to an arm? I'd like to hear that exclamation

  5. SO !
    What about they using it when they where very small babys eating and living differently as being a big predator ?
    What about it ?
    I wanna know !!

  6. Thinking more about it, there is no bipedal vertebrate species known with a complete loss of forelimbs – this would indicate that forelimbs are adaptive and did serve some function in TRex. The scapula-sternum could still have been used as a balancing fulcrum, and the arms for both grasping onto mates and fighting, maybe for handling eggs etc.

  7. To me t Rex arms makes them look like they had a disability lmao, either we probably picturing them wrong somehow, maybe we missed a couple extra bones, or nature really had a failure there lmao

  8. answer is genetic cloning and bringing back dinosaurs.

    mosquitoes that sucked the blood of T-rex then flew into sap are preserved. we'll eventually get it, but probably not in my lifetime.

  9. well, when the world was ending ( climate change, asteroids, finding out you're the last 2 Rexes on the planet…) what do you think they did? Seems like that shagging-Rex fossil must exist somewhere…

  10. How to survive a tyrannosaurus rex attack.

    1. Go back a few hundred million years.
    2. Have a T-Rex chase you
    3. Keep dodging its teeth and mouth
    4. Trip it over and make sure it falls on the floor on its side.
    5. Laugh at it and run off and it will take a long time to get back up because of its puny arms.
    6. Return to 2019 and celebrate your achievements here on youtube.

    achievement award

  11. Hi eons crew this episode & others about ancient fauna are not in the ancient fauna playlist so could you please add them to that playlist please

  12. Prosauropods (an outdated term) were NOT the ancestors of T-Rex nor any other theropod; theropods and sauropodomorphs split from each other, and no member of either is an ancestor/descendant of the either. How does PBS not get actual experts?

  13. Or…is just one idea…his little arms is used to raise his offspring.

    Eating carrion "could" preventing T-Rex to eat his own children.

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