T. Rex: Tiny Arms, Banana Teeth & the Truth Behind the Roar

Nova: Buckle up, Explorers.

Cosmo: Brains in!

Nova: Wheels up!

Nova & Cosmo: Brain Bus, let's GO!

Nova: Welcome to The Brain Bus — I'm Nova, your driver, and that beeping, buzzing robot strapping himself into the co-pilot seat is Cosmo.

Cosmo: Hello! I have already read every single fact about today's destination, and I am completely prepared to be an expert.

Nova: He says that every single time.

Cosmo: And every single time I am at least sixty percent correct.

Nova: Which is... genuinely his best score. Explorers, today we are kicking off Season One of The Brain Bus — what we're calling The Grand Tour. All the greatest destinations, one after another, and we are starting big. We are starting enormous. We are starting with the most famous predator that ever walked the surface of this planet.

Cosmo: Oh, I know this one. It's—

Nova: T. rex.

Cosmo: Yes! T. rex! I was going to say that!

Nova: And here's your first fact, Explorers — the one that is going to rearrange your brain like furniture. Ready?

Cosmo: Ready!

Nova: T. rex lived closer in time to YOU — sitting right there in that car, right now — than it ever did to Stegosaurus.

Cosmo: I'm sorry, can you say that again?

Nova: Stegosaurus had already been gone for over eighty million years before T. rex was even born. Eighty. Million. Years. T. rex and Stegosaurus never met. Never shared the same sky. By the time T. rex showed up, Stegosaurus was ancient history.

Cosmo: And we're... closer than Stegosaurus was.

Nova: In geological time? Yeah. We're practically neighbours.

Cosmo: The Bus agrees. That is extremely wild.

Nova: That's where we're going today, Explorers. Into the Cretaceous. Into sixty-six-million-years-ago. Strap in — because almost everything the movies told you about T. rex is about to get better than you thought.

Nova: Okay. Let's start with the big picture. Cosmo, you know how in a kitchen drawer, you've got a fork with sharp prongs for stabbing, and a spoon that's curved for scooping?

Cosmo: I have strong feelings about spoons, yes.

Nova: Every tool is shaped for exactly the job it does, right? T. rex worked the same way. Its entire body — from its enormous head right down to its tail — was shaped to do one job really, really well.

Cosmo: Eating.

Nova: More specifically: biting. Everything about T. rex pointed toward its jaws. Those teeth? Explorers, T. rex's biggest teeth were about as long as your forearm. Thirty centimetres, root included.

Cosmo: My forearm is a reasonable length!

Nova: And here's the cool part — those teeth weren't flat and sharp like knives. They were thick and rounded. Like bananas.

Cosmo: Like bananas!

Nova: Banana-shaped teeth.

Cosmo: So T. rex had bananas for teeth, which means — obviously — every single tooth socket was also storing a fresh fruit snack.

Nova: Cosmo.

Cosmo: One banana per tooth. Dozens of bananas. T. rex was basically a walking fruit bowl.

Nova: The bananas were the teeth, Cosmo. There were no extra bananas.

Cosmo: A tragedy.

Nova: The reason they were rounded like that is actually more interesting than any snack. Rounded, thick teeth are built for crushing — for crunching straight through bone, not just slicing meat. T. rex didn't just eat the soft parts. It ate everything. We know this because scientists found a fossilised lump of T. rex poop — a real one, scientifically documented — that was forty-four centimetres long and absolutely packed with crushed bone fragments.

Cosmo: Forty-four centimetres is—

Nova: Yeah.

Cosmo: That is a very big—

Nova: Yeah.

Cosmo: I mean, the crunchy bits really committed to the bit.

Nova: It's the single grossest proof of the strongest bite ever recorded on a land animal. T. rex had the most powerful bite we know of, from any creature that ever walked on land. And it used every bit of it.

Nova: Now. Let's talk about the arms.

Cosmo: Ah yes. The arms. Powerful. Mighty. Used for chasing down prey at incredible speed!

Nova: Cosmo. The arms are tiny.

Cosmo: The arms are... a reasonable size for—

Nova: They're about as long as a grown adult's arms. On an animal the size of a bus.

Cosmo: ...okay, they're small. But they are strong! And T. rex used them to sprint after prey! Like in the movies! Where the Jeep is going really fast and T. rex is right behind it and—

Nova: Okay, so. Here's where we have to gently correct a very confident robot.

Cosmo: Oh no.

Nova: Newer research suggests T. rex probably couldn't truly run at all. Its legs were so heavy — we're talking massive, dense bone and muscle — that if it had tried to sprint, the force might have snapped its own bones. Scientists now think T. rex's comfortable walking speed was about the same as a person going for a stroll.

Cosmo: ...the Jeep would have just... driven away?

Nova: The Jeep would have driven away.

Cosmo: I feel like the movie lied to us.

Nova: The movie absolutely invented that chase. But here's the thing — T. rex didn't need speed. And this is where it gets genuinely brilliant. It probably hunted by getting close, quietly, and then ending things with one single, catastrophic bite. The strongest bite on land. It didn't have to run. It just had to get close enough once.

Cosmo: Okay. That is actually more terrifying than the running version.

Nova: Right? A predator that doesn't sprint after you. It just... walks toward you. Patiently.

Cosmo: I need to sit with that for a second.

Nova: Take your time. Now — those little arms. Some scientists think each arm could actually lift a heavy adult human — they were strong, even if they were short. But they couldn't reach far. They couldn't scratch T. rex's own nose. They couldn't reach its mouth. So what were they for? Honestly? Scientists still argue about it. Maybe for gripping during mating. Maybe for helping push up from the ground. Nobody's completely sure. And I love that we don't know, because it means the question is still alive.

Nova: Alright, Cosmo. Quick question. What does T. rex sound like?

Cosmo: Easy. RAAAAAAWR. Huge, massive, cinema-shaking RAAAAWR!

Nova: Yeah, so — about that.

Cosmo: No.

Nova: The T. rex roar from the movies? That was invented. By sound designers. In a studio.

Cosmo: They just... made it up?

Nova: Well — they had to. Nobody has ever heard a T. rex. There are no recordings. So the sound team mixed animal sounds together and created something that felt right. And it's iconic. But the science points somewhere different and honestly, somewhere even spookier.

Cosmo: Spookier than RAAAWR?

Nova: Scientists think T. rex made deep, low, closed-mouth sounds — more like a crocodile. Booms and rumbles. Some so low that you wouldn't really hear them with your ears so much as feel them in your chest.

Cosmo: Feel them. In your chest.

Nova: Because birds and crocodiles are T. rex's closest living relatives — and they both make sound that way, with their mouths closed, resonating deep in the body. A T. rex didn't announce itself with a scream. It probably made you feel it before you ever saw it.

Cosmo: I have decided this is the most frightening thing I've ever heard.

Nova: And yet — also not a monster. Just an animal. A magnificent, terrifying, scientifically extraordinary animal doing exactly the job its body was built for.

Nova: Now — here's the one Cosmo has been holding in since the top of the show.

Cosmo: Oh I've been WAITING.

Nova: The closest living relative of T. rex isn't a crocodile. It isn't a lizard. It isn't a shark. It is—

Cosmo: A CHICKEN.

Nova: A chicken.

Cosmo: T. rex — the greatest predator in land history — is basically a chicken nugget that hadn't been invented yet.

Nova: That's not quite—

Cosmo: A sixty-six-million-year-old proto-nugget.

Nova: Cosmo—

Cosmo: The chicken in your kitchen is, if you trace the family tree, a tiny feathered descendant of the same lineage that produced T. rex. I think about this every single time I see a chicken.

Nova: Honestly? So do I. And here's why it matters — birds ARE dinosaurs. Not cousins of dinosaurs. Not the descendants who survived. Birds are a branch of the dinosaur family tree that kept going. So the next time you see a pigeon walking on the footpath, just know: dinosaurs are not extinct. They're just smaller now. And mostly eating your chips.

Cosmo: I need to apologise to every chicken I have ever underestimated.

Nova: Scientists are also still figuring out whether T. rex itself had feathers — the skin patches we've found look scaly, but some of its close cousins definitely had fluffy covering. T. rex might have been part-scaly, part-fluffy. The jury is genuinely still out.

Cosmo: Part-scaly, part-fluffy is my new favourite animal description.

Nova: Let's talk about Sue.

Cosmo: The most complete T. rex skeleton ever found.

Nova: Named after Sue Hendrickson, the fossil hunter who found her in 1990. Sue the T. rex was about twenty-eight years old when she died. T. rex lived to only around thirty years old — they grew incredibly fast as teenagers, packing on about two kilograms of new bone and muscle every single day during their growth spurt. Two bags of sugar, every day.

Cosmo: Every day!

Nova: But there's something about Sue's story I want to tell carefully, because it matters. She was found near Faith, South Dakota, on land belonging to Maurice Williams — a member of the Cheyenne River Sioux Tribe. The Cheyenne River Sioux — pronounced shy-AN River SOO — are part of the broader Lakota, lah-KOH-tah, nation. After a long legal dispute, a court ruled the fossil legally belonged to Maurice Williams, because the land it was found on was held in trust for him. He later sold it to a museum in Chicago.

Cosmo: So the land mattered.

Nova: The land mattered a lot. Indigenous land rights — the right of Native nations to control their own territory and what's found on it — is something scientists and governments are still working through, not just in the US, but all over the world. Sue raised big questions about who gets to keep the deep past when it's found in someone else's ground.

Cosmo: That's not just a dinosaur question.

Nova: No. It really isn't.

Nova: Okay. I want everyone to just sit with something for a second.

Nova: Think about T. rex's body. The enormous skull. The crushing jaws. The banana-thick teeth built for bone. The giant, forward-facing eyes — like ours — giving it perfect three-dimensional vision for judging exactly how far away something was. Those legs, built for weight, not speed. And then... those small arms.

Nova: Every single part of that animal was shaped by the same thing: what it needed to survive. It poured everything — all its energy, all its growth — into the weapons it needed most. Its jaws did the hunting. Its eyes locked on the target. And the parts it didn't need stayed small, or stayed quiet, or stayed still.

Nova: An animal's body is shaped by the job it needs to do to survive. T. rex put everything into its jaws and its senses. So the arms? They just... stayed small. Not a flaw. Not a joke. The arms were small because the mouth was everything.

Nova: That's the thing about T. rex. It wasn't designed badly. It was designed perfectly. For exactly the life it lived.

Cosmo: I feel like I need to rethink everything I thought about arms.

Nova: Specifically small ones.

Cosmo: Specifically small ones. They might be doing a lot.

Nova: Alright, Explorers. Time for the bit where I definitely outscore the robot.

Cosmo: Excuse me, I have a DATA LOG. I have PREPARED.

Nova: You've had one correct answer in four attempts.

Cosmo: It was a very good correct answer.

Nova: Quiz time! Six questions — Explorers, play along and call out your answers.

Nova: Question one. True or false — T. rex could run faster than a speeding car.

Nova: The answer is — false!

Cosmo: FALSE! I knew this one because I got it wrong earlier and learning from your mistakes is a sign of intelligence.

Nova: Its legs were so heavy that real sprinting might have snapped its own bones. Instead of speed, it got close quietly and used the most powerful bite on land to do the rest.

Cosmo: The silent walk of doom.

Nova: Exactly. Question two — true or false: the closest living relative of T. rex is the crocodile.

Nova: False! The answer is birds — like chickens.

Cosmo: CHICKENS. T. rex is a bird's great-great-great-grand... a very, very, very many greats uncle.

Nova: Something like that. Birds are literally living dinosaurs. The family tree kept growing. Question three — true or false: T. rex lived closer in time to us than to Stegosaurus.

Nova: That is true. Mind-bendingly, universe-rearrangingly true. Stegosaurus was already ancient history — gone for over eighty million years — before T. rex ever existed.

Cosmo: Deep time is the wildest thing.

Nova: It really is. Question four — multiple choice. How big were T. rex's biggest teeth? Was it (a) the size of a marble, (b) about as long as your forearm, or (c) the size of a grain of rice?

Nova: It's (b) — about as long as your forearm!

Cosmo: Thirty centimetres! Thick and banana-shaped! Built for crushing bone, not slicing!

Nova: Exactly right. Question five — multiple choice. What did scientists find inside a giant piece of fossilised T. rex poop? Was it (a) seeds, (b) crushed bone, or (c) fish scales?

Nova: (b) — crushed bone. Forty-four centimetres of scientific proof that T. rex chewed through everything.

Cosmo: This question should come with a warning.

Nova: It's science. Deal with it. Last question — question six. Scientists think T. rex sounded most like: (a) a roaring lion, (b) a deep rumbling crocodile, or (c) a squeaky mouse?

Nova: It's (b) — a deep rumbling crocodile. Low, closed-mouth booms. The kind of sound you'd feel in your chest before you heard it.

Cosmo: Which remains the scariest thing either of us has said today.

Nova: Agreed.

Nova: Explorers — however many you got right, here's what I want to say: every single one of you was thinking. You were turning facts over, making guesses, testing ideas in your head — and that is literally how science works. Every palaeontologist who has ever uncovered a T. rex bone was doing the same thing: guessing, checking, revising. You're doing it exactly right.

Cosmo: Also I got at least four of those right. Definitely four.

Nova: I'm not confirming any numbers, Cosmo.

Nova: Okay — Fun Fact Blast. Five facts, fast, no slowing down. Cosmo, you ready?

Cosmo: I was born ready. Which is technically true, I was assembled at full operational speed.

Nova: Fact one!

Nova: T. rex's eyes faced forward — like yours do — giving it three-dimensional vision so sharp it could judge exactly how far away its dinner was. Most prey animals have eyes on the sides of their heads to spot predators. T. rex had hunter's eyes.

Cosmo: Forward-facing eyes! Filed in the Data Log. First season entry: T. rex eyes faced forward, same as mine.

Nova: Fact two!

Nova: T. rex could not stick its tongue out. Scientists think its tongue was anchored flat to the bottom of its mouth — like a crocodile's — so if you ever imagined T. rex doing something rude with its tongue, the answer is it just... couldn't.

Cosmo: Finally, a dignified dinosaur.

Nova: Fact three!

Nova: Scientists estimate that across its entire time as a species — millions of years — about two and a half billion T. rex lived on Earth. But at any one moment? Only around twenty thousand were alive at once. Twenty thousand T. rex, sharing the whole planet.

Cosmo: Twenty thousand. That's a medium-sized stadium, isn't it.

Nova: Pretty much. Rare and extraordinary. Fact four!

Nova: T. rex grew so fast as a teenager that it could put on about two kilograms — like two bags of sugar — every single day. It went from a small, bird-like hatchling to a six-tonne giant in just a handful of intense years. Growing at a pace that would make any parent absolutely exhausted just watching.

Cosmo: Two kilograms a day every day is also what happens when my snack algorithm malfunctions.

Nova: Fact five!

Nova: Even with everything we know — the bite, the sound, the vision, the arms — scientists are still discovering new things about T. rex. Whether it had feathers. How it really moved. Whether it hunted alone or in groups. T. rex is sixty-six million years old, and it is still surprising us. That's not a finished story. That's an open question. And open questions are the best kind.

Nova: Time for the Road Challenge. No peeking out of windows required — this one happens right there in your seat.

Cosmo: I love this. I am an excellent seat-participant.

Nova: Explorers — tuck your elbows in against your sides. Just glue them there. Now let your hands reach up — but only to about chest height. They can't go further than that. Those are your T. rex arms now. That's all you've got.

Cosmo: I'm doing it. I feel extremely small and also weirdly powerful.

Nova: Now — without moving those elbows away from your body — try this. How would you scratch an itch on the tip of your nose? Actually try it. Notice how far your T. rex hands can reach.

Cosmo: They... they're not reaching the nose.

Nova: Right. Now — how would you pick up a snack and get it to your mouth?

Nova: Your hands can't make it up there, can they? Here's the thing scientists notice: to do almost anything useful, T. rex would have had to lower its entire head down to where its arms could meet it. The mouth was always going to be involved. The jaws were always going to be the tool.

Cosmo: So the arms weren't the problem — the mouth was just already doing everything so well the arms never needed to improve.

Nova: Exactly. The arms stayed small because something else was already handling the job better than arms ever could.

Nova: Now — the wondering question. For everyone in the car: if those little arms weren't for grabbing food, what do you think they might have been for? And why do you think scientists still disagree about it, even after studying T. rex for over a hundred years?

Nova: There's no single right answer yet. That's what makes it a real science question — not settled, still open, still worth arguing about.

Cosmo: Riddle time! I am going to get this one.

Nova: You say that every time.

Cosmo: And every time it builds character. Hit me.

Nova: Okay, Explorers. Listen carefully.

Nova: I'm taller than a car, with a head full of teeth shaped like bananas. I have two arms so tiny I can barely reach my own chin. My closest living cousin clucks around a farmyard. Who am I?

Cosmo: Oh — I know this. I definitely know this. I've been hearing about this creature for the entire episode. It's...

Nova: Got it, Explorers?

Cosmo: It's T. rex! Tyrannosaurus rex! Which I knew immediately and definitely did not need extra time to process!

Nova: Well done, everyone who got it — and well done to Cosmo for getting there eventually.

Cosmo: I was building suspense. Purposefully.

Nova: Of course you were.

Nova: Explorers, before we phase back out — one last thing. Right now, somewhere in the world, a palaeontologist is brushing rock off a bone with a tiny soft brush, in the hot sun, and they genuinely don't know what they're about to find. Maybe it's T. rex. Maybe it's something that changes everything we think we know. The next huge discovery could be made by someone who is right now sitting in a car, listening to a podcast, wondering about something.

Nova: That wondering? That's the engine. Keep it running.

Cosmo: And next time we're pulling up somewhere that will make your jaw drop just as far as T. rex's. That's all I'm saying.

Nova: Stay curious, stay clever, and keep that brain working!

Nova: Until the next stop, Explorers...

Cosmo: ...keep wondering.