T. rex’s arms are the joke everyone makes, but each one was about a metre long, thick and muscular, and by some estimates could lift around 200 kilograms. They only shrank compared with the head, which grew long, wide and strong enough to do the catching and the killing on its own. Other dinosaurs went the opposite way. Megaraptorans had huge arms with claws so big that the first one found was thought to belong on a foot.

This week’s word is palaeobiology. Professor Emily Rayfield, who has clearly been asked at a lot of parties, calls it “the biology of long dead extinct animals”. It’s the difference between what an animal was and what it was like: what it ate, how fast it could run, whether it got enough food.

Lauren and Cassius Morrison hear from Emily, Professor of Palaeobiology at the University of Bristol. She took finite element analysis, the engineering maths used to test bridges, planes and hip replacements, and used it to work out how hard extinct animals could bite. The first dinosaur she tested was Allosaurus. Its skull is about 80 centimetres long but only about 25 wide, with no room for big jaw muscles, and its lower jaw turned out to be quite weak. T. rex’s skull is well over a metre long and massively wide at the back, with teeth like bananas to match. Even so, Allosaurus could still bite three or four times harder than you can. Emily also explains why the answer is always a range rather than a single number, and how her student Sydney tested whether the little alvarezsauroids could use their arms to dig into termite mounds.

Fresh from the Dig links that study back to last week’s owl dinosaur, Shuvuuia, which turns out to be an alvarezsauroid too. Then Deinocheirus, the “terrible hand”: a pair of arms two and a half metres long, found in Mongolia in 1965, that had people imagining a giant killer for nearly fifty years. When the rest turned up in 2014, it was a duck-faced, humpbacked dinosaur with no proper teeth that ate plants and fish. In the Dino-Mite Question, Callum, seven, from Newcastle, asks why T. rex had such tiny arms. There are three good answers and no winner yet: the head took over, the arms were short, strong hooks for holding on, or they shrank so other T. rexes couldn’t bite them off at a shared meal.

Then it’s Build a Dino Lab. Week ten is the arms, and Emily says heads and arms sit at opposite ends of a scale: a T. rex-style crushing head with stumpy arms, or a tiny head with long digging arms. The first is ruled out by episode three, when our dinosaur got thin, slicing steak-knife teeth. Those are the Allosaurus kind, and they come with a narrow skull and a modest bite. So for the first time Lauren picks neither: short, thick, seriously strong arms with a big shoulder blade and three curved claws, to pin prey down while the teeth do the slicing, and a bite about three or four times a human’s. Cassius points out that arms like that are also good for moving a lot of soil. Next week, footprints, and a 166-million-year-old motorway in Oxfordshire.

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