A dinosaur’s tail could be a third of the whole animal, about as heavy to lug around as a concrete block, and for well over a century people drew dinosaurs dragging theirs along the ground. They didn’t. Palaeontologists have found thousands of dinosaur footprints and almost no tail marks, so a healthy dinosaur held its tail up and out behind it. The tail-dragging monsters in old paintings and old films were what scientists thought at the time, and that’s fine. Being wrong and then finding out is how science works.

This week’s word is counterbalance. It’s the seesaw, the tightrope walker’s long pole, and the heavy block on the back of a tower crane that stops it tipping over. A long tail stopped a two-legged dinosaur falling flat on its face. But a crane’s counterweight just sits there, and a dinosaur’s tail didn’t. Underneath it ran an enormous muscle, a giant bottom muscle, that ran forwards from the tail and down on to the thigh bone and pulled the leg back with every single step. So the tail was a counterbalance and an engine.

Lauren and Cassius hear from Pasha van Bijlert, a PhD researcher at Utrecht University in the Netherlands, who builds dinosaurs inside a computer and teaches them to walk. He 3D-scans a skeleton, adds muscles using the rough patches where they once gripped the bone, then drops the whole thing into a physics simulator that works out for itself how to walk and run. To check it works, he built an emu the same way, as if it were a fossil, and it topped out at about 40 kilometres an hour. Then he went to the Australian outback to film real emus with drones, and two of them ran alongside the car at 45 kilometres an hour and simply kept going.

Fresh from the Dig starts with Pasha’s own team, who worked out how fast Tyrannosaurus rex walked from the way its tail bobbed. Everything that swings has a rhythm it prefers, like a playground swing, and the rhythm of a T. rex tail gives a walking speed of about four and a half kilometres an hour. That’s your walking speed too, which is not a reason to go for a stroll with one. Then Stegouros from Chile, a small, cute armoured dinosaur whose tail ended in seven pairs of flat bony plates locked together like a fern leaf, and was about as friendly as an Aztec war club. And the footprints that settled the tail question turn out to be trace fossils, the same family as fossil nests, last meals and dinosaur poo.

Then it’s Build a Dino Lab, where one real fact becomes one real piece of the perfect dinosaur. Week six is the tail: a long, heavy, muscular one held out straight, or a light little stump like a pigeon’s. Lauren picks the long, heavy tail held off the ground, because episode one promised this dinosaur could run, and without it there is nothing to swing the thagomizer. Next week, wings, for a dinosaur that can’t fly.

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