A skeleton is what’s left after something died. A footprint is a moment from a living animal: it pressed its foot into the mud, lifted it out and carried on, and then the mud set. One dinosaur might take a few thousand steps a day for twenty years, so there are vastly more footprints than dinosaurs, and they turn up where no bones survive, because a bone needs burying fast while a footprint needs to dry slowly and be covered gently. They also answer a question a skeleton can’t. Not how fast dinosaurs could go, which was episode one, but how fast they were actually going on an ordinary Tuesday. For most of them it was two or three miles an hour, about the speed you walk to the shops.

This week’s word is trackway. Scientists say track rather than footprint, because a foot pressed into sloppy mud collapses into something that doesn’t look like a foot at all. One track is one step. A trackway is a run of them, and a trackway is a journey: which way the animal was heading, how fast, where it stopped, whether it turned round and whether anything was walking alongside it.

Lauren and Cassius Morrison hear from Professor Peter Falkingham, who studies dinosaur tracks at Liverpool John Moores University and was on his hands and knees brushing mud out of the Oxfordshire footprints. He explains how speed comes out of a trackway: measure the stride, from one left foot to the next left foot, and the length of the foot, then plug them into equations worked out in the 1970s. You can’t get the sprint from the stroll, though, because a walking lion and a walking cheetah look practically identical. Peter looks for tiny scratches on the walls of a track, left by the scales on the foot, which turn a hole in the ground into a little film of a foot moving. The longest trackway in the world runs to about 200 metres and nothing interesting happens along any of it, but some sauropod trackways show a herd on the move with the youngsters in the middle and the adults on the outside, something no skeleton could ever tell you. And if every dinosaur lined up for a race, his money is on something the size of Velociraptor or Deinonychus, because across living animals top speed peaks at about the weight of a cheetah.

Fresh from the Dig goes to Dewars Farm Quarry in Oxfordshire: around 200 footprints and five trackways, one more than 150 metres long, made about 166 million years ago by long-necked sauropods, probably Cetiosaurus, and a meat eater, probably Megalosaurus, the first dinosaur ever named to science and an Oxfordshire local. It’s a working quarry, so every time rock comes off more turns up, and the same layer carries on for kilometres under roads, houses and farmland. Then swimming tracks from places including Spain and Utah, just a few toe-tip scratches where a big meat eater’s feet brushed the bottom, which feed straight into the argument over how well Spinosaurus could swim. In the Dino-Mite Question, Rosa, eight, from Aberdeen, asks whether she can work out how fast she goes from her own footprints. She can, this weekend: walk normally across wet sand or mud with a grown-up, measure your foot from heel to toe, then measure from one left footprint to the next left footprint, and send in your numbers.

Then it’s Build a Dino Lab. Week eleven is the feet, and it’s the quickest decision yet: three forward-facing toes with claws, a theropod foot, rather than the round, elephant-like column of a sauropod, which is superb if you weigh 40 tonnes and stand on four legs. That leaves the question dodged since episode one: how big is it? Cheetah sized, around 50 or 60 kilograms, would be fastest, but it couldn’t carry the plates, throat armour, shoulder spikes and thagomizer. Six or seven tonnes like T. rex would need a far bigger head than last week allowed, and would break the week one promise that our dinosaur runs. So Lauren goes middling: about the weight of a large pony, around 350 kilograms and four metres long, most of it neck and tail. Not the fastest thing around, but big enough that the small fast things leave it alone, and fast for its size. A heavily armoured pony. Next week, where it lives, and the answer is underground.

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