The Science of Discworld Revised Edition

creatures, or for sudden escapes. But as the descendants of these creatures came out on land, they experienced real problems; they sagged and flopped. Compare a salamander with its sprawled-out legs, which cannot support its weight, with a lizard of the same size that has strong enough muscles and bones to run. (The tuatara still sprawls). The frog’s trick, one jump at a time, is much less effective than really well-designed legs, strong limb girdles, an effective bellows system around the lungs to supply the muscles with oxygen, and a four-chambered heart to keep the aerated and depleted blood apart – all the tricks that make a monitor lizard such a good predator.
    When you’ve achieved all these, then feeding on the seashore, or even in the sea like today’s marine iguanas on the Galápagos, becomes really easy. Instead of having muscles and respiratory systems that are just adequate for marine life, you’ve got the supercharged version that terrestrial gravity required of your ancestors. Going back into the sea is now a very viable option; only sharks and octopuses are any match for previously-terrestrial lineages. So the ichthyosaurs and plesiosaurs, like today’s dolphins and whales (but less so because mammals had become warm-blooded too while they were on land) found that living in the sea was easy.
    Until they evolved further and became their own best enemies, of course, after they had radiated into hundreds of species. The reptile-eating pliosaurs were like today’s killer whales, whose main diet is other whales.
    Meanwhile, several rather different lineages of Triassic land reptiles, somewhat prematurely called archosaurs (‘ruling reptiles’), had evolved really good limb girdles, of several patterns. Two very different lineages, ‘bird-hipped’ and ‘lizard-hipped’, went on to produce all those great big dinosaurs. The lizardy lot evolved into gigantic herbivores like
Diplodocus
and brontosaurs (now, alas, named apatosaurs; ‘thunder-lizard’ was so much more appropriate) and gigantic carnivores like the allosaurs and the tyrannosaurs. These all arrived at least sixty million years later, though: the early archosaurs were as far in the past of tyrannosaurs as we are in their future.
    The bird-hipped lot eventually produced those spectacularly armoured beasts that make such good film shots when tyrannosaurs are fighting them: ankylosaurs with spikes on a knobby tail, like that spiky ball on a chain that the villain wields in chivalric films; stegosaurs with bony plates, spikes down their backs; triceratops with the bony frill and three horns.
    Filmmakers always seem to make almost-purposeful mistakes: the innumerable eight-year old boys who’ve learned all the lovely names could correct them. It’s a pity that that otherwise lovely fight in Disney’s
Fantasia
, accompanied by Stravinsky’s
Rite of Spring
, is between a tyrannosaur and stegosaur. It couldn’t have happened, they weren’t contemporary. And the stegosaur never had the ankylosaur’s tail-armour, either. The late Cretaceous landscape, with the few very big dinosaurs that lived then, was doubtless an impressive scene; but the film producers’ version is no more accurate than
Fantasia
.
    This is hardly a surprise; after all, Hollywood has been wrong about so many other things. Scientists don’t always fare better. Right now, palaeontologists believe that tyrannosaurs were scavengers, not predators. We’ll stick our necks out and dispute that conclusion. Yes, tyrannosaurs may not have been fearsome predators, hunters … but if they weren’t, that doesn’t make scavenging the only option. They probably did something that we can’t imagine instead. We simply can’t see these animals as enormous vultures, with their tiny front feet scrabbling at a decaying corpse and that great head hidden in the abdomen of a dead sauropod like
Diplodocus
. We’d run away from them, whatever the scientists are saying now.
    On the Rincewind principle, you understand. Just in case.
    Other archosaurs went on to produce crocodiles and pterodactyls, and perhaps the birds – or these may have arisen from the same stock as those deinonychids made familiar by
Jurassic Park
, namely
Velociraptor
and its ilk. These probably were intelligent creatures, agile carnivores much like their portrayal in
Jurassic Park
. We’d certainly run away from
them
.
    There are a few puzzles that biologists keep puzzling at, like whether some of the big