The Science of Discworld Revised Edition

scientifically as (in)accurate as
One Million Years BC
with Raquel Welch being chased by dinosaurian monsters, the Flintstone family with their tame dinosaurs powering household gadgets, or Hamlet with a PC.
    It’s quite difficult to get the span of geological time to make sense . In his book
In Search of Deep Time
, Henry Gee does an excellent job of reminding us just how flimsy the ‘fossil record’ is. A few bones here; a few others five thousand miles away and ten million years later; from these we attempt to tell a story of evolutionary ancestry. It’s like claiming to have reconstructed human history from one flint flake and a half-eaten hamburger. Well, not as coherent as that, actually.
    It’s more difficult still to put the range of creatures up against the tapestry of evolutionary time to get some idea of what the dinosaurian Earth might have been like. Take sharks. There have been shark-like sharks since way before there were
any
reptiles; there have been those odd-looking horseshoe crabs for longer. Coelacanth fishes have been grubbing about in the dark depths off the continental shelves while the dinosaurs came and went. It’s not surprising, perhaps, that yeasts and other fungi, bacteria of several modern kinds, have been around for more than a thousand million years. We don’t expect ‘blobs’ to notice the passage of time. But coelacanths and sharks and tuataras are vertebrates – you’d think they’d be a bit progressive, evolving and changing into … whatever. But they didn’t; they just kept on doing their own thing.
    Sharks have eaten mesosaurs, have been annoyed by plesiosaurs and ichthyosaurs and cautious about pliosaurs, have eaten little mosasaurs and been eaten by big ones. There were ammonites in their seas, and belemnites and all kinds of other shelled octopuses. Then, when the big reptiles went, and all the ammonites and their friends disappeared, the sharks had the top levels of the marine food chains to themselves for tens of thousands of years. Then the mammals produced dolphins, killer whales, big whales … and the sharks just went on being sharks.
    Why didn’t sharks change? They have wonderful immune systems, which we’re just beginning to understand; they don’t get bacterial infections or, apparently, cancers. Perhaps they are not made ill by viruses, either. Though they do have lots of wormy and flukey parasites. Are today’s sharky superfish the newest, latest-model sharks? Or did the ancient ones also have marvellous immunity to disease? Was that their trick? Was that what brought them unchanged, in outward appearance at least, through such a long span of time?
    Since we can’t answer those questions, yet, let’s move to one that we can answer. What was happening on land as the Permian became the Triassic? Well, for a start, the biggest mass extinction of all, 248 million years ago. The number of species on Earth dropped by about 93%. Only 7% survived. We’re talking big species here; nobody knows what happened to bacteria or even protozoa, to nematode worms, or to rotifers. Except that a lot of protozoa have mineral shells, and got fossilised as the white cliffs of Dover, and rotifers have tiny hard jaws of characteristic shapes that a few hardy fossil-hunters collect. So we can check these out, and they give a similar picture.
    The precise cause of this mass extinction is debatable. There may have been collusion between a comet impact and massive volcanic activity, as we describe shortly. In any event, the early dinosaurs – and mammal-like reptiles and turtles, even ichthyosaurs and early plesiosaurs (but not mesosaurs) – lived through it. They were among the lucky 7%. Whatever the disaster was, it seemed great for them; it gave them just the open ecological spaces that they needed to radiate wildly. The Triassic seas were just as full of reptiles as today’s seas are full of mammals, and they stayed that way right up to the early Cretaceous. Those ocean reptiles had largely vanished before
Tyrannosaurus
appeared, though.
    Why were the reptiles so successful as sea creatures? There’s a persuasive biological explanation for this invasion of the seas by land animals. The story starts in the sea, moves on to land, and reverts to the sea again.
    Creatures that live in the seas experience very little gravity, if any; even heavily-armoured creatures like crabs produce forms that swim. In fact their muscles are for swimming, or for closing jaws on other