The Science of Discworld Revised Edition

dinosaurs were warm-blooded. And why everything was so damned big in the Cretaceous. The biggest teleost (bony) fish that there has ever been lived in Cretaceous seas; it was as big as today’s whale shark. And dinosaurs flew, too. There are lots of fossil pterosaurs like
Pteranodon
with eight-yard (7m) wingspreads, bigger than any of today’s birds, and there are a couple of fossils of
Quetzalcoatlus
with wingspreads twice as big. That’s bigger than the fighter-planes of World War II, like spitfires. We have no idea how these creatures lived, but there’s no doubt that they existed. 1 Unless you believe in planetary engineers with a sense of humour, as in
Strata
…
    This is where we should warn you about the most tempting way of thinking about these ancient creatures, and how we get it wrong all the time. Gee’s
In Search of Deep Time
takes us to task, showing how all of our pretty guesses about evolutionary scenarios – however well they seem to be based in the fossil record – are simply wrong, impossible.
    A classical mistake is the way we habitually think about that fish that came out of the water, whose descendants were the land vertebrates. We imagine it flopping out on the beach (and Rincewind encouraging it to go back into the water) with its developing lungs and its evolving legs … No. It had fairly well-developed legs while it was still living an underwater life, with internal gills like any perfectly respectable fish. It must have done, or it wouldn’t have
worked
.
    We have little idea what its legs were for at that stage, though: certainly not for walking on land. But there’s no question that these hands that we’re typing with are the descendants of those fishy legs … Just as our cough is a legacy of that fish’s crossover of its airway and foodway. It’s the pictures in our heads, of what we suppose happened, that are mistaken. They are lies-to-children that we can’t correct yet. But humans definitely evolved from a fish, and it had legs. It just didn’t use them to walk along the beach.
    Another interestingly wrong lie-to-children about evolution concerns the origin of birds. When those lovely
Archaeopteryx
fossils were found in the Solenhofen limestone, so well preserved that the feather imprints could be clearly seen – and the teeth and the claw-digits on the wings – it was clear that we had found the halfway stage between a reptile and a bird. It was a super Missing Link.
    Think about that for a moment: how can you find a Missing Link? Oops.
    Archaeopteryx
had a long tail like a lizard, and no keel in its chest for strong flight muscles. Were it not for the feathers, it would have been classified as a small pseudosuchian dinosaur like
Ornithomimus
(bird-mimic). In the late Jurassic there were many little dinosaurs with bird-like features, and some really well-developed diving-bird fossils had been found in early Cretaceous rocks from about fifteen million years later. These were real birds, called
Ichthyornis
, and they had already lost their flight abilities, reduced their wings to (very bird-like) vestiges.
    So
Archaeopteryx
was a bit ‘late’, and in the 1950’s zoologists thought that it probably represented a primitive reptile-bird lineage hanging on, perhaps contemporary with much more bird-like creatures. That’s another scenario that doesn’t make sense now. Many bird-like dinosaurs have been found recently, fossils from South America and, especially,
Caudipteryx
and
Protoarchaeopteryx
from China, and these make the problems worse. 2 They have feathers, but
they didn’t fly
. They have no wings, but they have arms with hands, sometimes with only two digits.
    So what were feathers ‘for’?
    Feathers are really remarkably complicated. They’re not at all like the scales of lizards and snakes, and it’s not easy to invent an evolutionary route by which feathers (or hairs, come to that) can have developed from scales. Some biologists who haven’t looked at feathers very carefully have imagined scales growing rather like the stage witch’s fingernails, so that they stick out like a pangolin’s scales. (That’s that silly mammalian tree-climbing anteater that looks like a big pine cone.)
    Feathers aren’t like that, though. They develop as cylinders: you can see baby feathers, called pin-feathers, on a plucked chicken from the supermarket. The scales on birds’ legs are respectable reptilian scales; perhaps surprisingly, no modern birds have structures