The Science of Discworld Revised Edition

readily to mind is the kangaroo – possibly because it springs most readily to almost anything, as for example in
The Last Continent:
    ‘And … what’s kangaroo for “You are needed for a quest of the utmost importance”?’ said Rincewind, with guileful innocence.
    ‘You know, it’s funny you should ask that –’
    The sandals barely moved. Rincewind rose from them like a man leaving the starting blocks, and when he landed his feet were already making running movements in the air.
    After a while the kangaroo came alongside and accompanied him in a series of easy bounds.
    ‘Why are you running away without even listening to what I have to say?’
    I’ve had long experience of being me,’ panted Rincewind. ‘I
know
what’s going to happen. I’m going to be dragged into things that shouldn’t concern me. And you’re just a hallucination caused by rich food on an empty stomach, so don’t try to stop me!’
    ‘Stop you?’ said the kangaroo. ‘When you’re heading in the right direction?’
    Australia alone has over a hundred species of marsupials – in fact most native Australian mammals are marsupials. Another seventy or so are found in the same general region – Tasmania, New Guinea, Timor, Sulawesi, various smaller neighbouring islands. The rest are opossums and some diminutive ratlike creatures, mainly in South America, though ranging into Central America and for one species of opossum right up into Canada.
    It looks as though placental mammals generally win out against marsupials, but the difference isn’t so great, and if there
aren’t
any competing placental mammals then marsupials do very well indeed. There are even some close parallels between marsupials and placental – a good example is the koala ‘bear’, which isn’t a true bear but looks like an unusually cuddly one.
    Most marsupials resemble ‘parallel’ placentals; a very curious case is the thylacine, otherwise known as the Tasmanian tiger or Tasmanian wolf, which is distinctly wolflike and has a striped rear. The thylacine was officially declared extinct in 1936, but there are persistent reports of occasional sightings, and suitable habitat still exists, so don’t be surprised if the thylacine makes a comeback. National Park Ranger Charlie Beasley reported watching one for two minutes in Tasmania in 1995. Similar sightings have been reported from Queensland’s Sunshine Coast since 1993: if these sightings are genuine, they are probably of thylacines whose recent ancestors escaped from zoos.
    Why such a concentration of marsupials in Australia? The fossil record makes it clear that marsupials originated in the Americas – most probably North America, but that’s not so certain. Placentals arose in what is now Asia, but was then linked to the other continents, so they spread into Europe and the Americas. Before placental mammals really got going in the Americas, marsupials migrated to Australia by way of Antarctica, which in those days wasn’t the frozen wasteland it is now. Australia was already moving away from South America, but hadn’t yet gone all that far, and neither had Antarctica, so presumably the migration involved ‘island hopping’, or taking advantage of land bridges that temporarily rose from the ocean. By 65 million years ago – oddly enough, the time that the dinosaurs died out, though that’s probably not significant – Australia was well separated from the other continents, Antarctica included, and Australian evolution was pretty much on its own.
    In the absence of serious competition, the marsupials thrived – just as ground birds did in New Zealand, and for the same reason. But back in the Americas and elsewhere, the superior placental mammals ousted the marsupials
almost
completely.
    Until a few years ago it was assumed that the placentals never made it to Australia at all – except for the
very
late arrival of rodents and bats from South East Asia about 10 million years ago, and subsequent human introduction of species like dogs and rabbits. This theory was demolished when Mike Archer found a single fossil tooth at a place called Tingamarra. The tooth is from a placental mammal, and it is 55 million years old.
    From the form of the tooth it is clear that this mammal had hooves.
    Did a lot of placental mammals accompany the marsupials on their migration Down Under? Or was it just a few? Either way, why did the placentals die out and the marsupials thrive?
    We have no idea.
    Early