Here are two I found very amusing:
Myths, not least Jurassic Park itself, have suggested that dinosaurs are not "really" extinct at all. ... Significantly, no one has made a film bringing back dodos, moas, pygmy elephants, or mosasaurs -- only dinosaurs and Hitler are popular for the reawakening myth. Both at the same time would be a good trick.
You can calculate how much energy a kangaroo uses when it makes a jump, count how many jumps it makes in a day, and deduce a lower limit on its daily energy requirements. ... Do the sums, and you find that the kangaroo's daily energy requirement is about ten times as big as anything it can get from its food. Conclusion: kangaroos can't jump. Since they can't jump, they can't find food, so they're all dead.
Strangely, Australia is positively teeming with kangaroos, who fortunately cannot do physics.