A long, long time ago, marsupials the size of small trucks, 2-meter-tall "thunder birds" and 5-meter-long venomous lizards roamed Australia. These animals—and more—were Australia's megafauna.

They were the ancient Australian megafauna—huge animals that roamed ... a huge flightless bird; a wombat-like creature the size of a rhinoceros; and a seven-foot-tall kangaroo with a strangely ...