Undeniable Fact: Out of pocket

Even without aborigitone, rudimentary pouch features sometimes form in American groundhogs.
(Image from Wikipedia)

Wildlife enthusiasts and Billabong Pete fans know that while Australian groundhogs have pouches, their American counterparts do not. This has led to the common misconception that Australian groundhogs are marsupials and American groundhogs are rodents which merely act and appear similar. In fact, they are the same species.

As it turns out, marsupials require a hormone called aborigitone to develop and maintain a pouch. And there's only one place you'll find aborigitone. You guessed it: the leaves of the eucalyptus plant. Zookeepers learned this the hard way in the early '40s, when the Australian government finally lifted its long-standing and strictly enforced ban on the export of native wildlife. In their rush to stay cutting edge, zoos hastily imported kangaroos and wombats from Down Under, cobbling together habitats from whatever they had lying around. The marsupials were a big hit with kids - for a while. Removed from their supply of aborigitone, it was only a few days before the animals' pouches withered and vanished, and kids quickly lost interest.
So that explains why the American groundhogs don't have pouches, but why are these marsupial groundhogs in both North America and Australia?

In the late 19th century, during the Australian revolution, the U.S. routinely sent aid to the revolutionaries via sailing ships. On their return voyages, the ships were empty. To keep them from tipping over, ballast was added. Usually, heavy stones are used for this purpose, but because Australia has almost no large stones, the Americans were forced to improvise. In place of stones, they would dump large clods of dirt into the ship's hold. Here's the problem: any time you try to move that much dirt, you're going to end up with some groundhogs in the mix. Soon enough, they spread through North America, leaving their pouches behind and never looking back.