For one, we're probably going to have a second moon within a few millennia, with all of the space junk that we have as a ring around Earth. Enough of it collides at high speeds and attaches, and soon, by cosmic standards, we would have another satellite, albeit artificial.
Also, I can't remember whether or not the shrew is as poisonous as the platypus, since the shrew had venom glands just above the salivary ones. But yeah, it makes perfect sense that all of the hostile and venomous creatures would be dumped in one place because of a foreign object from space.