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spjennifer Well, a few things there.
Young earth creationists don't believe in an Earth that is billions of years old, so they wouldn't accept your appeal to what you think things were like millions of years ago.
As I already said, the argument is that Noah wouldn't have to collect every variety of bear. One variety would count as one "kind."
You are assuming again that Australia existed as it does now and that all of the animals who live there now, did so then. I have seen it argued that the climate was somewhat constant throughout the Earth prior to the flood, so maybe platypuses wouldn't have needed to swim that far after all. They may have existed nearer to where Noah was located.
I also imagine that YECs would also not so easily dismiss the possibility of divine intervention in these matters, which not unreasonable within a theistic worldview.