Marsupial Biogeography: a huge problem for YEC
One of the biggest problems for YEC is mammalian biogeography, especially the fact that nearly all marsupials are concentrated in Australia, with only a few species occurring in South America. And these marsupials are all radically different from one another: a Tasmanian wolf is as different from a kangaroo as a rabbit is from a lion.
Creationists claim that, after leaving the Ark in Turkey, the animals could have traveled on natural rafts of vegetation and wood, which can form after major catastrophes. But the problem is that there is no natural mechanism that would cause all these very different kinds of marsupials to somehow end up almost exclusively in Australia—and only in Australia!
So they ultimately have to appeal to magic: a highly intelligent and organized God must have miraculously transported all the marsupials to one particular place, apparently for the sake of neat geographical organization. One obvious problem with this argument is the existence of marsupials in South America, extremely far from Australia, which ruins this supposed “neatness” of God’s arrangement.
Another problem is more theological: why would a benevolent, truthful, and omniscient God intervene miraculously in such a way as to create precisely the geographical pattern that scientists would naturally interpret as evidence of evolution and common ancestry? That sounds less like the work of a truthful God and more like the work of a trickster god.