Great American Interchange
The top native South American mammal success for a long time was the superorder Xenarthra, which had two different groups with large representatives in North America. One group was the giant ground sloths, such as the megalonychid Megalonyx. This group lived in North America for over 10 million years, well before the Great Interchange. How they got there is not known. They got as far north as Alaska and the Yukon.