Why do scientists think rodents originated in Eurasia despite early fossils found in North America?

While the earliest known fossil records of rodents date back 56 million years to the Late Paleocene Epoch in North America, scientists consider these species to have originated in Eurasia.1

The belief that the order Rodentia originated in Eurasia stems from the understanding that the group's evolutionary history is certainly older than the North American fossil evidence suggests. Because there is a lack of fossil evidence prior to the Late Paleocene, the exact origins and early evolutionary relationships of rodents remain a subject of ongoing research. Scientists rely on various methods, such as the analysis of comparative anatomy and DNA, to test hypotheses regarding these relationships and the geographic origins of the order.1