Urson
The North American porcupine migrated from South America, where all New World porcupines or hystricomorphs evolved. "Erethizon" appeared in North America shortly after the two continents joined together in the later Tertiary period. Other hystricomorphs also migrated, but "Erethizon" was the only one to survive north of Mexico. No known fossils are attributed to hystricomorphs prior to the late Tertiary period. Some fossils, such as species from the family Paramyidae, show resemblance to the porcupine, but they are so primitive and generalized that they could be ancestors to all later rodents.