One living South American marsupial , the tiny Monito del Monte , is more closely related to Australian marsupials than to other South American marsupials . Since it is the most ` basal ' ( = primitive ) australidelphian we know , its group probably evolved in South America and then colonized Australia . One living South American marsupial , the Monito del Monte , has been shown to be more closely related to Australian marsupials than to other South American marsupials ; however , it is the most basal australidelphian , meaning that this superorder arose in South America and then colonized Australia after the Monito del Monte split off .