A time-dependent, fundamental change in function for a sectorial tooth in a group of extinct, propleopine kangaroos is reported. In juvenile Ekaltadeta ima (Marsupialia, Hypsiprymnodontidae, Propleopinae) the second premolar (P2) functions as a serrated blade at the anterior end of the cheek tooth row. In adults, this tooth drops far below the occlusal plane of the cheek tooth row where it assumes a completely different function, that of a buttress, anterolingual to the base of the crown of the much larger, newly erupted third premolar (P3). This pattern of diphyodonty-related change in dental function is unique within Mammalia. It also represents an extraordinary example of biological recycling of a normally discarded tooth.