Novel features of tooth development and evolution in the honey possum, Tarsipes rostratus

The honey possum, Tarsipes rostratus, is a unique marsupial in several respects, from its smallest size at birth to its fully nectarivorous diet. A striking feature is the degree of reduction of its teeth, with fewer, smaller teeth in its jaws than for any other marsupial. The few peg-like teeth of honey possums cannot be clearly differentiated into premolars or molars. Here we used a developmental series of 21 honey possums, ranging in size from 2.8 to 30 mm head length, to uncover the identities of the postcanine teeth. We used histology, diceCT microCT and synchrotron microCT to find developing tooth buds and mineralising teeth. We found that there are up to two deciduous premolars in each quadrant, with between zero and two molars behind the deciduous premolars. In addition, there is no evidence of tooth replacement events at any locus. This confirms that the honey possum is one of the most extreme cases of tooth reduction in marsupials, even though it retains long lower incisors and an exceptionally long tongue to support its nectar-feeding habit.