Ontogenetic Evidence for Dental Homologies and Premolar Replacement in Fossil and Extant Caenolestids (Marsupialia)

It is generally accepted that the South American marsupial family Caenolestidae is characterized in part by the absence or noneruption of the third deciduous premolar (dP3) in both jaws, although juvenile stages have rarely been identified in extant or fossil representatives of the family. Published illustrations of the dentary of the Miocene caenolestid Stilotherium suggested to us, however, that P3 erupted relatively late during ontogeny, after the eruption of M4. In extant marsupials, this eruption sequence appears to represent the plesiomorphic state and this pattern is generally associated with the eruption of dP3 earlier in ontogeny, and its subsequent replacement by the erupting P3. Therefore, we suspected that a dP3 erupted in earlier ontogenetic stages of caenolestids; to test this hypothesis we searched the mammalogy collections of three museums for evidence of dP3 in juvenile specimens of caenolestids. Examination of more than 180 specimens of the three extant genera of caenolestid marsupials resulted in the identification of only nine juvenile or subadult stages of dental eruption. Seven specimens of Caenolestes and Rhyncholestes corroborated our hypotheses of late eruption of P3 in Caenolestidae. In addition, the two youngest specimens of Caenolestes possessed a tiny, rudimentary dP3, measuring about 0.4 to 0.5 mm in greatest length, associated with a mesiolingual eruption pit containing the apex of the larger P3 in very early phases of eruption above the alveolar margins. The tiny dP3 is clearly nonfunctional in occlusion, and it is questionable whether it erupted above the gun margins in life. Comparison of the dentaries of subadult caenolestids with four dentaries of the Miocene genus Stilotherium corroborated our initial impression that the fossil genus exhibited evidence of a late-erupting P3, comparable to the condition in extant caenolestids. We suggest that examination of other specimens of juvenile dentitions, skulls, and skeletons in museum collections can provide additional insight into the developmental and evolutionary biology of mammals.