Supernumerary molars in neotropical opossums (Didelphimorphia, Didelphidae)

Dental abnormalities, such as the occurrence of extra teeth, are recurrently found in many groups of mammals. Supernumeray molars were found in Didelphis aurita, D. albiventris, D. marsupialis, Philander andersoni, P. frenata, P. opossum, Chironectes minimus, and Caluromys philander. Frequencies of occurrence of supernumerary teeth in these marsupial species remained within a range similar to that found in other species. Four hypotheses are proposed and discussed to explain the origin of these teeth: appearance of extra teeth due to excessive development in size of the skull, reappearance of an atavistic condition, retention of the third deciduous premolar at the eruption of the permanent premolar, or some sort of ontogenetic disturbance that lead to the duplication of a tooth germ. The first hypothesis is discarded as all individuals have normal sizes for the species. No evidence in the marsupial fossil record supports the second. The morphology of the teeth observed does not support the third, as all teeth are apparently permanent (except for one specimen). Finally it is hard to find evidence against or in favour of the fourth, as there is no information available of the development of the museum specimens observed.