Supernumerary molars in neotropical opossums (Didelphimorphia, Didelphidae)

Dental abnormalities, such as the occurrence of extra teeth, are recurrentl y found in many groups of mammals. Supernumeray molars were found in Didelp his aurita, D. albiventris, D. marsupialis, Philander andersoni, A frenota, P. opossum, Chironectes minimus, and Caluromys philander. Frequencies of o ccurrence of supernumerary teeth in these marsupial species remained within a range similar to that found in other species. Four hypotheses are propos ed 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 erup tion of the permanent premolar, or some sort of ontogenetic disturbance tha t Lead to the duplication of a tooth germ. The first hypothesis is discarde d as all individuals have normal sizes for the species. No evidence in the marsupial fossil record supports the second. The morphology of the teeth ob served does not support the third, as a[[ teeth are apparently permanent (e xcept 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 developme nt of the museum specimens observed.