Thylamys is a genus of opossums in the family Didelphidae. The premaxillae are rounded rather than pointed. The females lack a pouch. The females' nipples are arranged in two symmetrical rows on the abdomen.[2] All species but T. macrurus store fat in their tails.,[3] although this is not necessarily true for all species in the genus.[4] Fossils belonging to the genus date back to the Miocene, with the oldest specimens being found in the Cerro Azul Formation of Argentina and the Honda Group of Colombia.[5] Genetic studies indicate that the genus may have originated around 14 million years ago.[6]
^Steiner, C.; et al. (2005). "New DNA data from transthyretin nuclear intron suggest an Oligocene to Miocene diversification of living South American opossums (Marsupialia: Didelphidae)". Molecular Phylogenetics and Evolution. 35 (2): 363–379. Bibcode:2005MolPE..35..363S. doi:10.1016/j.ympev.2004.12.013. PMID15804409.