immersed in water. The male also has a pouch (although not as watertight as the female's), where he places his genitalia before swimming. This is thought to prevent them from becoming entangled in aquatic vegetation and is probably helpful in streamlining the animal as well. Reproduction Water opossums mate in December and a litter of 1-5 young is born 12 to 14 days later in the nest. By 22 days the offspring are beginning to show some fur, and by 40 days or so their eyes are open, their bodies protruding from the mother's pouch. At 48 days of age, the young opossums detach from the nipples, but they still nurse and sleep with the mother. Fossil record The water opossum seems to have a history dating as far back as to the Pliocene Epoch. The first Pliocene occurrences were recorded in 1902, 1906, and 1917 when two fragments of left mandibular ramus with alveoli (characterized as MACN 2464 and MACN 3515), as well as the fragment of the distal end of a humerus (MACN 3675) were collected near the city of Parani, Entre Rios Province, Argentina. The mandibular fossils, MACN 2464 and 3515 were known to be from the water opossum because their teeth were nearly indistinguishable from living populations of the organism in present day. Additionally, water opossum teeth are characteristically different from other didelphids, likely because they are the only semi-aquatic marsupials, making these teeth easy to identify. Holocene subfossil fragments of Chironectes have been discovered in São Paulo, Brazil. Also, there are fossil specimens from the late Pleistocene-Recent cave deposits in Minas Gerais, Brazil, as well as from the late Pliocene in Entre Ríos Province, Argentina. Subspecies Chironectes minimus argyrodytes Chironectes minimus langsdorffi Chironectes minimus minimus Chironectes minimus panamensis References Footnotes Bibliography External links Oxford University Press, Journal of Mammalogy, Volume 90, Issue 1, 18 February 2009, Pages 93–103 Opossums Aquatic mammals Marsupials of North America Marsupials of Central America Marsupials of South America Mammals of Argentina Mammals of Bolivia Mammals of Brazil Mammals of Colombia Mammals of Ecuador Mammals of French Guiana Mammals of Guyana Mammals of Mexico Mammals of Paraguay Mammals of Peru Mammals of Suriname Mammals of Venezuela Mammals of Trinidad and Tobago Mammals of the Caribbean Mammals described in 1780 Taxa named by Eberhard August Wilhelm von Zimmermann