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Chacoan pygmy opossumThe Chacoan pygmy opossum is the smallest known species of opossum. The head-body length is 68 mm, the tail is 55 mm long and the hind foot is 11 mm. It differs from other "marmosine" or mouse opossums in having a long third manual digit, no distinctly tricolored pelage, a long fourth pedal digit, and a tail shorter than the head-body length. No other marmosine genera have this combination of characters.
Until 2004, it was only known from one specimen collected in the Chaco of Formosa Province, Argentina in 1920. It is classed as Vulnerable.
It was originally described as Marmosa muscula Shamel (1930a); as this name is preoccupied, Shamel (1930b) renamed it M. formosa. George Tate (1933) considered it a member of his "Elegans group" (=Thylamys) of Marmosa. It was treated as a species of Marmosa or Thylamys until 1989, when Gardner & Creighton (1989) listed it as a synonym of Gracilinanus agilis and later separated it from this species as G. formosus. Voss et al. (2004) erected the new genus Chacodelphys for the species.