This is the smallest of the Paraguayan mouse opossums:
The generic name Cryptonanus is taken from the Greek meaning 'hidden dwarf', referring to the fact that it had been overlooked for so long (Smith, 2009).
The small didelphid marsupials formerly classified as Marmosa (sensu Tate, 1933) include numerous superficially similar species distinguished from other confamilial taxa by:
Despite such resemblances, ‘marmosines’ are now widely recognised as a polyphyletic group, some members of which may be more closely related to Didelphis and other large opossums than to Marmosa sensu stricto (see Kirsch and Palma, 1995; Patton et al, 1996; Jansa and Voss, 2000; Voss and Jansa, 2003).
Although considerable progress has been made toward a monophyletic classification of these animals, the unrevised contents of several genera remain problematic (Voss et al,2005).
W Foster collected the type specimen for the species on 11 of September 1903 in Sapucay, Paraguay.
G H H Tate described this species in 1931 as a new subspecies of Marmosa agilis. The type specimen used for the original description is a skin and a skull which are part of the Museum’s collections.
However, it was not until 2005 that the consistent differences in characters were recognised as belonging to a different genus by Voss, Lunde and Jansa (Voss et al, 2005). These included: