Cryptonanus is a genus of opossums from South America. It includes five species found from Bolivia to Uruguay and eastern Brazil, one of which is now extinct. Although the first species were discovered in 1931, the genus was not recognized as distinct from Gracilinanus until 2005. It includes small opossums with generally grayish, sometimes reddish, fur that are mainly distinguished from other opossums by characters of the skull.
Species of Cryptonanus were first described in 1931 by George Henry Hamilton Tate,[1] who described Marmosa microtarsus guahybae (now Cryptonanus guahybae) as a subspecies of Marmosa microtarsus (now Gracilinanus microtarsus), Marmosa agilis chacoensis (now Cryptonanus chacoensis) as a subspecies of Marmosa agilis (now Gracilinanus agilis),[2] and Marmosa unduaviensis (now Cryptonanus unduaviensis) as a separate species.[3] In 1943, another species was described, Marmosa agricolai (now Cryptonanus agricolai).[4] Species of Cryptonanus were then included in a broadly defined genus Marmosa until the genus Gracilinanus was described in 1989. The fifth currently recognized Cryptonanus species, C. ignitus, was described as a species of Gracilinanus in 2002. At that time, the species of Cryptonanus were variously regarded as separate species or as synonyms or subspecies of other species of Gracilinanus.[5]
Robert Voss and others noticed that some of the animals then classified in Gracilinanus had an additional foramen ovale, an opening in the skull that is formed by an extension of the bone of the alisphenoid tympanic wing towards the middle and front. They looked for other characters that correlated with the presence of the foramen and found them easy to find, defining a group of species distinct from Gracilinanus.[6] A phylogenetic analysis corroborated the distinctness of Cryptonanus and Gracilinanus.[7] Voss and colleagues first noted the discovery in a footnote in their 2004 paper on Chacodelphys[8] and subsequently described the group of species with the additional foramen as a new genus, Cryptonanus.[1] The generic name, Cryptonanus is derived from the Ancient Greek words κρυπτος kryptos (hidden) and νανος nanos (dwarf) and was chosen because Cryptonanus species are small and their true identity was long hidden by taxonomic synonymy.[9] Cryptonanus is currently classified in the tribe Thylamyini of subfamily Didelphinae within the opossums.[10]
Voss and colleagues recognized each of the five names they referred to Cryptonanus—agricolai, chacoensis, guahybae, ignitus, and unduaviensis—as separate species, although they could find few distinguishing characters between them.[1] Further research in this matter is needed.[11]