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Cryptonanus
''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.
==Taxonomy==
Species of ''Cryptonanus'' were first described in 1931 by George Henry Hamilton Tate,〔Voss et al., 2005, p. 5〕 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''),〔Tate, 1931, p. 10〕 and ''Marmosa unduaviensis'' (now ''Cryptonanus unduaviensis'') as a separate species.〔Tate, 1931, p. 11〕 In 1943, another species was described, ''Marmosa agricolai'' (now ''Cryptonanus agricolai'').〔Gardner, 2009, p. 41〕 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''.〔Gardner, 2009, p. 40〕
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''.〔Voss et al., 2005, p. 2〕 A phylogenetic analysis corroborated the distinctness of ''Cryptonanus'' and ''Gracilinanus''.〔Voss et al., 2005, fig. 4〕 Voss and colleagues first noted the discovery in a footnote in their 2004 paper on ''Chacodelphys''〔Voss et al., 2004, p. 6, footnote 4〕 and subsequently described the group of species with the additional foramen as a new genus, ''Cryptonanus''.〔 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.〔Voss et al., 2005, p. 11〕 ''Cryptonanus'' is currently classified in the tribe Thylamyini of subfamily Didelphinae within the opossums.〔Voss and Jansa, 2009〕
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.〔 Further research in this matter is needed.〔Voss and Jansa, 2009, p. 128〕
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