The Argentine fat-tailed mouse opossum (''Thylamys sponsorius'') was formerly considered a species opossum in the family Didelphidae. It is found in the eastern foothills of the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

in northern

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

and southern Bolivia. Its dorsal fur is gray brown to dark brown. Its ventral fur is gray-based except for the white to yellowish chest hairs. It has been distinguished from ''T. cinderella'' by its postorbital ridges. ''T. cinderella'' has well-developed postorbital ridges in both juveniles and adults that extend laterally behind the eye sockets. Only adults of ''T. sponsorius'' have fully developed postorbital ridges, and these do not extend laterally behind the eye sockets. However, mitochondrial DNA sequence analysis does not support the population being distinct from ''T. cinderella''.

References

Opossums Marsupials of Argentina Mammals of Bolivia Marsupials of South America Mammals described in 1921 Taxa named by Oldfield Thomas Taxobox binomials not recognized by IUCN {{marsupial-stub