The Argentine fat-tailed mouse opossum (''Thylamys sponsorius'') was formerly considered a species of

opossum Opossums () are members of the marsupial order Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 126 species in 18 genera. Opossums originated in South America and entered North A ...

in the family Didelphidae. It is found in the eastern foothills of the

Andes The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...

in northern

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

and southern

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

. Typically reach around 10-13 cm (4-5 inches) in body length, with a bushy tail adding another 12-15 cm (5-6 inches). 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. While the

American Society of Mammalogists The American Society of Mammalogists (ASM) was founded in 1919. Its primary purpose is to encourage the study of mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence ...

maintains this as a distinct species,

mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

sequence analysis does not support the population being distinct from ''T. cinderella''.

References

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