The Chacoan Pygmy Opossum (Chacodelphys formosa) is a recently described genus and species of didelphimorph marsupial

Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

. The only species in Chacodelphys, C. formosa, is known from only one specimen collected in 1920 in the Chaco

Gran Chaco

The Gran Chaco is a sparsely populated, hot and semi-arid lowland region of the Río de la Plata basin, divided among eastern Bolivia, Paraguay, northern Argentina and a portion of the Brazilian states of Mato Grosso and Mato Grosso do Sul, where it is connected with the Pantanal region...

 of Formosa Province

Formosa Province

Formosa Province is in northeastern Argentina, part of the Gran Chaco Region. Its northeast end touches Asunción, Paraguay, and borders the provinces of Chaco and Salta to its south and west, respectively...

, Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

.

Description

The Chacoan Pygmy Opossum is the smallest known species of didelphid. It has a head-body length of 68 mm, a tail of 55 mm and a hind foot of 11. It differs from the other "marmosine

Marmosa

The nine species in the genus Marmosa are relatively small Neotropic members of the order Didelphimorphia. This genus is one of four that are known as mouse opossums. The others are Micoureus , Thylamys and Tlacuatzin, the grayish mouse opossum...

" genera (Marmosa

Marmosa

The nine species in the genus Marmosa are relatively small Neotropic members of the order Didelphimorphia. This genus is one of four that are known as mouse opossums. The others are Micoureus , Thylamys and Tlacuatzin, the grayish mouse opossum...

, Micoureus

Micoureus

Micoureus is a genus of Neotropic opossums of the family Didelphidae. It includes the following species:* Alston's Mouse Opossum * White-bellied Woolly Mouse Opossum...

, Monodelphis, Thylamys

Thylamys

Thylamys is a genus of opossum in the family Didelphidae. The premaxillae are rounded rather than pointed. The females lack a pouch. The females' nipples are arranged in two symmetrical rows on the abdomen All species but T...

, Tlacuatzin, Gracilinanus

Gracilinanus

Gracilinanus is a genus of opossum in the family Didelphidae. It was separated from the genus Marmosa in 1989, and has since had the genus Cryptonanus removed from it...

, Marmosops

Marmosops

Marmosops is a genus of Neotropic opossums of the family Didelphidae. It includes the following species:* Genus Marmosops** Bishop's Slender Opossum ** Narrow-headed Slender Opossum...

, Lestodelphys) in having a long third manual digit, no distinctly tricolored pelage, a long fourth pedal digit, and a tail shorter than head-body. No other marmosine genera has this combination of characters.

Taxonomic history

C. formosa was originally described as Marmosa muscula Shamel (1930a); however, this name is preoccupied, so Shamel (1930b) renamed it M. formosa. Afterwards, George Tate

George Henry Hamilton Tate

George Henry Hamilton Tate was an English-born American zoologist, who worked as a mammalogist for the American Museum of Natural History in New York....

(1933) considered it a valid member of his "Elegans group" (=Thylamys) of Marmosa, whereafter it has been variously synonymized or treated as a distinct species of Marmosa or Thylamys until 1989, when Gardner & Creighton (1989) listed it as a synonym of Gracilinanus agilis, and then later separated from this species as G. formosus. Finally, Voss et al. (2004) erected the new genus Chacodelphys for the species.