Caenolestestoidea
| Shrew opossums Temporal range: Late Oligocene–Recent |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Infraclass: | Marsupialia |
| Order: |
Paucituberculata Ameghino, 1894 |
| Family: |
Caenolestidae Trouessart, 1898 |
| Genera | |
The order Paucituberculata /ˌpɔːsᵻˌtjuːbərkjᵿˈlɑːtə/ contains the seven surviving species of shrew opossum: small, shrew-like marsupials that are confined to the Andes mountains of South America. The order is thought to have diverged from the ancestral marsupial line very early. They are considered to be part of Ameridelphia alongside true opossums. (Ameridelphia is paraphyletic, having given rise to Australidelphia, and thus could be considered an evolutionary grade.) Genetic studies indicate that they are the second most basal order of marsupials, after the didelphimorphs. As recently as 20 million years ago, at least seven genera were in South America. Today, just three genera remain. They live in inaccessible forest and grassland regions of the High Andes.
Shrews were entirely absent from South America until the Great American Interchange three million years ago, and are currently present only in the northwestern part of the continent. Traditionally, it was thought that shrew opossums lost ground to these and other placental invaders that fill the same ecological niches. Evidence suggests, however, that both groups not only overlap, but do not seem to be in direct competition, and the marsupials' larger size seems to imply that they prey on shrews and rodents. Several opossums, such as Monodelphis, also occupy small insectivore niches.
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