Shrew opossum

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.

Shrew opossums (also known as rat opossums or caenolestids) are about the size of a small rat (9–14 cm long), with thin limbs, a long, pointed snout and a slender, hairy tail. They are largely carnivorous, being active hunters of insects, earthworms, and small vertebrates. They have small eyes and poor sight, and hunt in the early evening and at night, using their hearing and long, sensitive whiskers to locate prey. They seem to spend much of their lives in underground burrows and on surface runways. Like several other marsupials, they do not have a pouch, and it appears that females do not carry the young constantly, possibly leaving them in the burrow.

Largely because of their rugged, inaccessible habitat, they are very poorly known and have traditionally been considered rare. Recent studies suggest they may be more common than had been thought.