Shrew Opossums: Paucituberculata

The New World marsupial order Paucituberculata contains only one surviving family, the Caenolestidae, or shrew opossums (the true shrews, family Soricidae, are not marsupial but placental mammals). Shrew opossums are mouse-sized marsupial mammals with dense, dark gray or brown fur. Also called mouse opossums and rat opossums, their appearance is more suggestive of shrews. Shrew opossums have long, conical, pointed snouts and tiny, beady eyes. There are three genera (JEN-uh-rah; plural of genus, JEE-nus) and five species: the living members of order Paucituberculata and family Caenolestidae are relicts, little-changed survivors of a once far more diverse assemblage of genera and species. Information about them is spotty, since the animals are nocturnal and secretive, and live for the most part in remote, difficult terrain.

Head and body length across species runs from 3.5 to 5.5 inches (9 to 14 centimeters), and tail length is 3.5 to 5.5 inches (9 to 14 centimeters), the tail length being about the same as the head and body. Males are larger than females, males weighing from 1 to 1.5 ounces (25 to 41 grams), females from 0.5 to 1 ounce (16.5 to 25.5 grams). A shrew opossum has five digits on each foot, the two small outer toes of the forefeet with blunt nails, the remaining three equipped with curved, sharp, strong claws. The hind feet have strong, curved claws on all toes except the so-called great toe, which is small and carries a small nail.

The fur of shrew opossums is thick and soft, covering the entire body and less dense on the tail. The coat has a disorderly look because different areas of the coat have different textures. The fur may be dark on the dorsal (upper) parts of the body, with lighter-colored under parts, or dark all over the body. Dark colors vary from gray-brown, black-brown, to near-black. The ears are shaped much like those of typical mice, and large enough to protrude well above the thick fur. Hearing is acute, as is the tactile (touch) sense of the long whiskers.

The upper lips bear small flaps of skin on both sides, a feature found only in the Caenolestidae. The Chilean opossum has these and similar flaps on its lower lips. Their function is, so far, unknown. They may be barriers to prevent blood, pieces of flesh, and dirt from collecting on the sides of the jaws. The tail is about as long as the combined head and body. The tail fur, sparser than on the body, is the same color as the upper pelt, but may include a white tip in some individuals. The tail is not prehensile, meaning it is not able to curl around and grasp objects. The tail of the Chilean shrew opossum swells up with stored fat for the southern winter months. Females do not have pouches and have four nipples, except for females of the Chilean shrew opossum, which have seven nipples, the seventh located on the midline of the underbelly.

The rostrum, or the front part of the skull including the jaws, is long and tapering. Each of the lower incisors has only one cusp, or protruding bump on its crown, unlike most mammal teeth, hence the order name, Paucituberculata, meaning, in Latin, "few bumps," since this feature is found in all species, living and extinct, in the order.