Didelphimorphia

The order Didelphimorphia contains the common opossums of the western hemisphere. Opossums probably diverged from the basic South American Ameridelphian stock in the latest Cretaceous or early Paleocene. A sister group is the Paucituberculata, or rat opossums.

Didelphimorphs are small to medium-sized marsupials. They tend to be semi-arboreal omnivores, although there are many exceptions. Most members of this taxon have long snouts, a narrow braincase, and a prominent sagittal crest (a bone crest running longitudinally along the center of the skull). The dental formula (one side of one jaw) includes 5 incisors (4 on the lower jaw), 1 canine, 3 premolars and 4 molars. By mammal standards, this is a very full jaw. The incisors are very small, the canines large. The molars are tricuspid.

Didelphimorphs have a plantigrade stance (feet flat on the ground). Most unusually, the hind feet have an opposable digit with a nail—not a claw. Nails are otherwise rare in the animal world and are common only among primates. Also like primates, the tail may be prehensile. The stomach is simple, with a small cecum. Opussum reproductive system are extremely basic, with a reduced marsupium. This means that the young are born at a very early stage. Females are usually somewhat larger than males.

Didelphimorphs are opportunistic omnivores with a very broad range of diet. Their unspecialized biology, flexible diet and reproductive strategy make them successful colonizers and survivors in unsettled times. Perhaps this is why their range has been expanding rapidly into North America.

• SUBCLASS MARSUPIALIA
  • Magnorder Ameridelphia
    • Order Didelphimorphia
      • Family Didelphidae
        • Subfamily Caluromyinae
          • Derby's Woolly Opussum, Caluromys derbianus
          • Equadorian Woolly Opussum, Caluromys lanatus
          • Bare-tailed Woolly Opussum, Caluromys philander
          • Black-shouldered Opossum, Caluromysiops irrupta
          • Busy-tailed Opussum, Glironia venusta
        • Subfamily Didelphinae
          • Yapok or Water Opossum, Chironectes minimus
          • White-eared Opossum, Didelphis albiventris''
          • Big-eared Opossum, Didelphis aurita''
          • Southern Opossum, Didelphis marsupialis''
          • Virginia Opossum, Didelphis virginiana''
          • Aceramarca Gracile Mouse Opossum, Gracilinanus aceramarcae
          • Agile Gracile Mouse Opossum, Gracilinanus agilis
          • Wood Sprite Gracile Mouse Opossum, Gracilinanus dryas
          • Emilia's Gracile Mouse Opossum, Gracilinanus emilae
          • Northern Gracile Mouse Opossum, Gracilinanus marcia
          • Brazilian Gracile Mouse Opossum, Gracilinanus microtarsus
          • Patagonian Opossum, Lestodelphys halli
          • Lutrine Opossum, Lutreolina crassicaudata
          • Anderson's Mouse Opossum, Marmosa anersoni
          • Grayish Mouse Opossum, Marmosa canescens
          • Little Rufous Mouse Opossum, Marmosa lepida
          • Mexican Mouse Opossum, Marmosa mexicana
          • Common Mouse Opossum, Marmosa murina
          • Pale-bellied Mouse Opossum, Marmosa robinsoni
          • Red Mouse Opossum, Marmosa rubra
          • Tyler's Mouse Opossum, Marmosa tyleriana
          • Dryland Mouse Opossum, Marmosa xerophila
          • (list to be completed)
    • Order Paucituberculata: rat opossums
  • Magnorder Australidelphia
    • 5 orders (not listed here)