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.