What's the Difference between a Possum and an Opossum?

Possums, opossums, or both?

Though some people view these terms as interchangeable, at least in the United States, they actually aren't. In fact, they describe quite distinct groups of animals.

Common brushtail possum (Trichosurus vulpecula), native to Australia

Possums are a group of marsupials found only in the Eastern hemisphere, mainly in Australia and New Guinea. All species that can technically be called possums are included in the Suborder Phalangeriformes, which includes six different Families: Phalangeridae, Acrobatidae, Petauridae, Burramyidae, Tarsipedidae, and Pseudocheiridae. There are also a number of species in this Suborder that don't include "possum" in their common names: the gliders and cuscuses.

Suborder Phalageriformes is included in the Order Diprotodontia, one of the many Orders to be found in Class Mammalia, and one of seven orders of marsupials. Order Diprotodontia includes the largest number of marsupial species, including wombats, bettongs, and the ever-charismatic kangaroos and koalas. Often, a diprotodont will be the first thing someone thinks of when they hear the word "marsupial."

Elegant fat-tailed mouse opossum (Thylamys elegans), native to Chile

Opossums, in contrast, are different enough from possums that they are classified in a completely separate order: Order Didelphimorphia. This Order has only one Family: Family Didelphidae. This group includes all of the marsupials to be found in the Americas. Though most of the species are found in Southern and Central America, there is a single species that lives quite happily farther north: the Virginia opossum (Didelphis virginiana).

What are some of the differences between possums and opossums? Beyond a number of morphological differences, possums are mainly herbivorous, nectivorous, or insectivorous while opossums are mostly omnivorous or carnivorous. This ties into the morphological differences, as the teeth of the two groups are quite different.

It remains true, however, that opossums are colloquially referred to as possums by many people. Some consider all of these species to be possums, with just the American species being opossums. However, since there are significant differences between the two groups I don't think this is appropriate.

Sources are Animal Diversity Web and Merriam-Webster. Images are from Wikimedia Commons under Creative Commons licenses: one, two, three.