The Australasian arboreal marsupials of suborder Phalangeriformes are also called possums because of their resemblance to opossums, but they belong to a different order.

[4] The word opossum is derived from the Powhatan language and was first recorded between 1607 and 1611 by John Smith (as opassom) and William Strachey (as aposoum).

Both men encountered the language at the English settlement of Jamestown, Virginia, which Smith helped to found and where Strachey later served as its first secretary.

[8] Opossums are often considered to be "living fossils",[9] and as a result they are often used to approximate the ancestral therian condition in comparative studies.

[9][10] But this is a mistake, because the oldest opossum fossils are from a more recent epoch, the early Miocene (roughly 20 million years ago).

[11] The last common ancestor of all living opossums dates approximately to the Oligocene-Miocene boundary (23 million years ago) and is at most no older than Oligocene in age.

[13][16][17] Large opossums like Didelphis show a pattern of gradually increasing in size over geologic time as sparassodont diversity declined.

[16][17] Several groups of opossums, including Thylophorops, Thylatheridium, Hyperdidelphys, and sparassocynids developed carnivorous adaptations during the late Miocene-Pliocene, before the arrival of carnivorans in South America.

[18] It has been suggested that the size and shape of the ancestral didelphid's jaw would most closely match that of the modern Marmosa genus.

[20][21][9] Although all living opossums are essentially opportunistic omnivores, different species vary in the amount of meat and vegetation they include in their diet.

[22] The water opossum or yapok (Chironectes minimus) is particularly unusual, as it is the only living semi-aquatic marsupial, using its webbed hindlimbs to dive in search of freshwater mollusks and crayfish.

[27] Metachirus nudicaudatus, found in the upper Amazon basin, consumes fruit seeds, small vertebrate creatures like birds and reptiles and invertebrates like crayfish and snails, but seems to be mainly insectivorous.

[28] As marsupials, female opossums have a reproductive system that includes a bifurcated vagina and a divided uterus; many have a pouch.

[30] Opossums do possess a placenta,[31] but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional.

When an opossum is "playing possum", the animal's lips are drawn back, the teeth are bared, saliva foams around the mouth, the eyes close or half-close, and a foul-smelling fluid is secreted from the anal glands.

The animal will typically regain consciousness after a period of a few minutes to four hours, a process that begins with a slight twitching of the ears.

Males make a clicking "smack" noise out of the side of their mouths as they wander in search of a mate, and females will sometimes repeat the sound in return.

Many large opossums (Didelphini) are immune to the venom of rattlesnakes and pit vipers (Crotalinae) and regularly prey upon these snakes.

[46] This adaptation seems to be unique to the Didelphini, as their closest relative, the brown four-eyed opossum, is not immune to snake venom.

Some authors have suggested that this adaptation originally arose as a defense mechanism, allowing a rare reversal of an evolutionary arms race where the former prey has become the predator,[48] whereas others have suggested it arose as a predatory adaptation given that it also occurs in other predatory mammals and does not occur in opossums that do not regularly eat other vertebrates.

Opossum oil (possum grease) is high in essential fatty acids and has been used as a chest rub and a carrier for arthritis remedies given as salves.

Caluromysiops Caluromys Glironia Hyladelphys Tlacuatzin Marmosa Monodelphis Metachirus Chironectes Lutreolina Philander Didelphis Marmosops Cryptonanus Gracilinanus Lestodelphys Thylamys Caluromysiops Caluromys Glironia Hyladelphys Tlacuatzin Marmosa Monodelphis Chironectes Lutreolina Philander Didelphis (incl.