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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 shrew 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, opossums may have prehensile tails. The stomach is simple, with a small cecum. Opossum reproductive systems 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.
Classification
Within the family of the Didelphidae, the following species exist: - 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 or Thick-tailed Opossum, Lutreolina crassicaudata
- Anderson's Mouse Opossum, Marmosa andersoni
- 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
- Marmosops cracens
- Marmosops dorothea
- Marmosops fuscatus
- Marmosops handleyi
- Marmosops impavidus
- Marmosops incanus
- Marmosops invictus
- Marmosops noctivagus
- Marmosops parvidens
- Brown Four-eyed Opossum, Metachirus nudicaudatus
- Alston's Woolly Mouse Opossum Micoureus alstoni
- Pale-bellied Woolly Mouse Opossum, Micoureus constantiae
- Long-furred Woolly Mouse Opossum, Micoureus demerarae
- Short-furred Woolly Mouse Opossum, Micoureus regina
- Monodelphis adusta
- Monodelphis americana
- Monodelphis brevicaudata
- Monodelphis dimidiata
- Monodelphis domestica
- Monodelphis emiliae
- Monodelphis iheringi
- Monodelphis kunsi
- Monodelphis maraxina
- Monodelphis osgoodi
- Monodelphis rubida
- Monodelphis scalops
- Monodelphis sorex
- Monodelphis theresa
- Monodelphis unistriata
- Black Four-eyed Opossum, Philander andersoni
- Gray Four-eyed Opossum, Philander opossum
- Thylamys elegans
- Thylamys macrura
- Thylamys pallidior
- Thylamys pusilla
- Thylamys velutinus
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