Living didelphimorphs, the opossums, are a diverse group of marsupials, including only one family but over 60 species. Most occupy Central and South America, but one species, Didelphis virginiana, occurs through most of the continental United States. Living didelphimorphs are small to medium in size. Their morphology is often referred to as "generalized," and they probably differ little in most respects from their Cretaceous ancestors. They have 5 upper and four lower incisors (that is, they are polyprotodont). Their canines are large. Molars are tritubercular with well developed talonids. The dental formula is 5/4, 1/1, 3/3, 4/4 = 50. Their feet are not syndactylous, and the first toe of their hind feet (hallux) is partially opposable. All toes except the hallux have claws; thehallux has a nail. The tail is prehensile and usually long and scaly. Opossums have relatively long rostrums, a small braincase, and often, a prominent sagittal crest.
Many opossums have a well-developed pouch, but some lack this structure. Their stomachs are simple.
Most members of this group are omnivorous or carnivorous. They can be found in most neotropical habitats from sea level to over 3000m, from dry thornscrub and grassland to tropical forest. Most are at least partially arboreal, but one species has become aquatic. Opossums are generally solitary, not interacting with conspecifics except to reproduce.
Didelphidae
(New World opossums)
Restricted to North and South America, didelphids have radiated into a wide variety of forms. Most are omnivorous or carnivorous. Several species are arboreal; one is aquatic and has fully webbed hind feet. Didelphids can be found in most habitats from sea level to over 3000m, from dry thornscrub and grassland to tropical forest. The extinct relatives of didelphids were even more varied in their morphology and habits; one group specialized as large carnivores, with one species actually resembling sabre-toothed cats, while another group apparently converged on kangaroo rats and other desert rodents.
Didelphids have a full complement of teeth (five upper and four lower incisors on each side of the jaw, one canine, three premolars, and four molars). Opossums are small to medium in body size; all have five digits on fore- and hindfeet, with the first toe on the hindfoot partially opposable; all digits except the first toe on the hindfoot have claws (it has a nail). The tail is long, scaley, and prehensile in most species. Other characteristics are described under the order Didelphimorphia.
 didelphis virginiana |
 koala bear |
oppossum |
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ARTIODACTYLA
CARNIVORA
CETACEA
CHIROPTERA
DASYUROMORPHIA
DERMOPTERA
DIDELPHIMORPHI
DIPROTODONTIA
HYRACOIDEA
INSECTIVORA
LAGOMORPHA
MACROSCELIDEA
MICROBIOTHERIA
MONOTREMATA
NOTORYCTEMORPHIA
PAUCITUBERCULATA
PERAMELEMORPHIA
PERISSODACTYLA
PHOLIDOTA
PRIMATES
PROBOSCIDEA
RODENTIA
SCANDENTIA
SIRENIA
TUBULIDENTATA
XENARTHRA
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