By Phil Myers
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; the hallux 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.
Technical characters ( skull, basicranium, lower jaw); see also Metatheria
Literature and references cited
Aplin, K. P., and M. Archer. 1987. Recent advances in marsupial systematics with a new syncretic classification. Pp. xv-lxxii in Archer, M. (ed.), Possums and Opossums: Studies in Evolution, Vol. I. Surrey Beatty and Sons PTY Limited, Chipping Norton. lxxii+400 pp.
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Marshall, L. G. 1984. Monotremes and marsupials. Pp 59-115 in Anderson, S. and J. Knox Jones, eds, Orders and Families of Recent Mammals of the World. John Wiley and Sons, NY. xii+686 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vi+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
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Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
