Elsewhere we've spoken of those "living fossils," the Duck-billed Platypus and Spiny Anteater. The American opossum is nearly as curious -- despite its being fairly common.
If you open just about any field guide to North American mammals, the opossum will probably be the very first of the 380 or so mammal species described. That's because opossums are our only marsupial, and marsupials are considered very ancient forms of mammals. Field guides typically place the "primitive" organisms first. The vast majority of the world's marsupials occur mainly in Australia, where kangaroos are the best known of the group.
You know that kangaroo mothers carry their young in pouches on their bellies. So do opossums. Opossum babies are born after only eleven to thirteen days of gestation (gestation is the time lapsing between when the sperm unites with the egg, and the creature is born). At birth, newborn opossums are so tiny that an entire litter, consisting of up to 14 babies, can fit into a single teaspoon. They are so undeveloped that it's impossible for them to survive outside the mother.
However, their front legs are developed all out of proportion to the rest of their bodies, and are strong enough for the completely blind babies to pull themselves from their birth opening, across the mother's belly, into her pouch, or marsupium. They do this by instinct. If the mother is disturbed and moves during the babies' pouch-searching process, the babies may well lose their way, fall off, and perish. Once inside the pouch, the babies find a nipple, attach themselves to it very securely, and during the next two months ride in the mother's pouch continuing their development.
Why did this round-about manner of producing babies evolve? One answer is that marsupials are considered primitive forms of mammal. Scientists have seen that in the evolution of animals it's a general trend for species to evolve toward producing few children, but taking good care of them. Of course, that's the way we very modern, recently evolved humans do it. The opossum's 14 or so babies clearly represent the primitive approach of producing lots of offspring with maybe only one or two ultimately surviving. But, think of this: If all 14 opossum babies were to develop to full term inside the mother, the poor mother wouldn't be able to move around much during her pregnancy. As it is, however, the young settle in a pouch that stretches as they grow, and the pouch is emptied more and more as the young mature and begin learning about the outside world.
The opossum's tail is long, thick, hairless, and very talented. It can wrap around things and hold onto them. Such useful tails are said to be prehensile. One of the most remarkable sights ever to hope for in our backyards is that of a mother opossum with all her babies riding on the mother's back, with their tails grasping hers, to keep from falling off.
Sometimes when opossums are cornered in a dangerous situation, they simply keel over into a dead faint. Many predators, such as dogs, then don't know quite what to do with a dead opossum. If the predator decides to just leave, in a few minutes the opossum will raise its head and look around, and if no danger seems to be threatening, it'll rush to cover. It's been "playing possum." Research indicates that the opossum isn't really "playing" when it does this. It actually seems to undergo a kind of nervous collapse. Since opossums aren't well equipped for fighting, their "delicate nervous system" may be an adaptation for surviving attack by predators such as dogs, who may be more interested in a good fight than in eating a sick-acting opossum.
Probably the main reason that opossums are so successful is that they aren't fussy about what they eat -- they're real omnivores, consuming fruits, vegetables, nuts, flesh, eggs, insects, carrion, and just about anything that can be chewed and swallowed in the garbage can.
If you'd like to introduce a young person to opossums, these books from Amazon.com might help you: Opossum at Sycamore Road (the Smithsonian Backyard series) and Opossums (the Nighttime Animals series)