Over on the other side of the world, in Australia, the word "possum" is, however, used to refer to an entirely different animal. These are nocturnal, tree-dwelling creatures, typically with large eyes and long tails, and the majority of the seventy or so species are herbivorous. Early settlers, who had probably only vaguely heard of the American animal, nonetheless decided to give it the same name. Like the Americans, over time they confused "opossum" with "a possum", and shortened the word. Unlike the Americans, their shortened word became not merely colloquial, but the one formally used in zoological texts.
The two animals are not particularly closely related, which is unsurprising, when you consider that they live so far apart. But they do, nonetheless, have one important thing in common: they are both marsupials.
So what are marsupials, and what unites these two animals at opposite ends of the world? (Yes, I know I've sort of covered this before, but that was seven years ago, and I'm going for a slightly different angle this time).
The word "marsupial" first entered the scientific literature in 1811, when Johann Illiger used it as the name for a family of mammals that he thought were closely related to the primates. Unlike primates, however, the females of these animals, such as kangaroos, had a pouch, in which they carried their young. Illiger borrowed the word marsuppium, meaning a pouch or purse, from Latin, as the name for his newly described group.
The concept of "family" in the modern taxonomic sense was very new at the time, and Illiger was more of a specialist in insects than in mammals, and it was only a few years later, in 1816, that Georges Cuvier revised his scheme. An expert in vertebrate anatomy, he realised that marsupials had nothing much to do with primates, and, in any event, there were too many kinds, with too much variation, to justify them being a mere family, and so he moved them to the next rank up, that of "order". Since then, just how different they are from other mammals has become increasingly clear, and, from about the 1990s onwards, they have been placed even higher up, as a group of related orders that branched off from the main mammalian line very early on.
How early? Well, almost ridiculously so, as it turns out, but before explaining that, we need to know how we identify marsupials, and how they differ from other mammals.
In fact, I've met people who weren't entirely sure whether marsupials even are mammals at all. This is probably because there's no commonly used word in English for the 95% of mammals that aren't marsupials. People therefore confuse what would technically be called "placental mammals" with just plain old mammals, and have a vague understanding that marsupials are something else. (There are also, of course, a tiny number of living mammals that are neither).
However, the defining features of mammals are that they give milk to their young, and that they have three bones in the middle ear, both of which are true of marsupials. That they have fur is also evidence for their mammalian nature, although the existence of dolphins proves that this isn't a mandatory requirement.
There are a number of anatomical differences between marsupials and other mammals, but the most significant are in the reproductive system. As the name itself suggests, of course, the females typically have a pouch, in which they carry their young. This however, is not as universal as one might think. In many cases, the supposed pouch is nothing more than a fold of skin that only partially protects the young, and one group of admittedly obscure South American marsupials doesn't even have that much.
More accurately, then, we would say that the key feature of marsupials is that they give birth to very undeveloped young, after an unusually short pregnancy. The young, which are tiny, must crawl their way through the mother's fur to reach her teats, which they then cling to as they grow into a more developed form, and eventually achieve independence. In most marsupials, of course, these teats are inside the pouch, which helps to protect them, as well as keeping them warm until they can regulate their own body temperature.
The problem with this, of course, is that it doesn't fossilise. There's no way we can identify whether a given fossil is a marsupial by looking for its pouch, or by examining the structure of its reproductive system. Fortunately, there are features that we can look for, which help us trace the history of the marsupial lineage.
Unfortunately, it turns out that these aren't unique to marsupials, and, indeed, placental mammals are the only ones that don't have them. That may be 95% of all living species, but it does mean that the presence of the bones isn't a sure-fire indicator of marsupial heritage when you're looking at really old fossils whose relationship to living species is unclear. In fact, it appears that all early mammals had the bones, and the ancestor of placentals lost them at some point, possibly because they got in the way of the expanding uterus during pregnancy.
This leaves us with that great fall-back in mammal palaeontology; looking at the teeth. The original placental mammals had three incisors, a single canine, four premolars, and three molars, in each side of each jaw. Most species have lost at least some of these since, but the underlying pattern remains, outside of highly specialised animals such as dolphins and anteaters. However, as the living opossum indicates, marsupials have many more teeth than this, with a starting total of 50 teeth, rather than 44.
Moreover, marsupials don't have milk teeth, as placentals typically do. It's not that they don't form; they do, but most of them never erupt, and vanish before the adult teeth appear. Using these sorts of features, especially the number and shape of the teeth, we can trace marsupial ancestry back to long before the extinction of the non-avian dinosaurs, to at least the Early Cretaceous, over 140 million years ago. Technically, because they lived so long before the last common ancestor of all the living forms, these very early ancestors aren't marsupials as such, but they do form a distinct lineage that stretches back just as far as the ancestors of placental mammals.
This brings us to one of the myths about marsupials: that they are somehow more 'primitive' than placental mammals. In fact, both lineages have an equally long history, existing side-by-side for millions of years. Marsupials first appeared in the Northern Hemisphere, before heading to South America, and eventually, via what was then an Antarctic land bridge, Australia. While it's true that all of those in the north died out, they survived in South America, and a number of species still live on that continent today, evidently not out-competed by their placental neighbours. One of them, of course, subsequently headed north again, which is where the Virginia opossum comes from.
Having said which, it is likely that the marsupial mode of reproduction is closer to that of the early mammals, once they had given up egg-laying. The widespread presence of those epipubic bones, including some in the very earliest ancestors of placental mammals, makes this, at least, quite likely.
How many different kinds of marsupial are there? There are over 300 species, which, as I've noted above, is around 5% of the 6,500 mammal species currently known. They fall, broadly speaking, into three main sub-groups.
Largest are the diprotodonts, the large (ish) herbivorous marsupials of Australia, New Guinea, and neighbouring islands. These include the true possums, and also such familiar animals as kangaroos, wallabies, koalas, and wombats, alongside more obscure creatures such as cuscuses, potoroos, and the wonderfully-named noolbenger. The scientific name for the group means 'two front teeth', and, indeed, they are characterised by the presence of large clipping incisors at the front of the mouth, that look a lot like rabbit teeth.
Secondly, there are the carnivorous marsupials, also found only in and around Australia. By far the most famous of these is the Tasmanian devil, with the great majority being much smaller animals resembling shrews or mice, including such things as dibblers, dunnarts, and the red kaluta.
The third group are, in fact, the opossums, over in the Americas. There are over 90 species of these animals, descendants of an early branch of the marsupial family tree that likely pre-dated the first trip of their relatives to Australia. These are the ones that retain the full set of teeth, and include, of course, that one species that reaches the USA and Canada.
Even these aren't quite the entirety of marsupial species, because there are a few that are sufficiently peculiar not to fit neatly within any of the three main groups. While the ant-eating numbats are clearly related to the carnivorous marsupials, the long-snouted, semi-hoofed, bandicoots are something else entirely, and the marsupial moles are just plain weird. Over in South America, there are those odd, pouchless, shrew-opossums, and the monito del monte, which is more closely related to its kin in Australia than to any of its own neighbours.
[Photos by Andrew Mercer and Pierre-Yves Beaudouin, from Wikimedia Commons.]