Because not every animal in Australia wants to kill you
Marsupials are diverse animals, in many cases, paralleling their placental counterparts in some key aspects of their anatomy or behaviour. True, they are not quite as diverse as their placental kin - there are no marsupials bats or dolphins, for instance. There are around 350 known living species of marsupial, commonly grouped into 19 "families", and, while is this undeniably small compared with the 5,000+ species and 120 or so families of placentals, it's hardly insignificant.
In fact, over the last 25 years or so, we have discovered somewhere around two new species of marsupial every year. It's hard to know how many of these will stand the test of time and, in many cases, we're talking about a previously known subspecies being raised to species level, but it seems safe to say that there are still many more species we don't know about. This is particularly true where they live in inaccessible places, with the mountainous jungles of New Guinea being perhaps the best such example.
It's not just new species that have been discovered; the expanding role of molecular and genetic studies over the last few decades, combined with traditional anatomic studies of less well-known groups, has greatly improved our picture of just how diverse marsupials are, and how the different kinds relate to one another. So, with the aid of a recent review of the subject, let's take a look at where we are, and what we know now that we didn't know... well, when I was studying this sort of thing at university, anyway.
The majority of marsupial species are, of course, native to Australia, and to its neighbouring islands, such as New Guinea. These animals fall into two broad groups - basically, the ones that are herbivorous and the ones that aren't (the reality is, of course, a little more complex). The bulk of the latter belong to the dasyurid family, the most famous member of which is the Tasmanian devil. It's also the largest living animal in its family, which is shared with a small number of cat-sized animals and absolutely loads of things that are basically marsupial shrews.
Since around the late '90s, it's been clear on a broad scale, how these different shrew-like animals relate to one another, although some significant debates remain on the finer scale, such as exactly where some of the New Guinean species fit. Many of the new species of marsupial discovered have also belonged to this group, since, being small, they're quite hard to spot.
It's not quite so simple with their relatives, the bandicoots. These are moderately small omnivorous animals with long snouts, and they used to be considered to all belong to a single family. In 1999, they were split into three families - although one of these had been entirely extinct since the 1950s. That study was conducted on analysis of variation in a single gene - entirely reasonably, given the technology of the time - but has been broadly confirmed by larger studies since. Nonetheless, just last year, it was suggested that the bandicoots of New Guinea might represent a separate family from the Australian sort, rather than the latter being an offshoot of the former, as previously thought.
There are two other families that belong to this mostly-carnivorous branch of "Australian" marsupials, both of which represent very small groups of distinctive and odd-looking animals. The numbats, which are basically marsupial anteaters, are clearly related to the main, dasyurid, group, although some complex issues with genetics mean that how either relates to the now-extinct thylacines isn't 100% clear.
The other puzzling group are the marsupial moles. with studies over the last 20 years or so showing no consistent pattern of where they might fit in the evolutionary tree. For what it's worth, the largest study to date puts them closer to the bandicoots than the other mostly-carnivorous marsupials, but there's no reason to suppose that this has been finally settled as yet.
As for the "herbivorous" marsupials (which do, in fact, include a few extinct carnivores), these include all of the most familiar Australian marsupials, so it's perhaps not surprising that they have received a larger share of the scientific attention, too. It has been clear for a long time that koalas and wombats are related, but the wide range of all the other herbivorous marsupials has proven harder to disentangle.
Not least because there's rather a lot of them. We do know that there are three evolutionary branches here, one of which contains the kangaroos, wallabies, and so on, and the other two of which constitute the Australian possums, mostly inoffensive, tree-climbing animals quite unlike the many-toothed giant rat-like thing that Americans have to put up with.
|The families of Australian marsupial|
Of the two possum groups, one includes the gliding possums, which look and behave rather like flying squirrels, along with some close and not-so-close relatives... the latter including the rather odd noolbenger, an animal that only feeds on nectar and pollen. The other includes a number of non-gliding species some of which are relatively large; there remains considerable confusion as to how these species should be arranged within the family.
Which leaves us, at least in Australia, with the kangaroos and their kin. It has long been known that the potoroos and bettongs, both of which look rather like rabbit-sized wallabies, represented an early branch away from the kangaroo family proper. In the late '90s, however, one particular kind of potoroo proved to be something else entirely. In retrospect, this animal, now called the musky rat-kangaroo, was always a bit odd, having a much simpler stomach and being incapable of bounding around on its hind legs. It now gets its own family, and presumably diverged from its relatives before they figured out how to hop.
The kangaroos diversified quite rapidly starting around 10 million years ago, presumably because that's when Australia started drying out, and they were better adapted to living in a sparse open Outback than dense forest. Just as with the ultimate origin of the group, this makes it difficult to resolve some of the fine detail of how different kangaroos and wallabies relate to each other, and, again, the matter isn't entirely resolved yet.
A couple of points are worth noting, however. One is that we still don't know exactly what a quokka is, with at least five different theories as to the identity of its closest living relative having what looks like reasonable evidence backing them. One thing it isn't related to, though, are the rock wallabies, which we know have pretty convincing evidence are closely related to the tree kangaroos.
Yes, tree kangaroos. The ones that live up trees. And that don't look at all like rock wallabies. Those ones.
As an aside, while new possums and the like are often described, only one member of the kangaroo family has been described in the last quarter-century or so. This is the dingiso, the only known tree kangaroo that doesn't like climbing trees.
I'll just let that sink in, I think.
Not all marsupials live in Australia or New Guinea, however, since there are over a hundred known species in South and Central America, with new examples being named almost every year. Almost all of these belong to the opossum family, which also has a single representative in the US and Mexico. They are all at least partly omnivorous, and tend to be good at climbing. Probably the strangest is the yapok, the only marsupial that spends much of its life in the water - females have a closeable, watertight pouch to stop their young drowning when they swim.
Just eight species of American marsupial do not belong to the opossum family as generally recognised. Seven of these are shrew-opossums, an obscure group about which we know virtually nothing - one was only discovered in 2013.
The other is the monito del monte, a small, furry, tree-dwelling animal that, unusually, is more closely related to marsupials in Australia than it is to its own neighbours. This apparent oddity is explained by the fact that marsupials only reached Australia from South America. The only known fossil relatives of the monito del monte were discovered in Antarctica, and date back to a time when that continent was still green. It's unclear whether they represent Australian animals heading back to their original homeland, or something that got left behind when everyone else headed Down Under, although the latter seems more likely.
Of course, there are also many other fossil marsupials, which reveal, as one might expect, a long and ancient history of the group, including many kinds of animal that are no longer with us. But that will have to be a matter for another time...
[Photo by Andrew Mercer, from Wikimedia Commons.]