Saturday 30 July 2011

The Strangest Marsupial?

So, I finally reach the end of the mini-series on mole-like mammals. I've looked at the moles themselves, two different kinds of mole rat, and the golden moles of Africa, and that leaves just one group. But its perhaps both the strangest and the least studied of them all.

As you can see (sort of) from the picture, these animals look remarkably like the golden moles. There's probably a good reason for that; like most golden moles, they burrow their way through dry, sandy soils, and they have a very similar lifestyle. Yet, apart from both being mammals, they are entirely unrelated. These animals dig through the sands, not of Africa, but of Australia. For these are the marsupial moles (Notoryctes spp.)

The marsupial mole family contains just two species, imaginatively named the northern and southern marsupial moles. They're pretty much impossible to tell apart just by looking at them, and for a long time it wasn't clear that they were two separate species at all. They live only in the deserts of central and north-western Australia, where they like especially sandy soils, and feed primarily on ants and termites.

They are, if anything, even more adapted to this life than the golden moles are. Their forelegs are reduced to mere pointed stubs ending in pick-like claws, and their hind limbs are flattened into spades for shovelling sand behind them. They have no visible eyes or ears, and they can close their small nostrils to prevent them filling with sand. The snout and forehead are covered by a shield of thick horny material that acts like the blade of a plough, enabling them to push their way forward. To help further, the bones of their neck are fused solid, allowing them to keep their head rigidly positioned.

Like the golden moles they travel by 'swimming' through the sand, but, unlike their African counterparts, they travel through sand so loose that the tunnels simply collapse behind them - which means they don't need to make molehills. They do shelter in underground chambers to sleep, but even these don't seem to be permanent.

But, if you're a marsupial, travelling through loose sand presents an additional problem that golden moles and the like don't have to face. We've seen how mole-like mammals have small ears, permanently closed eyes, and so on, to prevent them filling with dirt. But female marsupials also have a pouch, and if that filled with sand, the young would all suffocate. Although some marsupials don't have much, if anything, in the way of a pouch, that approach won't work for moles, because then the young would be even more exposed to abrasion. In the absence of evolving a zipper, the marsupial moles have, nonetheless evolved an effective solution: the pouch faces backwards.

And the strangeness doesn't end there. For one thing, even male marsupial moles have a pouch, something unknown in any other marsupial. Apparently, they have no optic nerves, which suggests that they can't detect light at all (which does seem rather odd, given that you'd think they'd want to avoid it). Their cheek teeth have an odd V-shaped crest, something unknown among any other marsupial, and fairly rare among placental mammals - although, interestingly, the golden moles are among the few to share the feature.

All of which raises the question - just how are they related to other marsupials? The picture was confused for a long time, with lots of different theories. It was clear that they weren't closely related to anything else, and one theory was that they represented the very first split within the Australian marsupials, not long after they had arrived on that continent from South America. Modern genetic analysis seems to be giving us a clearer picture, however, and it now seems that they represent an early branch within the group that led to the various other insect and flesh-eating marsupials, probably not long after that group split from the herbivorous marsupials, represented today by the likes of kangaroos and koalas.

 Tasmanian     Marsupial     Bandicoots
Devil, etc.      Moles          etc.
     ^             ^             ^
     |             |             |
     |             |             |       Herbivorous
     ---------------             |       Marsupials
            |                    |           ^
            |                    |           |
            ----------------------           |         Monito
                       |                     |       del Monte
                       |                     |           |
                       -----------------------           |
                                  |                      |
                       (Arrival in Australia)            |
                                  |                      |

The genetic evidence suggests that this split happened very early in the history of the Australian marsupials, probably during the earliest epoch of the Age of Mammals, the Paleocene. However, until very recently, we had no fossils of this small and enigmatic group to go on. The closest thing we had, discovered in the 1980s, was a beast named Yalkaparidon. It had features in common with the marsupial moles, but also several that resembled those of bandicoots, and others that resembled possums. Perhaps it was an ancestor of the group, something that showed what marsupial moles had evolved from?

In retrospect, that sounds a bit desperate, and, in fact, the animal lived long after the common ancestor of marsupial moles and bandicoots, let alone possums. More likely. it represented some other line that has since vanished altogether. However, we do now have a fossil marsupial mole, first officially described in October of last year.

This is Naraboryctes, known from a small number of specimens, all dug up in north-west Queensland. The fossils date, as near as we can tell, from the early Miocene, long after when the genetics tell us the group originated, but also well before most modern animals had begun to take hold. In many respects, it resembled the modern species, and the bones of its arms in particular were clearly well-developed for digging. Yet, while the adaptations were present, they were not so extreme as in the living forms.

So, this animal clearly did spend much of its life digging, although it may not have been as subterranean as the living species. However, there was a clear difference between it and its descendants: the animal lived before the Australian deserts came into existence. Our best bet is that, in those days, even the interior parts of Queensland were covered by rainforests (as parts of the coast still are). Indeed, other fossils found with Naraboryctes include tree-dwelling possums, forest bats, and tropical frogs.

So the marsupial moles evolved, not to swim through sand, but to burrow through the soft, rich, soil of the jungle floor. Again, note the parallel with golden moles here; most of them live in deserts, but some do like moist environments - so we know that the basic body shape works well in either. For the marsupial moles, it was only when the climate began to dry out that they evolved into what we have today.

Today, just two species survive, and we know relatively little about the populations of either, so that the IUCN, which normally decides such things, has declined to list them as either endangered or safe. Given their specialist requirements, though, its reasonable to suppose that they won't like - for example - advancing agriculture, and the Australian federal government has declared them both to be endangered species. But, even if they were doing well, they are sufficiently well hidden that its possible we just wouldn't know.

[Picture from Wikimedia Commons. Cladogram adapted from Beck 2008.]

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