Sunday, 25 May 2014

The Teeth of Dead Kangaroos

Not exactly lush vegetation
How to tell what long-extinct animals ate?

Aside from those that died recently enough to leave us mummified, rather than fossilised, remains, your best bet is probably fossilised dung. In the case of carnivores you might even be lucky enough to find the bones of their kills. Of course, both do need matching up to the correct animal, but the former, in particular, is not especially common. Absent such direct clues, then, we have to deduce what we can from the skeleton, and that generally means examining the teeth.

The overall shape of the teeth can give us some pretty clear indications of whether an animal was a carnivore or a herbivore. This is even more true for mammals than it is for dinosaurs, since we have plenty of clear examples alive today. Powerful stabbing canines and flesh-shearing molars indicate a carnivore, while flat grinding plates and leaf-clipping incisors imply a herbivore. There's obviously some gradation in between, in the case of omnivores, insectivores, and, for that matter, weird specialists such as vampire bats, but it's a good starting point.

But can we narrow it down further? Large herbivores, for example, can be divided into two main groups: grazers and browsers. The former munch down large quantities of grass or other low-quality food, while the latter snip away at tasty foliage from bushes or trees. In today's world, for example, cows are grazers, while deer are mostly browsers. There's plenty of blurring between the lines, and some animals, such as fruit-eating monkeys, that aren't really either, but, for hoofed animals at least, it's an important dietary distinction. (Among small mammals, there are also more of those weird specialists - animals that feed largely on tree gum, or nectar, say).

How to tell the grazers from the browsers? One way is to look at the shape of the jaw, or the shape of the teeth. This can be helpful, especially where you have close relatives alive today to compare your fossil with. Grazers, by their nature, tend to scoop up large quantities of grass, and therefore have broader muzzles, while browsers pick at specific bits of food, and tend to have longer, narrower, ones. Similarly, since grazers are eating tougher forage, their teeth should be better suited to long-term wear, which generally means higher tooth crowns.

These are important clues, but they are indirect, and they're also indications of moderately long-term evolutionary change, rather than what a particular animal was eating at a particular time. To get more direct, and shorter-term evidence, we need to look at the teeth rather more closely.

A widely used technique here is microwear analysis. Here, we examine the teeth under a microscope, and look for tiny pits and scratches on their surface. Grass defends itself against being eaten by incorporating microscopic pieces of silica in its leaves - enough, at least, to deter the browsers. Furthermore, anything scooping up mouthfuls of grass is bound to get bits of grit and dirt in its teeth, which also leave their mark. More selective browsers have different issues, and comparing the exact ratio of pits to scratches on the enamel surface can tell us a lot. Moreover, it can really zoom in on the last season of the animal's life, or on even shorter time periods.

We've been doing microwear analysis since the 1970s, and it's been very useful, especially in trying to figure out the diets of our own ancestors. A newer technique, however, is so-called mesowear analysis, first developed in about the year 2000. It's based on a similar principle to microwear analysis, but, rather than looking at microscopic features, examines the overall shape of the tooth to see how it has been worn down over the years. In particular, we want to figure out how much of the wear is due to grinding against tough food and grit (suggesting a grazer), and how much due to grinding against teeth in the other jaw (suggesting the cutting action of browsers).

To prove that this works, this has been tested against the results achieved from overall tooth and jaw shape, against those of microwear analysis, and against a fourth technique that measures stable isotope ratios. And, in general, it seems that it does work, albeit with the same sort of caveats that messy biology always throws in our way. It's also said, unlike jaw shape, to work in the same way regardless of the kind of animal we're looking at.

But does it? It's been used on a wide range of hoofed animals, and seems to broadly hold so far on animal such as horses, antelopes, and giraffes. It's even been applied to an extinct group of ungulates found only in South America. Which, when we're talking about large herbivores, should about cover it, right? Well, no, because there's another group of large, grazing and browsing mammals.

In Australia, the native animals never evolved hooves. Following the general rule that plants are more abundant than animals (and easier to catch), it's nonetheless the case that most Australian marsupials are herbivores. And the biggest grazing marsupials are also among the most familiar: kangaroos and wallabies.

So does mesowear analysis work on kangaroos, and, if so, can it tell us anything about how extinct species lived? There are reasons to think that it might not, and that's because kangaroo teeth are actually fairly odd.

In hoofed mammals, the teeth are arranged broadly like this: At the front there are clipping teeth, usually incorporating the canines, which aren't needed for anything else; behind them is a toothless gap that allows the animal to manipulate food with its cheeks, and then, at the rear, a number of grinding teeth to do the chewing.

Superficially, the teeth of kangaroos do look rather like this. They have clipping incisors at the front, which are a different shape than those in hoofed animals, but which aren't used in mesowear analysis anyway. Then there's that gap, and then the chewing teeth. Yes, they have four molars in each jaw, whereas no placental mammal has more than three, but they have less premolars to compensate, and honestly, it's hard to tell.

No, the difference is in how they grow. You see, the rear teeth of kangaroos work like a conveyor belt. They get slowly pushed forward as the animal ages, until they drop out and get replaced by a new one bursting up through the gums form behind. Kangaroos are always teething. Granted, most of them only have a limited supply of tooth buds, so they do run out eventually (and die), but that's just not the way things work in hoofed mammals - although it is, as it happens, in elephants. Since this means that particular teeth change their position in the jaw as the animal ages, it's at least plausible that this might affect how they wear down.

A recently published analysis examined 43 different species of living marsupial, to test this out. Including koalas, wombats, and possums, as well as kangaroos, they were able to show that, in general, the method works just as well as it does for other mammals. It's not perfect, but then, it isn't in placentals, either. Confounding factors may include uncertainty as to what exactly the living species eat, and the fact that at least some of the wallabies browse on ground-level herbs that likely mean they do pick up the grit that's otherwise typical of grazers. (From the placental side of things, there are antelope that graze on tall grasses in rivers, which likely creates the opposite problem).

They went on to examine the teeth of six fossil kangaroos from sites in Queensland that are around two to three million years old. There is some dispute as to what this part of Queensland looked like at the time - whether it was just grassy plains or a more mixed environment with stands of woodland between the grass. Living kangaroos (as opposed to wallabies) are all grazers, which would tend to suggest the former. An environment, in other words, not entirely unlike that the interior of Queensland has today.

Yet the analysis showed that all but one of the species was a "mixed feeder", somewhere between a pure browser and a pure grazer, and evidence that biology really doesn't like sharp dividing lines. Since this would reflect not merely what the animals liked to eat, but what they had actually been eating, this would suggest a more fertile environment than we'd previously suspected. There was an exception, though: the giant kangaroo Protemnodon roechus really does seem to have been a grazer, at least most of the time.

But, otherwise, apparently not. That these species had all previously been thought to be grazers on the basis of their overall tooth shape may indicate that, in other parts of Australia, they really were, and that they could switch to a grassier diet if they needed to. Or that they'd only recently evolved from something that was more of a grazer - something that may also be true, for example, of the living pronghorn antelope, whose teeth also don't seem to quite match its diet.

Nonetheless, this may give us an insight into Pleistocene Australia. To which, as it so happens, I will be returning in a few weeks time...

[Picture by Jenny Smits, from Wikimedia Commons]

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