On Friday I gave you this skull as a mystery object, dredged as it was from the memory card of my severely concussed camera:

This was a slightly sneaky object, because I had a feeling that quite a few of you would make the same mistake as I did when I first saw this skull, by assuming that it’s from a large rodent. The large front teeth (the incisors) support this, since enlarged first incisors are a feature of the rodents.

Rodents also have a large gap behind those incisors called a ‘diastema’ – which this skull has for the lower jaw (or mandible), but you may notice that the upper jaw has three incisors in the pre-maxilla (that’s bone in the front bit of the upper jaw) before the diastema. This is easier to see in a side (or lateral) view:

You might notice that there’s a small tooth behind the third incisor in the upper jaw – that’s a canine. You might also notice the faint wiggly line in the bone of the jaw just above the canine – that’s the junction (or suture) between the maxilla and the pre-maxilla bones. The canine is the first tooth in the maxilla and all the incisors are in the pre-maxilla (this is the same for all mammal teeth).

Rodents only have two teeth in the pre-maxilla, not six. They also have no canine teeth in the maxilla. That means this mystery object cannot be a rodent. Here’s what a rodent’s diastema looks like (the suture between maxilla and pre-maxilla is really clear in this photo):

Lagomorphs (rabbits and hares) have teeth similar to rodents, except they have an extra pair of incisors behind the front pair in the maxilla – these are called ‘peg teeth’:

Peg-teeth in a Rabbit skull

Clearly the mystery object has more incisors than this, plus those canines, so it can’t be a lagomorph either.

So what beastie has six upper and two lower incisors? Several of you worked out that this was a marsupial from the dentition (namely Cromercrox, jonpaulkaiser, David Craven and Zigg), but only Prancing Papio and Jamie Revell managed to get it to species, namely the Koala Phascolarctos cinereus (Goldfuss, 1817).

These charismatic marsupials are arboreal (they live in trees) and they are folivores (they eat leaves). They fill a similar environmental niche to the sloths in South America, showing similar slowing of their metabolic rate in response to their low-energy food – namely Eucalyptus leaves in the case of the Koala. It should be stated that they are not bears, indeed they are not even remotely like bears, so why they get called ‘Koala bears’ is beyond me.

Female Koala (Phascolarctos cinereus) at Billabong Koala and Aussie Wildlife Park, Port Macquarie, New South Wales, Australia. By Quartl 2009

The teeth of Koalas are actually rather similar to those of Kangaroos, although the rest of the skull is somewhat differently shaped (less long and relatively higher). Their arrangement of teeth in the bottom jaw even gives the Order they belong to its name: the Diprotodontia, which translates as meaning ‘two front teeth’ in Greek. This pattern of teeth works very well for snipping off bits of plant or gnawing seeds, which is why it occurs in quite a lot of animals. So it’s worth remembering that buck teeth aren’t only for the rabbits and rodents – always check the other teeth too.