Can you spot what is unusual about this ?

One of the great things about seeing the bones of a particular animal often is that you get to spot when there are minor things which are different. As you will know if you read my blog regularly, the animal I most often see and find are deer, both roe and red deer. That's what this skull is of - a roe - that's the easy bit !

I found it a few weeks ago, and quickly spotted that something was a bit unusual. You probably won't spot it from the photo - unless you really know deer really well - but read on if you want to know what makes this roe deer a little bit special.

But first - a bit about deer teeth.

Deer have a big gap between the front incisorform teeth, and the "cheek teeth". They are normally born with three baby premolar teeth (pm1/pm2/pm3) and their first molar (M1) emerges fairly quickly. As they change into adults, which takes a bit over a year in roe and about two and a half years in red deer, the baby premolars get pushed out by the adult premolars (PM1/PM2/PM3 - the capitals means "the permanent teeth") and the back two molars come up (M2 and M3). This makes it easy to judge the age of a juvenile deer (see my guide), and also you know if you find a row of six teeth on a lower or upper jaw then it was an adult.

Unlike foxes or badgers, deer teeth are close together and it's difficult to see where one ends and one begins, so I use lines on my photographs to show where each tooth begins and ends. And here's how the jaw I found looks (at the top) with another jaw from another roe deer at the bottom. The bottom one is more worn down, as it was an older deer, but can you see what is different ?

If you haven't spotted it yet, here's the same jaws from the side, with the PM1/PM2/PM3 teeth pulled out:

The unusual roe deer skull that I found has an extra tooth, between the PM2 and PM3. And it has this extra tooth on both sides of the jaw. Very odd !

But what does this mean ?

This means I have found an entirely new species of deer, and I'm going to be incredibly rich and famous. Okay, it actually turns out I haven't, and people don't tend to get rich and famous for discovering new species anyway.

It means it's pretty unusual, although one of the first red deer skulls I found had an extra tooth in the roof of its mouth. But I hardly ever see this.

The other thing is that when you find something unusual on a skeleton, if it only occurs on one side, then it's usually abnormal. Animals are usually pretty much symmetrical along the spine, with each side a mirror image of the other. This extra tooth is on both sides, which means it might be a genetic abnormality, not an injury.

I needed to take a closer look at it.

The first thing was that although the jaw I was comparing it against was slightly longer (probably because the comparison jaw was from an older male, and the found jaw was from a younger female), the length of the six teeth was longer on the new found jaw.

There weren't any obvious signs of injury or disease, either. The odd tooth, which I called PM2½, was much smaller than the other teeth, but it didn't look that unusual.

This is the extra tooth. By itself, it does not match any of the juvenile or adult teeth.

The next thing to check was whether there was an extra tooth hole for it, or whether it slotted into another root hole. As you can see, there is definitely an extra hole which is unusual to see because PM1 and PM2 and pushed closer together on the newly found jaw. PM1, PM2 and PM3 all have two roots, but PM2½ only has one.

That it only had one root gave me a clue, because it reminded me of something unusual about deer lower jaws.

Four clues to solving PM2½

Here's the mystery jaw in the middle, with a normal adult jaw above and a normal juvenile (8 months old) jaw below.

Here's three things which I know about deer teeth:

1. The unusual thing sometimes about baby teeth is that the adult tooth is completely a different size or shape. The baby and adult first and second premolars are more or less the same shape, but the baby pm3 has three cusps and roots, and is quite wide, and the adult one which pushes it out from below has two cusps and two roots and is slightly narrower. You see it better from above:

2. You can see from the picture that the adult  PM3 and PM2½ together are about the same width as the baby pm3.

3. You can also see from the picture that the curve shape of PM2½ appears to be turned round 90 degrees to all the other teeth.

4. One other thing I know about the baby pm3 is that it wears very, very thin by the time it is pushed out. I have deer teeth with baby pm3s that are just a few milimetres thick, and almost worn all the way through. In some deer, they must wear through and fall out in pieces.

But what if they don't fall out ? What if the baby PM3  wears through, gets in pieces, then one piece gets wedged in the tight gap between PM2 and PM3 ? 

So my theory is that PM2½ is actually one third of the baby pm3 which wore through; two of the cusps fell out with the other teeth as normal, but the third cusp stayed in the mouth wedged between the other two teeth with its own root.

 I might be wrong and it' not a perfect answer, but it's not a stupid one either. If it happened it both jaws, it could be a genetic abnormality, or maybe it just had teeth that wore through faster than normal. Either way, this was a really good find ! 

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