How I identified a mystery bone

This week's post is a long one, about working out how to identify a fragment of bone I was given, and which was found in the village. I am sometimes asked how I identify bones, and it's difficult to explain because it's a mixture of guesswork, looking in books, and remembering details from other skulls I have.

So here's how I worked it out from a fragment of bone what it was from, even though I'd never seen any bones from that animal before. I actually worked most of it out while standing in my garden, so it wasn't quite as complicated as it seems here. So here it is...and do you think I got it right in the end ?

Which bone is it ?

I could tell straight away that it was the back end of a skull - what's called the braincase - where the front  half of the skull  had been broken off. Without knowing what it was, I could identify the spine hole in the braincase, the ear holes, and roughly where the orbits (eye sockets) were.

The point where the skull had broken made it much more difficult to identify, because it wasn't possible to know the exact length of the skull, or check the teeth, which are really useful for identification:

Here's the other side with a scale bar:

I could also tell it was from a mammal, not a bird, because of the rough feel of the bone, and similarities to other skulls I had. For example, a few mammals, from cats to squirrels, have those triangles of bone above the eye socket at top right.

How big was it ?

This is a mistake a lot of people make when they send pictures to me; there needs to be a ruler or coin in the picture to show scale.  This skull part was about 4.5cm long, but that's obviously not the entire length.

Most guides to skull sizes are based on adult skull sizes. Was this an adult ? I thought so because underneath the skull, the central part was smooth. In other mammals, there are two horizontal unfused lines in very young animals, one in young adults, and none in older animals.

I wasn't sure if whatever this animal was fused its bones in the same order, but other signs of fused bones made me think it was an adult, which would help. But I couldn't work out how big it was until I got an idea of the proportions of the skull.

At a guess, though, this skull looked bigger than a squirrel and smaller than a pine marten. But size doesn't always help. This is it next to some other skulls of similar sizes. It isn't massively like any of them so far:

Where was it found ? 

You would be amazed how many people email me with pictures of bones without saying what country they live in, or where it was found. Knowing a bit about where it comes from helps narrow down what it is likely to be. 

Luckily I know where it was from. Someone in my village came round to my house to give me it. She said that see found it beside the river near her house. So that started me thinking about what kind of animals lived there.

What bits looked like other animals ?

This bit is sort of guesswork, and sort of knowledge.

Animals are grouped into families, which really means close relatives. A domestic cat skull looks a lot like a lynx or puma because they are from the same family of felids, even though they live quite different lives. The same is true of beavers and mice, which have similar skulls but are massively different sizes, because they are in the rodent family.

So if this skull looked a bit like another skull I had, they might be related, even if they were different sizes.

One thing that was distinctive was the Y-shaped ridge between the eye sockets and the middle line of the top of the skull:

The raised ridge at the back of the skull was also unusual as well. (The broken bit at the back of the braincase was probably from a predator at or after death.)

I had seen both of these things before in two different animals. The Y-shape I had seen in badger skulls, although badgers also had a big ridge along the top of the braincase, called a saggital crest, which this skull didn't have. Here's the mystery skull next to the badger:

It was much too small to be a badger, and it had some difference, but this was the first clue.

The other part which looked similar to a badger was the underside:

The second clue was the ridge at the back of the skull. I had seen that before as well, on an otter:

The photo doesn't show it very well but the ridge is very similar in both. The proportions were very similar as well, including the same "Y" shape that was on the badger.

Working out the family

Looking a bit like a badger and a bit like an otter doesn't sound very useful unless you know that badgers and otters are from the same family: mustelids, which I'd thought when I first saw the mystery skull. The mustelid family is not very big, and in the UK that would include badgers, otters, pine martens, stoats, mink and weasels. Oddly enough, when I put the mystery skull next to my beech marten skull earlier, they didn't look very similar because beech martens don't have the ridge at the back of the skull, or the Y-shaped ridge above the orbits.

Here's the mystery skull next to (L-R) my beech marten, otter, and badger:

I was getting close ! But it looked liked the mystery skull was much smaller than these. Could I work out an exact size ?

Working out an exact size

Dad helped me with some of the maths on this.

If the mystery skull was the same proportion as the otter, as seemed likely, and I knew the length of the mystery skull portion, and I knew the percentage of the equivalent portion of the otter skull, I would be able to work out the entire length of the mystery skull.

My otter skull is 12.5cm long, and the part from the back to the spikes above the orbits was 8cm. So the length of the part of the otter skull  from the eye spikes backwards was 8/12.5 = 64% of the length of the skull.

The same part on the mystery skull was 44mm long. So if 44mm is 64% of the skull, then the entire skull would be 69mm long, if the proportions would be the same as the otter.

Skullsite.co.uk gives these lengths of skulls for the smaller mustelids:

• Ferret: 67mm
• Pine marten: 76mm
• Mink: 60mm
• Weasel: 40.4mm
• Stoat: usually over 42mm

So it looked most likely to be pine marten, ferret, or possibly mink, but not weasel or stoat.

Pine marten, ferret or mink ?

Pine marten would be VERY exciting. I have been filming one for six months, but never saw one for real. But the pine marten was on a moor much higher up, five miles away from where this was found. And after I checked, the best match was not with pine marten, but with mink. The ear bones, and the way the ridge at the back rises up made it look very similar:

After I suggested mink, Mrs Anderson, who had brought me the skull, said that she often saw mink swimming in the river. I knew there were mink some miles away, but not in the village.

So my final conclusion is that the skull was from a mink, Mustela lutreola ! 

But what do you think ? Have I got it right ?

I want to thank Mrs. Anderson for this skull.

Remember that on the 21st July it's my 5th anniversary of blogging, and I'll be doing a special post where I answer your questions about my book, my hobby and my blog. You can leave your questions to me here !

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