Last week I gave you this guest mystery object from Andy Taylor:

Those paired incisors and the large diastema (or gap) behind them are characteristic of a rodent, so that narrows down the possible options by not very much, since there are well over 2250 species.

However, this specimen also has a very large infraorbital canal (that big forward-facing hole in front of the eye socket), which is a feature of the hystricomorphs – the group containing capybaras, porcupines, chinchillas, guinea pigs, hutias and suchlike. The teeth are an additional give-away for the group, with their discrete little patterns on the grinding surfaces:

The scale of this specimen threw me a little at first, as I’m used to dealing with metric, but this scalebar has 16 subdivisions, so it muct be imperial. Which really helps, as it means this skull is quite large for a rodent – somewhere in the region of 5 inches or 12.7cm. That immediately narrows things down a lot more.

An additional clue is in the narrow skull profile:

A lot of rodents have a lower and broader skull and this kind of shape is what I expect to see from a species that moves through dense undergrowth rather than climbing, digging or swimming. This, combined with all the other clues, led me to the agoutis.

There are 12 species of agouti, so we’re still not quite there with the identification. Most species that I can find comparative material for have a significantly longer rostrum than this specimen (relative to the rest of the skull), but there are some species I’ve not been able to find a really reliable reference skull to compare against.

There is one species where I couldn’t find a great skull comparison, but which appears in living individuals to have a relatively short snout and the skull image I could find also appears to be less nasally advantaged than most agoutis. That was the Brazilian or Red-rumped Agiouti Dasyprocta leporina (Linnaeus, 1758).

I’d be keen to take a further look with some good quality reference material, but I’m reasonably happy with this. The advantage of good comparative material is that it really helps make those detailed observations about particular shapes and sizes that can be hard to properly recognise from a photo.

In a strange, but very welcome cooincidence, I had the good fortune to meet up with Andy last Friday evening. I was at the NatSCA conference in the UK – which was held just a short journey from where Andy lives. It offered a great chance to chat with a fellow bone geek and natural history fanatic and I hope to have some more guest objects from Andy and his amazing collection in the future!