Friday mystery object #427 answer

Last week I gave you a nice skull to have a go at identifying:

It proved to be more tricky than I thought, but I think that may be because there is a skull image on Wikimedia that may have misled people searching for a comparative skull of this species.

This is the skull of the humble Guinea Pig Cavia porcellus (Linnaeus, 1758), but if you tried searching for Guinea Pig skull, you may have seen this image:

Clearly this is not the same species as our mystery object – the incisors alone are an absolute give-away, with their striking orange enamel and the their much greater size. Those big incisors also bed deeply into the mandible, creating a pronounced ridge at the base of the mandible that props the entire skull at an angle. This one is the skull of a Coypu, regardless of the Guinea Pig identification given on the Wikimedia page.

There were also quite a few suggestions that the mystery object might be a Capybara, or one of several other South American rodents. The size suggests it’s not Capybara – I suppose a very young Capybara might just about be small enough, although they would certainly have less pronounced muscle scars and more open sutures.

There are plenty of other South American rodents, but most of those of a similar size and overall shape have a much more V-shaped exit to the nasal passage in the palate, rather than this very open and U-shaped structure.

When identifying skulls, it’s generally best to rule out the most common and likely species first, since this can significantly speed up the identification process. This is why misidentified comparative specimens can be a problem, so always try to check more than one example. I’ll certainly be suggesting an edit to the misleading Wikimedia entry to help prevent this issue in future, but this isn’t a criticism, since nobody is perfect and I know I’ve made mistakes myself in the past, especially early on, so I’m trying to fix them retrospectively!

4 thoughts on “Friday mystery object #427 answer

  1. As a lucky owner of a capybara skull, i can say pretty confidently it’s not a young capybara skull, as the ramus of the mandible on a capybara is distinctive in being below the axis of the mandible, rather than pointing up. extra leverage i’d wager.

  2. Found an image on the WWWeb
    showing the underside of the skull with the lower jaw removed: good for looking at (upper) dentition. The molars are certainly simpler than those on a Capybara, but maybe not as primitive as I initially thought: each “loph” (I’m not sure that’s the right term, but I’ll use it: the transverse structure that each molar has about two of) seems to be divided along its crest, and so has two cutting edges, fore and aft. So o.k. for herbivory. But if, as you say, its diet is mostly grass, why does it have such narrow incisors? At least larger grass eaters (think: Perissodactyls) tend to have wide snouts with a wide incisor array for cropping the stuff.
    Anyway: thanks once again for contributing to my education!

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