Vole incisors are ridiculously insanely huge

Mike Taylor of the always excellent Sauropod Vertebra Picture of the Week blog recently acquired the skeleton of a squirrel, and commented on the ridiculousness of the size of the single incisor in the lower jaw. In squirrels these run for about three quarters of the length of the entire jaw in addition to the crown of the incisor visible at the front end, and are heavily curved and so at some points lie just inside the bone margins of the jaw. Mike also commented that ‘the tooth literally could not be any bigger’. Here’s the blog post, and the killer image from it is reproduced below:

img_0800-squirrel-right-mandible-medial-tooth-juxtaposed

It just so happens that my colleagues at the University of Helsinki, Aki Kallonen of the Laboratory of Microtomography, Department of Physics, and Elodie Renvoise, my fellow postdoctoral researcher in the Jernvall EvoDevo lab, Institute of Biotechnology, have been X-Ray Computer Tomography (CT) scanning vole jaws in 3D. Mike Taylor was able to remove his squirrel incisor without damage to the jaw, but CT scanning allows a look inside the jaw with the teeth still in place. Aki and Elodie kindly allowed me to use their data to make a picture that shows the single incisor inside the vole jaw:

VoleJawLingualmirror

The image shows differing density values, in the same way a traditional X-ray image shows the denser bone as white with the less dense soft tissue, muscles, skin, fat etc, as transparent. In this case, it’s coloured from transparent = lowest density (in the vole jaw this is thin bone, as there was no soft tissue on the specimen scanned), through orange representing intermediate density, with yellow the highest density.

As can be seen, the incisor curves below the molar teeth and reaches all the way to the rear of the jaw, excluding the sticky out bits at the top and bottom (the condyle and angular process respectively). In comparison, in the squirrel it only reaches about three quarters of the way back. The last section of the incisor in the vole appears as a dark oval in the CT scan, indicating that although there is space at the back of the jaw for the end of the incisor, it isn’t mineralised and so shows as a region of very low density (and that the bone surrounding it here must also be thin). You can then see the developing tooth increasing in density as you go forwards, becoming orange where the tooth dentine has mineralised.

The lower part of the front half of the incisor ,which is the only portion covered in enamel, the hardest and densest tooth part, shows up as an orangey yellow grading into a bright yellow, indicating a very high density in the CT scan. Similarly, the outlines of the vertical ridges of the three molar teeth, also enamel covered, are a bright orange or yellow, and so high density. Their unusual shape is because, like a horse molar tooth, the vole molar teeth grow continuously for all, or nearly all, of the animals lifespan. The most rearward part of the incisor space is probably full of the stem cells that allow the incisor to continuously grow.

In addition to extending further rearwards than in the squirrel, I’d say that the vole incisor sticks out a little further than the squirrel incisor at the front of the jaw, and occupies more of the jaw in the diastema (the gap in front of the molar teeth) and below the molars. The incisor is actually longer than the entire lower jaw. As other parts of the vole jaw are also smaller than in the squirrel, I’d say that the vole incisor is larger than the squirrel incisor relative to the jaws containing them. This means that for the squirrel incisor, the tooth literally could not be any bigger the tooth could be considerably bigger! It also means that the vole jaw is reaching the absolute limits of how big the incisor can be. The entire jaw consists pretty much of just the incisor, with the molars and the three processes for muscle attachment (coronoid and angular) and articulation with the skull (condyle) basically hanging off the incisor’s edges. In fact, if you look closely, you can see that the molar tooth roots have to bend around the incisor to fit in the dentary bone…

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5 comments

  1. Pingback: Vole incisors: putting squirrels to shame | Sauropod Vertebra Picture of the Week #AcademicSpring
  2. Pingback: Voles not living in holes | Tetrapod Teeth & Tales
  3. Gloria Cuenca-Bescós

    The teeth seem to have roots. Is it the mandible of a Clethrionomys? Where does it have the mandibular foramen? Just at the rear of the incisor alveolus? Thanks. Gloria

  4. Ian Corfe

    Hi Gloria – The molar teeth do indeed have roots. My colleague Elodie Renvoise is doing a project on the roots of these teeth, so I deliberately didn’t make the image of them too clear so as to not steal her thunder! The mandible is indeed from Clethrionomys, though it seems as though the genus name Clethrionomys has been replaced relatively recently with Myodes, so this specimen is labelled as being from the genus and species Myodes glareolus.

    Interesting question about the mandibular foramen! In most mammals, this is found towards the rear of the jaw, often at a similar height to the last molar tooth but posterior to it, and on the lingual face of the jaw – the side where the tongue sits, rather than than the buccal face where the cheeks sit. In the mouse, the incisor is pretty big and extends a fair way towards the back of the jaw, and the mandibular foramen is located just to the rear of the incisor. In the squirrel picture above, it can be seen as the crescent shaped opening just behind the rear end of the incisor.

    As I suspect you were asking, if the vole incisor reaches all the way to the rear of the jaw, where on earth can the mandibular foramen fit! Almost certainly not at the rear of the incisor alveolus due to the lack of room, so presumably just above or below this. I’m afraid the rendering above from the CT scan doesn’t clearly show the foramen at all, as I made most of the bone semi-transparent to better show the teeth, so i will have to go back and check the data. It will be very interesting to see what has happened to the position of the foramen, so great question!

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