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:
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:
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…