Fossil Friday – Max’s mandible, CT-edition

Max dorsal

Last week we took Max to California Imaging and Diagnostics in Hemet, who donated a some time on their CT scanner to scan the lower jaw of Max the mastodon. A couple of days later CID provided us with disks with the scan data, and we’ve finally had a chance to start examining them. If you’re unfamiliar with CT scans, the term is short for X-ray computed tomography. CT scans take a large number of X-ray images from multiple angles. A computer can then combine those images to make cross-section X-ray slices through the object. The slices can be stacked to make a digital 3D model of the object, or removed to examine the internal structure at any particular slice. It’s an invaluable, non-destructive method of examining the inside of an object.

I’ve uploaded an initial video of the slices of Max’s jaw. The slices are transverse, run anterior to posterior, and are viewed facing anteriorly. Basically, imagine standing behind the jaw looking forward, and seeing cross-sections of the jaw gradually getting closer to you:

Since the video runs front to back, the first thing you see is the anterior tip of the jaw. (Well, actually the the first thing is the plaster cradle the jaw is sitting on, then the tip of the jaw.) As you move back, you pass though the mandibular symphysis where the two haves of the jaw are joined, through the teeth, and then to the coronoid process and mandibular condyles (the condyles are incompletely shown, because the jaw was a bit to large for the scanning area).

There are several slices that have especially noteworthy features. This is about 4 seconds into the video:

Max 1

The image of the jaw on the right is for orientation; the purple line is the approximate location of the slice. The blue arrow is pointing the pathological bone growth near the tip of the right dentary. Inside the bone, there are a series of fractures next to this bony growth, one of which is marked with the green arrow. I’m not sure if they are actually associated with the growth, or if they occurred after burial.

Moving a little further back, to about 6 seconds:

Max 2

We’re now behind the bony growth, but its effects are still present. The blue arrow is pointing to a foramen (a passage for nerves or blood vessels) that has been displaced anteriorly and ventrally, apparently in response to the growth. The green arrow is pointing to the mandibular symphysis, where the left and right dentaries are joined together. This was a surprise to me. These bones fuse together in older animals, obscuring the line of fusion. Max was a fully mature mastodon, middle aged at least. I would expect the symphysis to be completely fused, and on the outside it is; there’s no trace of the symphysis on the outside of the bone. Yet inside they’re still unfused. Is this typical of mastodons, or the Max’s symphysis never fuse (or pop open) due to the beating his jaw apparently went through?

Moving further back, to about 8 seconds:

Max 3

This slice shows how much that foramen was displaced, when compared to the left dentary (blue arrows). It’s also clear that the dentary is still messed up well behind the bony growth; note how asymmetrical the two dentaries are on the dorsal surface (they should be nearly mirror images). In fact, the entire mandible is quite asymmetrical. I originally assumed that this was mostly due to deformation that occurred after burial, but it’s now clear that at least some (maybe most) of the asymmetry occurred while Max was alive.

At around 14 seconds:

Max 4

We’re now getting into the tooth rows. The first root and cusp of the left 2nd molar is clearly visible (yellow arrow). The white arrow below the tooth is the opening of another foramen. One the right, the notch marked by the green arrow is the socket for the last root of the right 1st molar. Max was a mature mastodon, so the 1st molars (and all the premolars) had already worn down and fallen out; only the 2nd and 3rd molars remained, and the 2nd molars were heavily worn.

As we already knew from examination of the exterior, Max had a second injury on his right dentary, essentially a crease running obliquely below the anterior tooth row with ridges of secondary bone growth on each side of the crease. This injury is marked with the blue arrow, showing apparently compressed bone beneath the surface (the bright line on the inside) and the swelling associated with the secondary bone growth.

From this point in the video you can see the tooth cusps and roots grow and shrink as the scans run through the tooth rows. There is a change in one of the teeth at around 28 seconds:

Max 5

At this point we’re all the way back to the last root of the 3rd left molar. As indicated by the green arrow, the pulp cavity on this tooth is open (you can see the same thing on the last root on the right 3rd molar at 29 seconds). This suggests that this part of the tooth was still growing. Sure enough, if we look at the cusps, the last cusp on the 3rd molar is only very slightly worn, indicating that it had just erupted.

By 34 seconds the scans have passed completely through the tooth rows, and the remainder of the scans pass through the coronoid processes and the mandibular condyles.

We still have a lot to look at with these scans, and we’re hoping they’ll be useful to people that are working on mastodon anatomy. We’re going to keep working to make this data available online as we have time to process it. We also intend to include annotated versions of these scans in our upcoming exhibit, Stories from Bones.

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