Things to Make and Do, part 9b: the right to arm bears
November 7, 2011
This is the second post on the Wedel lab’s recently acquired skull of Ursus americanus, the American black bear. The first installment covered ended with the disinterred-but-still-filthy skull bits sitting on my dining room table. This post covers putting the teeth back in, and just enough anatomy to justify putting up more cool pictures.
About five minutes after I took the last picture from the last post, I put the cranium and mandible to soak in warm, soapy water and spent the rest of the day doing other things. Last night I got a couple of old toothbrushes and scrubbed off most of the dirt from the external surfaces of the skull. I alternated toothbrush work with running water from the bathtub faucet over and through the skull bits. I also used one of the rubber nasal aspirator bulbs (or “snot suckers”, as new parents in the real world invariably call them)–which make tremendous water guns–to sluice out some of the grimier cavities. It was fun to force water into the mandibular foramen and see it come shooting out the mental foramen, along the canal traveled by the inferior alveolar nerve and vessels.
All of the teeth were loose in their sockets, and the incisors and upper canines were either falling out or could be pulled out without too much trouble. I yanked all of the teeth that I could, figuring that it was better to yank-and-glue rather than leaving them loose. I set the loose teeth on separate plates, one upper, one lower, arranged in the same order they were in the alveoli, and let everything air dry overnight.
Shown above is my setup for replacing the teeth:
- a trash bag for protecting the table
- a dish towel to provide a soft surface
- the bear skull pieces
- loose teeth on their respective plates
- five minute epoxy
- toothpicks for mixing and spreading the epoxy
I had been going to use something less hardcore for the gluing, but fortunately Vicki got home and I was able to draw on her experience from reconstructing loads of human skulls from archaeological and forensic sites. She said just go with epoxy, and so I went.
Here we are about halfway through the process. A few tips, some obvious, some maybe less so:
- Even if you’ve done a perfect job of keeping the teeth in order, test-fit them anyway. If nothing else, this will give you a visceral sense of what the tooth feels like sliding into the socket, and it will help you figure out how much glue to apply. Also, test fit adjacent teeth together so you’ll know if they have to go back in in a particular order; sometimes one tooth is at a subtle angle and blocks the next tooth from coming out or going back in.
- Better to put the glue in the socket than on the tooth. The roots are not much smaller in diameter than the alveoli and the crowns stand out a bit from the bone (although the ‘exposed’ roots would have been covered with gums in life). If you put the glue on the tooth you’re liable to either have it bulldozed off by the alveolar rim as you slide the tooth into the socket, or you’ll put too much on and have a bunch of worthless glue on the exposed portion of the root.
- If the teeth are worn, like the incisors are here, it’s nice to do a bunch at once so you can get all of the wear surfaces lined up as they were in life. Better than having one tooth set up completely and then realizing it was all the way forward/back/in/out and the other teeth can’t match its orientation.
Everything back in. There are a couple of incisors still AWOL, and a few premolars, but the dentition is still reasonably complete (remember that I am used to working on Early Cretaceous North American sauropods, so a little completeness goes a long way). I ran a thin bead of epoxy around the bases of all of the teeth that had not come out of the alveoli, to hopefully rein in any future wanderlust on their parts.
Sweet action. I wish I had something more intellectual to say here, but I really don’t.
Nasal turbinates. Holy crap, were these a pain to get clean. I didn’t get them completely clean, there’s probably enough dirt up in there to germinate something. But I asymptotically approached the point where removing more dirt would have meant damaging the turbinates, which are at least manilla-envelope-thin if not laser-printer-paper-thin.
A closeup of the infraorbital foramen on the right, which transmitted the infraorbital artery and vessels in life. The neurovascular tracks on the external surface of the bone are pretty sweet; you can see them on the left side, in context, two photos up.
In addition to closing our jaws in a hinge-like motion, we can slide them fore and aft and also from side to side. Those kinds of motions are fine when you’ve got comparatively weak jaws like ours, and we still occasionally get into trouble–jawbreakers are so named for a reason. But those non-hinge-like motions would be disastrous for something that can close its jaws with several hundred pounds of force. So most big carnivores have wide, almost cylindrical jaw joints that constrain the motion to being almost purely hinge-like. In mustelids (weasels and kin), which have the strongest bites for their sizes of any mammals, the condyle is so cylindrical and the fossa so deeply enclosing (imagine a Q sitting inside a very slightly larger C–that’s the jaw joint seen from the side) that sometimes you simply can’t get the jaws to disarticulate after death. This ain’t quite that extreme, but it’s closer to the mustelid condition than the human. Not surprising, since weasels are united with bears and seals in the clade Arctoidea.
And here’s why bears need those cylindrical jaw joints: check out the muscle attachment scars on the back of the mandible. These are for the temporalis and masseter muscles, the same muscles you can feel bulging out on the side of your head and the corners of your jaw when you bite down forcefully or grit your teeth. IIRC, the maximum bite force a human can exert is around 180 pounds, and lions can do something like 900 pounds. Not sure where Ursus americanus falls, but definitely on the please-don’t-bite-me end of the scale.
And here’s why those muscle attachment scars are so big. The zygomatic arches are only partly complete here, but you can see how wide is the space between the left arch and the braincase. All of that space–two full inches, mediolaterally–was filled with temporalis muscle that provided most of the power for jaw-closing. This is why pit bulls have such wide, flat-looking heads: they have normal-sized dog brains and huge, thick jaw muscles. See also: my hyena dissection photos.
Looking very dog-like here in anterior view. There are some butchery marks on the skull, most noticeably across the external nares here, and along the mandibles. Not sure what that’s all about, since I can’t reconcile the stated backstory–cop shoots dangerous bear, buries head in backyard–with a need to make repeated cuts across the snout and jaws. And no, they’re not shovel marks. I knew that already, and Vicki confirmed that the marks are peri-mortem (around the time of death, but impossible to confirm as pre- or post-mortem). Anyway, I’m not complaining. Despite the damage, the skull is still an awesome thing, and the cut marks add a touch of mystery. I’ll post more pictures when I get the left temporal region glued back on.