Thursday, February 17, 2011

Premaxillary Bone

This week we had the opportunity to examine skulls at the Exhibit Museum of Natural History here at the University of Michigan. The skulls came from 20 different species similar to those that Johann Wolfgang von Goethe described in his research on the premaxillary bone. In his work, Goethe argues that the premaxillary bone is present in humans just as it is in horses, lions and primates1. Therefore, our class went to view a collection of skulls in order to compare the presence and shape of the premaxillary bone across many different species of animals and try to find an underlying reason for the different forms.

The first set of skulls I looked at were those from a bison, cow, and deer (pictures of a cow are shown from the side and below – the premaxillary bone is labeled 4).

All three of these animals had elongated premaxillary bones (about 15 cm in the bison) below the nasal cavity that wasn’t connected to any teeth. These animals are all herbivores, with a diet consisting mainly of grass, berries, and twigs. This led me to postulate that herbivores might all have similarly elongated premaxillary bones that weren’t connected to any incisors. Although incisors were present amongst the bottom row of teeth, the corresponding spot in the top row was completely void. Professor Phil Meyers, our host, explained that instead of having teeth here, these animals have a flat, hard patch due to the premaxillary bone. Although this explains the absence of the teeth, it still does not make much sense to me. The incisors are used for tearing, which would be important for pulling bark off a tree or grass up from the ground. Therefore, it seems that these animals should have incisors in both the bottom and top rows of teeth.

In order to further investigate the lack of incisors in herbivores, I moved on to examine the skull of a horse (pictured, left) and tapir, which are both herbivores as well. To my surprise, the premaxillary bones did not have the same form as in the previous three animals. In both the horse and the tapir, the premaxillary bones were elongated to the same degree, but were connected to 6 front teeth. Looking at different types of rodents, which are also herbivores, the premaxillary bones were elongated in beavers and capybaras (pictured, right), but instead of being connected to 6 teeth, the premaxillary bones were connected to only 2 teeth. Thus the hypothesis that the premaxillary bone was not connected to any teeth in herbivores and the idea that there is a relationship between the type of food eaten and number of teeth connected to the premaxillary bones were quickly proven false.

In omnivores, such as humans and a few types of primates, the premaxillary bone is extremely small and sometimes hardly noticeable. This is especially the case in adults, hence the difficulty Goethe faced in proving its existence in humans. I was able to observe this in the macaque, gibbon, gorilla and chimpanzee skull samples. All of these animals, which have similar eating habits to humans, have extremely similarly shaped premaxillary bones – their lines of fusion in the mouth were hardly noticeable and they were connected to the front 4 teeth. I found this to be the same for lions as well, which are primarily carnivores (pictured). To look at the connection between diet and size of the premaxillary bone, I looked at a sample of a sloth skull. Alas, sloths, which are herbivores, had premaxillary bones that resembled those of the primates and lion without front teeth. Therefore it seems that there isn’t a cut and dry connection between diet and the presence of incisors or the shape of the premaxillary bone.

After reflecting though, I came across the idea that there is a connection between the way animals find their food and the size of their premaxillary bone. Animals such as the cow, bison, horse, and rodents don’t use hands like primates and sloths to search for food. Instead they use their noses or snouts to poke around and identify foodstuff. This corresponds to the elongated premaxillary bones in these animals. The premaxillary bone functions to form a snout that can be used for digging or poking around. The skulls of a dugong (pictured) and manatee serve to fit into this mold. Both of these animals are herbivores, have well-developed premaxillary bones, and especially in the case of the dugong, use a snout to dig up sea grass for food. Although not entirely fool-proof, it seems a slight connection might exist after all.

1 Goethe, Johann Wolfgang von. “An Intermaxillary Bone is Present in the Upper Jaw of Man As Well As in Animals.” In Scientific Studies, edited and translated by Douglas Miller, 111–116. New York: Suhrkamp, 1988.

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