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Dental Function as a tool in Anatomical Differentiation
In the research paper titled “Mammalian dental function and wear: A review“, the author Peter S. Ungar,1 present a brief synopsis of work on relationship between mammalian tooth form and function, and considers the role of dental wear in studies of mammal teeth.
For our journey of validation, we start with teeth. Teeth, and how they developed to help us eat, is an important part of the anatomical differentiation framework required for our hypothesis. Tooth crowns are important bio-surfaces that receive considerable attention from biotribologists. Most focus on huuman teeth, given clinical implications of tooth wear and failure2. But Homo sapiens is just one of tens of thousands of vertebrate species with teeth, and one of hundreds of thousands of animal species that have hardened structures in or around the oral cavity used in food acquisition and processing3. We can learn a lot about dental biotribology by extending our studies to the teeth of other species, especially other mammals. Mammalian teeth are like natural experiments wherein dental form, structure, and function are varied4. They allow us to explore basic principles about how teeth work, and to discover alternative solutions to the fundamental challenges that they face – acquiring and processing foods without being broken or worn away in the process. The study of mammalian teeth is important both for “pure science” reasons (e.g., understanding how nature works and what animals in the past ate), and for applied ones (e.g., the development of bio-inspired designs).
The idea that tooth shape relates to the mechanics of chewing dates back at least to Ryder׳s5 work in the 19th Century, but the concept as we know it today was developed largely by George Gaylord Simpson early in the 20th Century.
Simpson combined chewing motions and occlusal features to define three types of food processing:
- shearing, wherein opposing crests slide past one another with vertical jaw movements;
- opposition (now called crushing), wherein cusps are pressed into basins with vertical movements; and
- grinding, wherein cusps slide across basins with horizontal movements.
Simpson was able to reconstruct food processing methods used by fossil species by combining studies of tooth shape, facet pattern, and wear scratch direction. The model is more complicated than the demonstration above, but the take-home message should be clear – teeth are basically guides for chewing.
For the purpose of our articles and hypothesis, the relationships between tooth form, masticatory movements, and diet do hold up reasonably well when comparing related species. For example, bats that consume tough insects have longer shearing crests, whereas those that eat pulpy fruits have larger crushing and grinding surfaces. Likewise, omnivorous bears and the raccoon have larger crushing surfaces on their cheek teeth whereas carnivorous cats have larger shearing areas. And among bears, the bamboo-eating panda has larger crushing areas than the carnivorous polar bear, which has longer shearing crests.
We use some of these concepts to help us explore anatomical differences. One final but important thought about dental form in the context of chewing is that teeth cannot break down food without being brought into opposition in very specific ways. We know today that the whole process is incredibly complex. Mastication [Chewing] involves many different elements above and beyond the teeth – neural control over movements and sensory feedback, the size, attachments, and actions of the muscles of mastication, tongue, and cheek, the external and internal architecture of the jaw, the temporomandibular [Wikipedia] joint, and supporting hard and soft tissues. Chewing must be coordinated with lubrication by saliva, placement and retention of items between the teeth, and ultimately, swallowing. These elements all act together in symphony and synergy to accomplish food breakdown and they are all important for understanding the context in which teeth function and wear.
References and Research
- 3.Ungar PS. Teeth: A Very Short Introduction. 1st ed. OUP Oxford; 1st Edition (March 27, 2014); 2014. https://www.amazon.com/Teeth-Very-Short-Introduction-Introductions-ebook-dp-B00ICN2ZJQ/
- 4.Ungar PS. Mammal Teeth: Origin, Evolution, and Diversity. 1st ed. Johns Hopkins University Press; 1st Edition (August 31, 2010); 2010. https://www.amazon.com/Mammal-Teeth-Peter-S-Ungar-ebook-dp-B004BDOSNW/dp/B004BDOSNW
- 5.Ryder JA. Further Notes on the Mechanical Genesis of Tooth-Forms. Proceedings of the Academy of Natural Sciences of Philadelphia. 2019;31(1):47-51. www.jstor.org/stable/4060276