Margot Michaud, UniLaSalle; Melvin Vankelst, University of Liège; and Narimane Chatar, University of California, Berkeley
THE ESSENTIALS
- Teeth are the primary interface between an animal and its food.
- The “tribosphenic” molar is used for slicing and grinding. How can these two opposing functions be combined?
- A new study reveals how these teeth have evolved to strike the right balance based on each animal’s diet.
Could you cut a steak and crack a nut with a single tool? A thin blade easily penetrates soft material but cannot withstand heavy loads. A crushing tool, on the other hand, must be wide and sturdy but does not cut well.
Over the course of their evolution, however, the vast majority of modern mammals have combined these two functions into a single structure: a specialized molar. But to what extent can a single tooth reconcile these two opposing functions?
To find out, our team compared these molars in modern and fossil mammals, then tested the grinding and cutting capabilities of their 3D-printed replicas. By combining high-precision scans, 3D printing, and mechanical testing, our new study, recently published in the journal Science, sheds light on the constraints placed on mammalian dentition.
A Tooth with a Dual Function
Teeth serve as a crucial interface between an animal and its food. Their shape thus partly determines how food can be processed before being swallowed, as well as the physical forces they can withstand.
Because they fossilize very well, these structures also provide insight into the diet and biomechanics of extinct species, allowing us, in particular, to trace certain major stages in the evolution of mammals, including the emergence of the “tribosphenic” molar.
This tooth, with its distinctive morphology, combines a tall, cutting region—the trigonid—with a lower region used for grinding—the talonid. When the jaw closes, these regions work together with the upper teeth to shear and crush food in a single motion.
By enabling mammals to both slice and grind food in a single bite, this architecture expanded the range of resources available to them and may thus have promoted their ecological diversification. However, these two functions are facilitated by dental morphologies that are largely contradictory: slicing favors tall, sharp points and well-aligned ridges, while grinding requires a large contact surface and a sturdy structure. It therefore remains to be understood how this geometric incompatibility shaped the evolution of molars in mammals.
The predator in all its forms
Not all mammalian molars are identical. Over the past 66 million years, carnassial teeth—molars specialized for cutting flesh—have evolved independently in several groups of mammals.
Currently, the Carnivora (a group that includes cats, dogs, and bears, among others) are the only true possessors of carnassial teeth. Despite their name, the Carnivora include not only specialized predators but also omnivorous species and even strictly herbivorous species, such as the giant panda. This dietary diversity is accompanied by a remarkable variety of dental forms. In felids, the carnassial is a pure blade, with a reduced or absent talonide. Dogs, bears, and raccoons, on the other hand, retain grinding surfaces suited to a varied diet, including the bamboo eaten by the giant panda.
Our team has therefore created 3D scans of the lower canine teeth of 250 extant and fossil species. Two major recurring forms emerge: tall, two-cusp teeth with a reduced or absent talonide, typical of specialized predators such as felines; and teeth with a well-developed talonide, associated with a greater dietary diversity.
On your marks. Get set? Bite!
A selection of teeth was reproduced using 3D printing and then tested in two devices made from materials designed to mimic the properties of biological tissues. To evaluate cutting performance, the models were pressed into layers of gelatin that mimicked soft tissues, such as skin and muscles. We measured the force required to penetrate the gel as well as the size of the lacerations based on the morphology of each tooth. To assess grinding, the same tooth shapes were tested on a printed material specifically designed to have properties and strength similar to those of bone.
Our tests show that few tooth shapes effectively cut through soft tissues, and all share the same morphology: two tall cusps connected by sharp ridges, a double-edged blade that concentrates pressure over a minimal surface area and penetrates the gelatin deeply with little force. Among hypercarnivores (which feed almost exclusively on meat), the carnivorous tooth even approaches the theoretical optimum, a sign that the margin for variation is narrow: to cut effectively, the height and orientation of the cusps, as well as the alignment of the ridges, must be precisely combined.
Conversely, many dental morphologies allow for effective grinding. A well-developed talonide generally helps the tooth withstand heavy loads, but it is not the only possible solution. In some blade-shaped teeth, a small notch located between the two cusps distributes stress and limits the risk of fracture, just like the openings found on a circular saw blade.
However, shapes that best balance these two functions remain rare: fewer than 1% of the shapes studied come close to the optimal compromise between the two functions. The trade-off, therefore, does not pit two equally important functions against each other. Rather, it contrasts a highly demanding function—slicing, which allows for only a small number of optimal shapes—with a more flexible function—crushing, which can be achieved by several different designs. Consequently, there are far more ways to design a good crusher than a good slicer.
When Specialization Closes Certain Doors
Over millions of years, this imbalance could have far-reaching consequences. The development of a larger talonid increases grinding capabilities and may allow for the consumption of a wider variety of foods. Conversely, the evolution of a blade-shaped carnivorous tooth is associated with a narrower specialization.
This specialization can thus produce an “evolutionary ratchet effect”: once the talonid has been greatly reduced and the tooth has become specialized for cutting flesh, a return to a more versatile tooth becomes difficult. Thus, hyaenodonts and oxyaenodonts—close relatives of Carnivora mammals—which had highly specialized teeth, eventually went extinct. While our study does not identify this as the cause of their extinction, it suggests that extreme specialization may limit access to new resources.
Evolution does not seek the perfect tool
So, can you cut a steak and crack a nut with a single tool? Among mammals, the answer is yes, but rarely with the same efficiency. The tribosphene molar made both of these functions possible, while also promoting specialization in one or the other.
This finding reminds us that, in evolution, an innovation is never synonymous with perfection: it opens up new possibilities while imposing certain constraints, and the resulting trade-offs help shape the diversity of life.
Margot Michaud, Lecturer and Researcher in Evolutionary Biology and Anatomy, UniLaSalle; Melvin Vankelst, Biologist and Paleontology Researcher, University of Liège; and Narimane Chatar, Postdoc, University of California, Berkeley
This article is republished from The Conversation under a Creative Commons license. Read the original article.
