Researchers have an intuitive understanding of the relationship between tooth shape and use: animals that eat meat have pointy teeth, and those that eat plant material have flatter teeth used for grinding. But many animals also have more specialized diets, which require more specialized teeth to go along with them. Durophagy is the consumption of hard-shelled prey items like bivalves, snails, or even nuts. This means that durophagous predators need to breach or completely break the shells of their food (Figure 1).
Fig. 1: Even invertebrates have durophagous "teeth": crabs that use their claws to crush shells can have pretty impressive denticles!
As with teeth "designed" to eat meat versus plants, durophagous teeth are also easy to recognize. According to the literature and our intuition, shell-crushing teeth are big, flat, and hammer-like. However, this isn't always the case. Hard-prey consumption has evolved at least five separate times within jawed vertebrate lineages, and there is an underappreciated diversity in durophagous tooth morphologies: teeth can be rounded, cupped, or even cusped (with a point on the crushing surface). This variation begs the question: what are the functional limitations of a crushing tooth?
To test this, Dr. Adam Summers and I modeled three sets of crushing tooth shapes, each varying by a single parameter, to isolate different aspects of tooth morphology. For the first series, we varied the overall concavity of the surface of the tooth, from concave to convex, and for the second and third series we modified the morphology of a central point, changing either the height of the cusp or the width of its base. To understand how well different tooth shapes break prey, we used an automated mill to create aluminum models of the three series of tooth shapes. We used these models to measure how much force was required to break snail shells created with a 3D-printer. While the printed shells did not have the same microstructure as natural snail shells, they did behave as brittle solids and there was no significant variation in size or material properties between individual shells.
Fig. 2: FEA of a concave tooth model. Warmer colors indicate locations that are most likely to fail.
An alternate explanation behind the evolution of hard-prey crushing tooth shapes is that they may allow teeth to withstand the high forces generated during shell-crushing bites. We used an engineering technique called Finite Element Analysis (FEA) to compare in-tooth strain distributions between the same three series of tooth models. FEA works by taking a complex shape, breaking it into a number of smaller, simpler shapes, and seeing how forces act on and are transferred between them. This allowed us to predict which teeth would be more likely to fail, and where on the tooth the failure would most likely happen (Figure 2).
Comparing the results of the physical model tests and the FEA study, the functional trade-off between the ability of teeth to break prey items and the need to prevent tooth failure becomes apparent. Pointed teeth and teeth with cusps are better able to break prey, but they are also themselves more likely to break. Cupped teeth aren’t as likely to fail, but more force is required for them to break shells. Given these functional trade-offs, the "least convex" and the "flat tooth" models seem to represent an ideal tooth.
Fig. 3: One of the placodonts that we studied, Placodus gigas. This specimen was from the collection at the Senckenberg Forschungsinstitut und Naturmuseum in Frankfurt am Main (SMF R.564), one of the eight collections that I visited to gather placodont data.
However, all hard-prey crushing teeth are not flat, so there must also be another factor at work in tooth design. To test the importance of evolutionary history as well as the effects of non-selective pressures, we investigated changes in tooth shape in the Placodontia (Figure 3) — an extinct group of durophagous marine reptiles from the Triassic period (about 250-200 million years ago). By collecting data from fossils and comparing tooth shape between species, we found that the earlier species of placodonts had pointier teeth, while most of the later occurring groups had teeth similar to the predicted ideal tooth morphology. But, get this: one of the last-occurring groups, instead of having the predicted tooth shape, had teeth with an overall concave surface and a small cusp. In this group we see a tooth with features that both reduce likelihood of tooth failure and break prey with less force: an alternate durophagous tooth design.
The epochs-long arms race between predators and prey helps drive the evolution of both groups, and the stereotypical flattened teeth of durophagous predators are their weapon of choice. But, while flat teeth crush well, they are not the only option for durophagous animals. Other factors, like tooth replacement rate, tooth material properties, or even the strength and shape of the prey can lead to variation and greater complexity in tooth shape. By understanding the functional pressures behind tooth shape we can begin to understand the relationships between long-extinct species, as well as predict how durophagous predator-prey interactions may change in the future.
Stephanie finished her Ph.D. in Summer 2015 working with Adam Summers at UW, where she studied the functional morphology (form and structure) of teeth that crush hard prey. She is now exploring swimming in reptiles (extant and extinct) with Brooke Flammang at the New Jersey Institute of Technology.
All photos by Stephanie Crofts.