UMass Amherst/Andrew Conith
3D-printed mammal teeth
Researchers used 3D-printed replicas of 200-million-year-old mammal teeth to prove the ability for teeth to damage prey is the most significant factor in the shaping of teeth
A team of scientists have proved the ability for teeth to damage prey is the most significant factor in the evolution of their shaping, using 3D-printed replicas of 200-million-year-old mammal teeth.
Polymers that mimic insect prey were also used during the studies.
The researchers at the University of Massachusetts Amherst have found bite force and the animal’s energy expenditure have been less important in the evolution process.
This unexpected finding should change the way biologists view natural selection as it is studied through dental morphology. Tooth shape is linked to diet and biomechanics of feeding. Much of what is known about early mammalian evolution comes from fossilised teeth.
In a bid to better understand how tooth shape influenced diet in early mammals, evolutionary biology doctoral student, Andrew John Conith and his advisor, Elizabeth Dumont, with polymer scientists, Alfred Crosby and graduate student, Michael Imburgia came together. Dumont and Crosby are both members of the Centre for Evolutionary Materials at UMass Amherst, where researchers apply biological thinking to engineering problems.
The team used 3D-printed replicas of 200-million-year-old molars in their tests to simulate a bite. Teeth from two shrew-like early mammal species, the primitive Morganucodon and the more advanced Kuehneotherium, were used. Both species were underfoot when dinosaurs roamed the earth in the Triassic Period, and are considered exemplars of early mammal evolution.
“The big question here is why teeth look the way they do,” said Conith. “Most of the work on early mammalian tooth evolution has been descriptions of what they look like and how they could potentially work as tools for biting and crushing insects. We took it one step further, to make these tools and test them. We merged two modern technologies and used 3D prints of teeth to ‘bite’ into polymer gels with a exoskeleton-like crust that accurately mimicked insects.
“Based on these experiments, we think the factor that natural selection worked on was the ability to break apart food, and that selection for maximum damage is the primary determinant of tooth shape.”
Most previous research has ignored damage in favour of investigating force and energy based on the assumption that selection favoured animals that expend the least fore and energy. But now Conith and his team think people need to reconsider these typical parameters and now think more critically about damage.
“We haven’t rewritten the book, but we have added a new chapter,” added Conith.
To imitate insect prey for the bite force experiments, energy efficiency and damage by Morganucodon and Kuehneotherium jaws, the researchers constructed gel-filled, candy bar-shaped rectangles coated with a polymer shell to mimic an insect’s exoskeleton. They then constructed two polymer ‘proxy insects’, one hard-shelled, one softer-shelled, and ran then experiments using both Morganucodon and Kuehneotherium molar shapes.
A force-testing machine was used to bite the proxy insects between 3D-printed teeth replicas from the upper and lower jaw. The force, energy and damage inflicted to the jells were then measured. To assess damage, they measured the cracks and fractures in the polymer coating from digital photographs following a biting trial.
The authors concluded that the more primitive Morganucodon model required less force and energy to fracture hard gels, while Kuehneotherium required less force and energy to fracture soft gels.
“More importantly, Kuehneotherium also inflicted more damage to both the hard and the soft gels. These results suggest that changes in tooth shape in some early mammals was driven primarily by selection for maximising damage, and secondarily for maximising biomechanical parameters such as force or energy,” Conith and colleagues write.
"When we started this project we thought we'd only report the force and energy results, we never thought about damage. It wasn't until we actually saw the destruction Kuehneotherium could inflict on our model insects that we thought it would be interesting to measure."