Early Changes in Dental Morphology

The evolution of differently-shaped teeth in mammals is thought to be one of the major morphological innovations behind their success. Teeth are the first point of contact with food, and so increasing their efficiency at breaking down food allows easier access to nutrients during digestion. At the origin of mammals in the Mesozoic, and their later radiation in the Cenozoic, the number of tooth shapes increased greatly, as did the diversity of the types of foods that mammals ate. One of the major milestones in the diversification of mammal tooth shapes came with the tribosphenic molar - a multifunctional tooth that had both a cutting surface (trigonid), and a crushing surface (talonid).

There were a gradual series of changes in molar morphology that lead to the evolution of the tribosphenic molar. The initial step involved a transition from a linear alignment of three cusps on the molar crown, to a triangular alignment. This step occurred during the Late Triassic - Early Jurassic and is represented by the insectivorous mammals Morganucodon (linear arrangement) and Kuehneotherium (triangular arrangement). The triangular arrangement of cusps in Kuehneotherium is homologous to the triangular cutting part of the tribosphenic molar. Therefore, this initial shift in cusp position was the first step towards forming a tribosphenic molar, and is though to have opened the door to an increased range of diets and a wealth of morphological diversity in tooth shape.

To understand the biomechanical implications of this transition, we built 3D models based on Morganucodon (linear alignment of molar cusps) and Kuehneotherium (triangular arrangement of molar cusps), and used them to puncture hard and soft proxy food items made from polymer gels that mimicked the material properties of insects. We measured three biomechanical parameters at the instant that the food item fractured (force, energy, and time) and the amount of damage that each model inflicted on the food item.

The main finding from our newly analysed data is that the Morganucodon and Kuehneotherium models differ in their biomechanical interactions with hard and soft foods. Specifically, Morganucodon was better able to process hard food while Kuehneotherium was better able to process soft foods. Perhaps more importantly, Kuehneotherium molars inflicted significantly more damage on food items regardless of their material properties.

These results suggest that changes in dental morphology in some early mammals was driven primarily by selection for maximizing damage, and secondarily for maximizing biomechanical efficiency for a given food material property.

This work was recently published by the Journal of the Royal Society Interface (see the Publications tab), and was picked up by several media outlets. Watch the video below to see this work featured on Daily Planet's 'Discoveries' section.