Homodonty and Heterodonty - an iconoclastic view of teeth

Abstract

Teeth are perhaps one of the most readily identifiable traits that categorize vertebrates. Indeed, much of what we know about human evolution is based on fossilized teeth. In a similar manner the teeth of other vertebrates can serve as an important clue in identifying the relationship between the morphology of the organism and its environment. Before this potential can be realized, we need a more integrative approach for examining the role of different tooth structures. Previous studies have explored the role of tooth shape largely divorced from the way teeth interact with each other, with food, and with different anatomical structures involved in feeding. My dissertation addresses how measuring dental mechanics can reveal nuances in shape, function, and evolution of teeth as well as how teeth function in combinations with lips, tongues, and jaw muscles. Tooth function changes with shape, orientation (which way the tooth is pointing), position, and the dynamic forces produced by the jaw muscles. We refer to dentitions with all the same shaped teeth as homodont (‘homo’ = same, ’dont’ = tooth) and those with different shapes and sizes as heterodont. Collapsing dentitions into such blunt and discrete categories, obscures both functional differences between teeth and insights into dental evolution. Consider that teeth that look the same may not function the same. Conical teeth are the most common tooth shape and prevalent in fishes which make up more than half of all vertebrate diversity, Conical teeth are superficially tasked with the simple job of puncture. However, there is a great deal of variation in the shape and placement of conical teeth. This variation suggests conical dentitions represent a single shape solution to different functional problems. To test this hypothesis I developed a statistical model – a ’functional homodonty’ metric – that lets us explore how different combinations of teeth work together. The model incorporates parameters relevant to conical teeth, but there is no reason that the model cannot be expanded to investigate incisive, molariform, or triangular tooth shapes. Indeed, other groups have already applied the metric in very different conditions than I proposed. The evolutionary history and variation of dental morphologies creates opportunities to explore the bounds of the functional homodonty metric. By doing so, we increase our understanding of how and why morphological heterodonty evolves, and the importance that individual teeth serve along the jaw. The latter half of my dissertation examines the coordinated actions of lips and teeth. Pacus are herbivorous cousins of piranhas and have to peel fruit and crack open seeds without the use of hands. My work showed how the morphology and behavior of the lips work in tangent with the teeth to hold, reposition, and break down complicated food items. In this published work I demonstrated the importance of a multidimensional approach to evolutionary questions and how quantifying variation along more than one axis (i.e space and time) provides the substrate for understanding dietary transitions.