Early Hominin Paleoecology

6. The Functional Morphology of Jaws and Teeth

203 6 The Functional Morphology of Jaws and Teeth: Implications for Understanding Early Hominin Dietary Adaptations Peter S. Ungar and David J. Daegling The famed French politician and gourmet JeanAnthelme Brillat-Savarin (1826) once wrote “tell me what you eat, I’ll tell you who you are.” Our diet choices are such an important part of defining us that an entire discipline, nutritional anthropology, has developed to study how and why we are what we eat. Paleoanthropologists take this one step further, suggesting that an understanding of the evolution of human diets can even inform us on how we came to be the way we are as a species. Diet is an important key to defining an animal species , its ecological role, and its relationships within a biological community. Primatologists, for example, recognize that “diet is the single most important parameter underlying behavioral and ecological differences among living primates” (Fleagle 1999). The same must have been true for extinct primate species, including our own ancestors, the early hominins. The aim of this chapter is to consider one of the most common approaches that paleontologists take to reconstructing the diets of fossil mammals: the study of relationships between jaw and tooth form on the one hand, and diet on the other. How do we do this? The most common approach is the comparative method. We observe that a given structure in living animals is used for a specific function. This allows us to hypothesize a relationship between that structure and its function. If all living animals that have this structure use it in the same way, we can “predict”(or retrodict) that fossil animals with that DOI: 10.5876/9781607322252:c06 peter s. ungar and david j. daegling 204 structure would have used it the same way (Kay and Cartmill 1975; Anthony and Kay 1993). Studies of the mechanics of anatomical structures and theoretical models further help us understand what a trait is best suited for, which can be very useful for evaluating hypothesized form–function relationships and for trying to figure out the function of a structure that has no modern counterpart. This chaper presents a comparative study of the shapes of jaws and teeth of living primates, particularly of those aspects that relate to food processing, with an eye toward reconstructing the diets of fossil species. Teeth and jaws are the most common elements found in most vertebrate fossil assemblages, including the early hominins. As durable parts of the digestive system, they seem ideally suited to being studied for the reconstruction of diet.Here we offer examples of how the process works for two of our hominin forebears, Australopithecus africanus and Paranthropus robustus. Craniodental adaptations of these two hominins are often compared and contrasted, and these suggest differences in the diets of these species. AUSTRALOPITHECUS AFRICANUS AND PARANTHROPUS ROBUSTUS Researchers have focused attention on the dietary adaptations of South African Plio-Pleistocene hominins for more than a half century, ever since Robinson’s (1954) classic study comparing Australopithecus africanus and Paranthropus robustus. Robinson noted that P. robustus had larger cheek teeth and smaller incisors, larger chewing muscles, more enamel chipping on their cheek teeth, and other features that suggested to him a specialized herbivorous diet compared with a more generalized, omnivorous A. africanus. Many subsequent workers have focused on differences in the sizes of the teeth of these two hominins (e.g., Tobias 1967; Groves and Napier 1968; Jolly 1970; Pilbeam and Gould 1974; Szalay 1975; Kay 1975a; Wood and Stack 1980; Walker 1981; Peters 1981; Kay 1985; Lucas et al. 1985; Wood and Ellis 1986; Demes and Creel 1988; McHenry 1988; Ungar and Grine 1991; Teaford and Ungar 2000). Many of these researchers suggested that the larger molars and smaller incisors of Paranthropus, compared with Australopithecus, reflect a diet that was dominated by large quantities of low-quality foods requiring little incisal preparation but more heavy chewing. While relationships between tooth size and diet are still not well understood (Teaford et al. 2002; Ungar et al. 2006), this interpretation does appear to be consistent with craniofacial functional morphology of these hominins (Du Brul 1977; Ward and Molnar 1980; Rak 1983; Sakka 1984; Daegling and Grine 1991). Most would agree that the facial architectures and the functional morphology of jaws and teeth 205 jaw shapes of these hominin species differ from one another and from modern primates in a manner that relates to their dietary adaptations. The...