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The African Plio-Pleistocene hominins known as australopiths evolved a distinctive craniofacial morphology that traditionally has been viewed as a dietary adaptation for feeding on either small, hard objects or on large volumes of food. A historically influential interpretation of this morphology hypothesizes that loads applied to the premolars during feeding had a profound influence on the evolution of australopith craniofacial form. Here, we test this hypothesis using finite element analysis in conjunction with comparative, imaging, and experimental methods. We find that the facial skeleton of the Australopithecus type species, A. africanus, is well suited to withstand premolar loads. However, we suggest that the mastication of either small objects or large volumes of food is unlikely to fully explain the evolution of facial form in this species. Rather, key aspects of australopith craniofacial morphology are more likely to be related to the ingestion and initial preparation of large, mechanically protected food objects like large nuts and seeds. These foods may have broadened the diet of these hominins, possibly by being critical resources that australopiths relied on during periods when their preferred dietary items were in short supply. Our analysis reconciles apparent discrepancies between dietary reconstructions based on biomechanics, tooth morphology, and dental microwear.evolution ͉ face ͉ finite element analysis ͉ hominin ͉ diet

The wide range of dietary niches filled by modern mammals is reflected in morphological diversity of the feeding apparatus. Despite volumes of data on the biomechanics of feeding, the extent to which the shape of mammal skulls reflects stresses generated by feeding is still unknown. In addition to the feeding apparatus, the skull accommodates the structural needs of the sensory systems and brain. We turned to bats as a model system for separating optimization for masticatory loads from optimization for other functions. Because the energetic cost of flight increases with body mass, it is reasonable to suggest that bats have experienced selective pressure over evolutionary time to minimize mass. Therefore, the skulls of bats are likely to be optimized to meet functional demands. We investigate the hypothesis that there is a biomechanical link between biting style and craniofacial morphology by combining biting behavior and bite force data gathered in the field with finite-element (FE) analysis. Our FE experiments compared patterns of stress in the craniofacial skeletons within and between two species of bats (Artibeus jamaicensis and Cynopterus brachyotis) under routine and atypical loading conditions. For both species, routine loading produced low stresses in most of the skull. However, the skull of Artibeus was most resistant to loads applied via its typical biting style, suggesting a mechanical link between routine loading and skull form. The same was not true of Cynopterus, where factors other than feeding appear to have had a more significant impact on craniofacial morphology. Key words: biting behavior; bone stress; adaptation; finite-element analysis; Chiroptera Mammal evolution is largely a story of the expansion of dietary niches from an insect-eating ancestor to include foods ranging from meat and bone to plankton. This diversity is clearly reflected in the morphology of the craniofacial skeleton. The association between skull structure and diet across distantly related mammals suggests that skull shape underwent selection over evolutionary time as new dietary niches were explored. Many excellent laboratory-based studies of feeding have provided a wealth of detailed information about the biomechanical behavior of bones and muscles under controlled experimental conditions. Building on this knowledge, morphologists are beginning to venture into the field to investigate how natural behaviors interact with morphology to define how animals function within their native environments. By

This article reviews the fundamental principles of the finite element method and the three basic steps (model creation, solution, and validation and interpretation) involved in using it to examine structural mechanics. Validation is a critical step in the analysis, without which researchers cannot evaluate the extent to which the model represents or is relevant to the real biological condition. We discuss the method's considerable potential as a tool to test biomechanical hypotheses, and major hurdles involved in doing so reliably, from the perspective of researchers interested in functional morphology and paleontology. We conclude with a case study to illustrate how researchers deal with many of the factors and assumptions involved in finite element analysis.

The morphology and biomechanics of the vertebrate skull reflect the physicalproperties of diet and behaviors used in food acquisition and processing. We use phyllostomid bats, the most diverse mammalian dietary radiation, to investigate if and how changes in dietary hardness and loading behaviors during feeding shaped the evolution of skull morphology and biomechanics.When selective regimes of food hardness are modeled, we found that species consuming harder foods have evolved skull shapes that allow for more efficient bite force production. These species have shorter skulls and a greater reliance on the temporalis muscle, The vertebrate skull is a classic example of a complex anatomical system that is composed of numerous highly integrated units (Cheverud 1982;Klingenberg 2008). The morphology and function of the skull are presumed to be under strong selection and shaped by the physical properties of the diet and behaviors that improve food acquisition and processing. These selective pressures act on the performance of the cranial system as a whole and ultimately shape the morphology of its individual parts. For example, the ecological advantages of consuming hard mollusks have been linked to the evolution of high bite performance (bite force) in Chamaeleolis lizards (Herrel and Holanova 2008). High bite forces in these lizards are the result of tall heads with a pronounced temporal ridge and large jaw adductors, which are traits that reflect the biomechanics of the jaw system (Herrel and Holanova 2008). Thus, examining the mechanisms through which the morphology of cranial elements translates into function is a powerful approach to understanding how the vertebrate skull evolves. To date, few studies have been able to integrate the behavioral and ecological factors shaping the skull diversity across vertebrate lineages, but an extensive and continuously growing body of research has helped us elucidate skull biomechanics in a vast array of vertebrate species, from suction feeding in fish (reviewed in Westneat 2005) to biting in a wide array of vertebrates (e.g., Cleuren et al. 1995;Freeman and Lemen 2008;Curtis et al. 2010;Davis et al. 2010). These studies have set the stage for much-needed comparative approaches which, using recent 2 5 8 7