Australopithecus anamensis: A finite-element approach to studying the functional adaptations of extinct hominins

Palaeontology Research Group, Department of Human Anatomy and Cell Biology, University of Liverpool, Liverpool L69 3GE, UK.
The Anatomical Record Part A Discoveries in Molecular Cellular and Evolutionary Biology 04/2005; 283(2):310-8. DOI: 10.1002/ar.a.20175
Source: PubMed

ABSTRACT Australopithecus anamensis is the stem species of all later hominins and exhibits the suite of characters traditionally associated with hominins, i.e., bipedal locomotion when on the ground, canine reduction, and thick-enameled teeth. The functional consequences of its thick enamel are, however, unclear. Without appropriate structural reinforcement, these thick-enameled teeth may be prone to failure. This article investigates the mechanical behavior of A. anamensis enamel and represents the first in a series that will attempt to determine the functional adaptations of hominin teeth. First, the microstructural arrangement of enamel prisms in A. anamensis teeth was reconstructed using recently developed software and was compared with that of extant hominoids. Second, a finite-element model of a block of enamel containing one cycle of prism deviation was reconstructed for Homo, Pan, Gorilla, and A. anamensis and the behavior of these tissues under compressive stress was determined. Despite similarities in enamel microstructure between A. anamensis and the African great apes, the structural arrangement of prismatic enamel in A. anamensis appears to be more effective in load dissipation under these compressive loads. The findings may imply that this hominin species was well adapted to puncture crushing and are in some respects contrary to expectations based on macromorphology of teeth. Taking together, information obtained from both finite-element analyses and dental macroanatomy leads us to suggest that A. anamensis was probably adapted for habitually consuming a hard-tough diet. However, additional tests are needed to understand the functional adaptations of A. anamensis teeth fully.

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    ABSTRACT: Au. anamensis es un hominino del Plio-Pleistoceno que habitó en un rango amplio de ambientes, que incluía ambientes cerrados y zonas más abiertas de sabana. El análisis de su patrón de microestriación vestibular (n=5) muestra una abrasividad superior (NT=220,60) al de Au. afarensis (NT: 150,69). En cambio, el patrón de Au. anamen-sis muestra claras afinidades con el de varias especies cercopitecoi-deas como Cercocebus , Mandrillus y Cercopithecus sp., tanto para el número total de estrías como en un análisis discriminante. Estos re-sultados indican que Au. anamensis presentaba un régimen alimen-tario de tipo frugívoro-granívoro similar al de los cercopitécidos ac-tuales (semillas, nueces, rizomas, tubérculos, raíces), a diferencia de Au. afarensis, una especie eminentemente frugívora.
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    ABSTRACT: The human skull is gracile when compared to many Middle Pleistocene hominins. It has been argued that it is less able to generate and withstand high masticatory forces, and that the morphology of the lower portion of the modern human face correlates most strongly with dietary characteristics. This study uses geometric morphometrics and finite element analysis (FEA) to assess the relationship between skull morphology, muscle force and cranial deformations arising from biting, which is relevant in understanding how skull morphology relates to mastication. The three-dimensional skull anatomies of 20 individuals were reconstructed from medical computed tomograms. Maximal contractile muscle forces were estimated from muscular anatomical cross-sectional areas (CSAs). Fifty-nine landmarks were used to represent skull morphology. A partial least squares analysis was performed to assess the association between skull shape and muscle force, and FEA was used to compare the deformation (strains) generated during incisor and molar bites in two individuals representing extremes of morphological variation in the sample. The results showed that only the proportion of total muscle CSA accounted for by the temporalis appears associated with skull morphology, albeit weekly. However, individuals with a large temporalis tend to possess a relatively wider face, a narrower, more vertically oriented maxilla and a lower positioning of the coronoid process. The FEAs showed that, despite differences in morphology, biting results in similar modes of deformation for both crania, but with localised lower magnitudes of strains arising in the individual with the narrowest, most vertically oriented maxilla. Our results suggest that the morphology of the maxilla modulates the transmission of forces generated during mastication to the rest of the cranium by deforming less in individuals with the ability to generate proportionately larger temporalis muscle forces.
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