Chewing function and jaw articulation in fossil and extant shrews

点击翻译按钮获取中文摘要

Teeth are the most important source of information for reconstructing the dietary adaptations of extinct mammals. The diversification of mammals during the Cenozoic was accompanied by diverse adaptations of tooth morphologies. A crucial selection-factor for the development of the tooth morphology are the physical properties of the ingested food which can range from soft and ductile to hard and brittle. These are reflected in tooth morphology and the wear facets on the dental surface caused by the chewing action. Besides molar morphology, chewing is influenced by the jaw articulation and motion of the mandible. The increasing complexity of molar morphology (tribosphenic molar) is accompagnied by modifications of the mandible, whereby the exact interplay of both components is essential for an efficient comminution of the food. Mandibular movements can be deducted from wear facets on the cheek teeth. An efficient mastication of food is enabled by the interplay of tooth morphology, shape of the mandible and jaw articulation, and the chewing musculature. In the proposed project this interaction shall be studied in fossil and extant shrews. Shrews have a unique, highly complex mandibular articulation, with a double-headed articular process that consists of a dorsal and a ventral part that articulate with separate articulation facets at the sqamosal. It is assumed that the double-headed articular process allows for more differentiated and more complex mandibular movements. The vertical arrangement of the glenoid facet at the skull allows for a distinct anterior- posterior movement of the mandible. The observed outward tilt of the lower jaw during closure might have increased the grinding or shearing action of the molars. In the project the masticatory cycle will be simulated with high-resolution virtual 3D surface models generated by micro computed tomography. The virtual simulation will be performed with the Occlusal Fingerprint Analyzer (OFA) software that has been developed in the Deutsche Forschungsgemeinschaft´s research unit and which has been very successfully applied for functional analyses since then. With the OFA the path of the masticatory cycle can be reconsructed and its efficiency can be quantified by measurement of the contact areas of the teeth. Goal of the project is a deeper understanding of the interplay between the unique jaw articulation and chewing movements in shrews. We hypothesize that the mandibles of soricine shrews that have clearly separated articular heads perform more differentiated and complex movements that the crocidurine shrews with less clearly differentiated articular heads. Since the molar morphology is generally similar in shrews, the shape of the articulation and the resulting jaw movements should be deductable from the wear facets and striations.