Skull Shape, Masticatory Apparatus, and Diet of Vassallia and Holmesina (Mammalia: Xenarthra: Pampatheriidae): When Anatomy Constrains Destiny

Abstract

The form and function of the masticatory apparatus of the fossil genera Vassallia and Holmesina are analyzed so that the possible dietary behaviors of these pampathere xenarthrans might be inferred. Analysis is based on comparisons of dental morphology and skeletal features (through RFTRA) associated with the masticatory musculature among the pampatheres, the extant dasypodids Euphractus and Dasypus, and the glyptodont Propalaeohoplophorus. A method is proposed for generating a moment arm of the massetericus independently of the muscle's line of action, which allows direct comparison among extant and fossil mammals. The masticatory apparatus of the pampatheres strongly resembles that of Euphractus among extant forms, but the development of muscular attachment sites indicates a more powerful musculature, particularly the massetericus; the taxa differ most markedly in dental morphology. Long moment arms about the jaw joint and large ratios of muscle to bite moments indicate forceful rather than quick movements. The various skeletal and dental features analyzed suggest that the masticatory apparatus of the pampatheres was more powerful and efficient in transverse chewing than in dasypodids and that they were primarily grazers consuming mainly coarse vegetation. These features, some shared with herbivorous ungulates, include wide, relatively flat mandibular condyles; condyles well dorsal to muscular insertion sites; expanded angular processes; unfused symphysis; a posteriorly extended tooth row; open-rooted teeth; mesial teeth that bear mainly transverse striations; distal teeth that are mesiodistally elongated, bear basined occlusal surfaces, and in Vassallia possess a central island of resistant dentine that acted as a functional analogue of an ectoloph; and teeth with a stepwise arrangement. The results of this study indicate that detailed analysis and comparison of morphology lead to useful predictions of behavior.