New well-preserved fossils from Peru reveal details of the dentition and morphology of the mandible and rostrum in 2 late middle to early late Miocene beaked whales (Cetacea, Odontoceti, Ziphiidae). Apical mandibular tusks are present in both Nazcacetus urbinai and Messapicetus sp. In the former the tusks are associated with a strong reduction of the postapical dentition, whereas in Messapicetus sp. a complete series of functional upper and lower teeth is retained. The larger sample of Messapicetus sp. from a single locality and age reveals intraspecific variation in size and shape of the tusks and surrounding structures. In addition, the rostrum of Messapicetus displays thickened premaxillae, dorsally closing the mesorostral groove. By comparison with modern beaked whales, most of them highly sexually dimorphic at the level of the tusks and rostrum, we propose that the tusks of Messapicetus were used in intraspecific fights between adult males. Strengthening of the rostrum through the dorsal closure of its transverse section would have reduced the risk of fractures when facing impacts.
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Vol. 91 • No. 1