9419898 Luo The phylogenetic relationships of cetaceans (whales, porpoises and dolphins) provide the basis for understanding their great diversity and unique aquatic adaptations. Recent molecular and paleontological studies of cetaceans have posed very different hypotheses for cetacean relationships that would lead to different evolutionary interpretations of their aquatic specializations. To test these competing hypotheses, the proposed research will examine the morphological characteristics of the cranium and the ear. Although the basicranium is a very useful source of systematic characters in studies of other mammals, it has been relatively under-explored in cetaceans. Therefore, a thorough phylogenetic analysis of the basicranium has a good potential to generate new data for inferring relationships of cetaceans. Two groups of modern cetaceans, the odontocetes (toothed whales) and the mysticetes (baleen whales), have very different underwater hearing mechanisms. Modern toothed whales have some inner ear structures specialized for hearing high frequency sound that is crucial for their echolocation (sonar). By contrast, extant baleen whales rely entirely on the low frequency sounds. The anatomical data that are related to these functional differences between modern odontocetes and mysticetes are very limited. Little is known of the ear structures of fossil cetaceans. As a result, several issues on the structural evolution of cetacean ears have not been resolved. This study will examine the inner ears of fossil and extant odontocetes, mysticetes and the extinct archaeocetes, using accurate techniques of serial grinding, the industrial CT scanning, and computer graphic reconstruction. A detailed study using these techniques will contribute new data to cetacean evolutionary biology, and test hypotheses on the structural evolution of the cetacean ear.
