Johannes G. Thewissen

This project investigates the transition of cetaceans (whales, dolphins, porpoises) from terrestrial quadrupeds to obligate marine swimmers. This transition is one of the few paleontologically documented examples of macroevolutionary change in mammals. The project will interpret patterns of macroevolutionary change in two systems that underwent pervasive modifications, locomotion and hearing, and evaluate its paleoecological context. Hypotheses.-- Seven hypotheses will be tested. Two of these make predictions about changes in the modes of locomotion in early cetaceans. Three hypotheses make predictions about evolution and function of the cetacean middle and inner ear. Another two hypotheses address the physical environment under which this transition took place, making prediction about paleoecology. New fossil material will be collected from freshwater and shallow marine sediments of the Kuldana and Kohat Formations of Northern Pakistan, the only area where the earliest and most plesiomorphic cetaceans are found. This material will allow testing of the hypotheses. Kuldana/Kohat cetaceans lived across a variety of depositional environments in what is now a structurally complex area, requiring detailed geological study. Sedimentological study will elucidate the physical setting of the transition. Related, ongoing work includes analyses of modern cetacean ear development and function, and an analysis of physiological constraints, based on stable oxygen isotopes. Significance.-- Cetacean origins is one of the best documented examples of land-water transitions among vertebrates, and showcases several form-function complexes that underwent pervasive change. Cetacean swimming evolved from quadrupedal paddling to dorsoventral oscillation of the broadened tail, and the Kuldana cetaceans display intermediate modes. The ears of cetacean ancestors were tuned to mid-frequency airborne sounds, whereas modern cetaceans hear underwater high- (odontocete) or low- (mys ticete) sounds. The position of the Kuldana cetaceansis unclear, but their auditory morphology is different from their terrestrial sister group and modern cetaceans. Fossil morphologies interpreted in an established phylogenetic context and against a background provided by modern aquatic mammals elucidatesthese evolutionary patterns. Studying the sedimentology of the rocks that document these morphological patterns clarifies the ecological setting of the land-to-water transition. This study is of broader significance for the higher phylogeny of mammals. It will provide morphological data to test phylogenies of the affinities of sperm whales (odotocete paraphyly) and the relationship of cetaceans to artiodactys (artiodactyl paraphyly). These phylogenetic hypotheses were proposed on the basis of molecular data, and are greatly disputed by morphologists. Studies of faunal evolution have suggested that Indo-Pakistan contacted Asia in the Cretaceous. Geological data dispute this claim. This study will provide a critical test for the Eocene paleontological data.