NEW WHALE FROM THE EOCENE OF PAKISTAN AND THE ORIGIN OF CETACEAN SWIMMING

MODERN whales (order Cetacea) are marine mammals that evolved from a l and-mammal ancestor, probably a cursorial Palaeocene-Eocene mesonychid (1-3).Living whales are streamlined, lack external hind limbs, and all swim by dorsoventral oscillation of a heavily muscled tail(4,5). A st eamlined rigid body minimizes resistance, while thrust is provided by a lunate horizontal fluke attached to the tail at a narrow base or ped icle(6). We describe here a new 46-47-million-year-old archaeocete int ermediate between land mammals and later whales. It has short cervical vertebrae, a reduced femur, and the flexible sacrum, robust tail and high neural spines on lumbars and caudals required for dorsoventral os cillation of a heavily muscled tail. This is the oldest fossil whale d escribed from deep-neritic shelf deposits, and it shows that tail swim ming evolved early in the history of cetaceans.