Origin of Whales in Epicontinental Remnant Seas: New Evidence from the Early Eocene of Pakistan

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Abstract
Pakicetus inachusfrom the early Eocene of Pakistan is the oldest and most primitive cetacean known. The dentition ofPakicetusresembles that of carnivorous mesonychid land mammals as well as middle Eocene cetaceans. The otic region of the cranium lacks characteristic specializations of whales necessary for efficient directional hearing under water.Pakicetusoccurs with a land-mammal fauna in fluvial sediments bordering epicontinental Eocene remnants of the eastern Tethys seaway. Discovery ofPakicetusstrengthens earlier inferences that whales originated from terrestrial carnivorous mammals and suggests that whales made a gradual transition from land to sea in the early Eocene, spending progressively more time feeding on planktivorous fishes in shallow, highly productive seas and embayments associated with tectonic closure of eastern Tethys.