Evolution of Whales from Land to Sea

Pakicetus sm

Museum of Paleontology curator Philip Gingerich has led paleontological expeditions to Pakistan and Egypt since 1975.  One of these, in 1978, yielded some surprising finds:  teeth and the partial cranium of a 50 million year old cetacean so primitive it wasn’t clear at first that it was a whale.  Gingerich and Donald Russell studied the new whale and in 1981 named it Pakicetus inachus.  The new archaeocete was the oldest fossil whale known at the time, with primitive ears making it incapable of the directional hearing in water that is so important for feeding and communication in all later whales.  The reconstruction of Pakicetus illustrated here is by the University of Michigan’s John Klausmeyer.  More of Klausmeyer’s reconstructions appear in a new review published by the American Philosophical Society (click here for pdf).

Creationists mocked Pakicetus at first as an implausible link between land mammals and whales, making it clear that more fossils would have to be found that are intermediate in time, form, and physiology.  Gingerich has been in the field searching for fossil whales in Pakistan or Egypt virtually every autumn for the past 30 years, and the results have been rewarding.  Early-to-middle Eocene Pakicetus and Pakicetidae gave rise to more advanced semiaquatic whales grouped as Protocetidae.  Protocetids, including early forms like Maiacetus, swam by pushing water with their feet, still had ankles like those of sheep and deer today, and still came out on land to give birth.  Advanced protocetids gave rise to middle-to-late Eocene Basilosauridae that were fully aquatic and the first to swim by pushing water with a powerful fluked tail.  Basilosaurids are the last common ancestor of the baleen and toothed whales living today.

The best of the new fossils, Rodhocetus, Maiacetus, Dorudon, and Basilosaurus, are represented by complete skeletons that together link land mammals to modern whales.  Filling a gap means there are now two, but each new gap is smaller than the first.  There will always be more to learn, but the evolution of whales from land to sea is an instance where the fossil record is proving gratifyingly complete and informative about evolution in the geological past.