One of the most significant advances in our understanding of early whale evolution was the result of using a technique from stable isotope geochemistry. Fossils do not commonly give us clues about the behavior of an animal, but using this technique it was possible to determine the drinking behavior of fossil whales.
The isotope technique made it possible to detect minute differences in the proportions of different isotopes of oxygen in the bones of fossil whales. These minute differences are the result of the differences in the proportions of these isotopes between seawater and freshwater, and they thus reflect the water ingesting behavior of the animals.
Most modern mammals, including humans, cannot live without a freshwater source. Modern whales, on the other hand, never have access to freshwater and appear to drink salt water. In collaboration with Dr. Lois Roe and several other specialists, Dr. Thewissen was able to determine that pakicetids probably drank freshwater, but that protocetid whales had no access to freshwater. The difference between pakicetids and protocetids is only 7 million years, apparently whales evolved the ability to do without freshwater in very little time.
Data for Ambulocetus suggests that it lived in waters of a large range of salinities (and thus oxygen isotope compositions). Remingtonocetids are particularly interesting because they are the only family that is found in restricted marine environments with a lot of freshwater input (think of estuaries), but also in nearshore marine environments that lack freshwater input. They might be the first family to be independent of freshwater.
The figure below (modified from Roe et al. 1998, in The Emergence of Whales, J.G.M. Thewissen, editor) shows that primitive pakicetids have oxygen isotope values similar to those of modern freshwater dophins, whereas protocetids have values similar to marine whales.
All pictures on Dr. Thewissen's pages are public access, although the source must be identified in publication.
Ambulocetidae
| Basilosaurids and
Dorudontids | Bibliography | Hearing
| India |
Locomotion | Mysticetes
| Odontocetes | Osmoregulation
| Pakicetidae |
Pakistan | Protocetidae
| Remingtonocetidae | Whale
| Whale Origins!