Like many people, I've always been interested in whales. My interest in whale evolution began when I was teaching about mammal evolution at Massey, and it really got off the ground when I read Carl Zimmer's excellent book, At the water's edge. Now there's a fossil that tells us even more about whales and their closest relatives (Thewissen et al. 2007).
Scientists have known for some time that whales are related to the group of mammals called artiodactyls (even-toed ungulates like cows and hippos). The fossil that Thewissen and his co-workers describe is important because it's an artiodactyl with morphological similarities to whales. New fossil material for Indohyus, a raoellid artiodactyl, shows that features of the ears, premolar teeth, and limb bones in this species were more similar to whales than to other artiodactyls. The researchers have also been able to make conclusions about Indohyus's habitat based on measurements of stable-oxygen-isotopes in its teeth.
Whales have unusual ears: the bony capsule (the bulla) that surrounds the internal ear has a particular shape. Until now this feature was found exclusively in cetaceans, but Thewissen et al. have also found it in Indohyus. (Whales' ears are also somewhat isolated from the rest of the cranium by a bony stalk – a feature related to their use of sonar.) Similarly, Indohyus premolars are similar in shape to those of early cetaceans.
The researchers have also concluded that Indohyus was aquatic, on the basis of bone density. The fossil's limb bones are very dense, a feature seen in other aquatic groups such as manatees, seat otters, hippos, and seals – and whales. In hippos this increased bone density (compared to other mammal groups) allows them to walk along the bottom of lakes and waterways. Further evidence of an aquatic lifestyle comes from measurements of oxygen isotopes (O-18) in tooth enamel. O-18 tends to be lower in aquatic organisms than for terrestrial animals, and the Indohyus teeth have typical 'aquatic' values.
Similarly, C-13 values were used to find out about the animal's diet. Organisms that consume freshwater phytoplankton (eg early cetaceans such as Pakicetus) tend to have lower C-13 values than those eating either large aquatic plants or terrestrial plants. The higher C-13 values found in Indohyus don't tell us whether it was feeding in water or on land, but they do show it had a different diet to the early whales.
Thewissen and his co-authors hypothesise that whales evolved from a raoellid artiodactyl ancestor, and that Indohyus is a sister species to those early cetaceans – i.e. early cetaceans & Indohyus were more closely related to each other than to any other taxon. They go on to suggest that the common ancestor to these groups was already aquatic, and that the evolutionary innovation in the whale lineage was a significant change in diet.
However, not everyone may agree with the taxonomic place given to Indohyus. Some scientists feel that molecular data, combined with other fossil material, place whales closer to hippos. Time will tell.
J.G.M. Thewissen, L.N. Cooper, M.T. Clementz, S. Bajpai & B.N. Tiwari (2007) Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450: 1190-1194