Fossil record: Terrestrial ancestors of whales


The marine mammals known as cetaceans - whales, dolphins and porpoises - originated about 50 million years ago in south Asia, but their terrestrial ancestor is something of a mystery. It is likely to have been a racoon-sized animal from India known as a raoellid, which probably took to the water in times of danger.

J Thewissen and his colleagues present evidence for a close relationship between whales and raoellids, which lived at about the same time as the earliest whales. The structures of the skull and ear region of raoellids are very similar to those of early whales, and their bone thickness and isotope evidence both indicate that these creatures spent much of their time in water. Raoellids, however, were mainly herbivorous on land, so the spur for the ancestors of whales to take to the water was probably an abundance of aquatic prey.

According to independent molecular evidence, hippos are the closest relatives of today’s whales. However, hippos don’t appear in the fossil record until some 35 million years after whales diverged from their terrestrial ancestors. Thewissen and colleagues’ raoellid Indohyus now provides the missing Eocene piece of the jigsaw.

CONTACT

J Thewissen (Northeastern Ohio Universities College of Medicine, Rootstown, OH, USA)
Tel: +1 330 325 6295; E-mail: thewisse@neoucom.edu