Encyclopaedia of the fossil ancestors of cetaceans

The evolution of cetaceans is one of the most extraordinary transitions documented in the fossil record. In 10 million years (Eocene, between 55 and 45 Ma), the land ancestors of today’s whales, artiodactyls close to the hippopotamuses, moved from a riverside way of life to the marine world. This gallery presents the 10 key fossils that mark out this transformation: from the modest Pakicetus, 50 cm tall, to the giant Livyatan melvillei at 17 metres.

The evolutionary journey of the cetaceans

The history of the cetaceans is one of the most extraordinary evolutionary transitions ever documented. In 10 million years, the blink of an eye in geological terms, the land ancestors of today’s whales moved from a semi-aquatic riverside way of life to a fully marine one. This gallery presents the main fossils that mark out this transition.

The archaeocete: tracing the return of mammals to the sea

Cetaceans descend from a four-legged land mammal, an artiodactyl related to today’s hippopotamuses. The archaeocetes of Pakistan and Egypt (Eocene, 53 to 34 Ma) document, over a few million years, a unique transition: from Pakicetus, still terrestrial, to the amphibious Ambulocetus, then Rodhocetus, Maiacetus and finally Basilosaurus, fully aquatic.

The key sites are Wadi al-Hitan (“valley of the whales” in Egypt, a UNESCO World Heritage site), the Pisco Basin in Peru (where Livyatan melvillei rivalled the megalodon in the Miocene) and Kazakhstan. The palaeontologists Hans Thewissen and Philip Gingerich gave this account its structure, confirmed by the molecular phylogeny that joins cetaceans and hippopotamids in a single clade: the Cetartiodactyla.

Also worth exploring

Our Evolution of cetaceans page offers an interactive timeline, and the molecular phylogeny traces the divergences between living species. To understand present-day diversity, see the IUCN database of 78 species.