A water-loving mammal that lived 50 to 60 million years ago was probably the "missing link" between whales and hippos, according to a new analysis.
Hippos are the last of a mammal group that thrived for 40 million years
Biologists have argued over the relationship between hippos and whales for a period of almost 200 years.
The findings come from an analysis of features in different animal groups carried out by a US-French team.
Their report is published in the science journal Proceedings of the National Academy of Sciences.
Jean-Renaud Boisserie, Michel Brunet and Fabrice Lehoreau found that the semi-aquatic ancestor of whales and hippos split into two groups: cetaceans and the anthracotheres.
Cetaceans eventually spurned land, lost their legs and became fully aquatic.
The pig-like anthracotheres, flourished over 40 million years and died out less than 2.5 million years ago. They left only one descendent: the hippopotamus.
The study places whales firmly within the cloven-hoofed group of mammals known as Artiodactyla, which includes cows, pigs, sheep, antelopes, camels and giraffes.
Scientists had assumed hippos were cousins of pigs because they shared distinctive ridges on their molars.
The similarities may not be immediately obvious
But then genetic analyses indicated that hippos had more in common with cetaceans, the group to which whales and dolphins belong.
"If you look at the general shape of the [hippo] it could be related to horses, as the ancient Greeks thought, or pigs, as modern scientists thought, while molecular phylogeny shows a close relationship with whales," Dr Boisserie explained.
"But cetaceans - whales, porpoises and dolphins - don't look anything like hippos. There is a 40-million-year gap between fossils of early cetaceans and early hippos."
"Cetaceans are artiodactyls, but very derived artiodactyls."