Raoellidae

Raoellidae (/reɪoʊˈɛlɪdeɪ/) is a family of extinct semiaquatic artiodactyls from the middle Eocene (early Lutetian) closely related to cetaceans. They are known primarily from northern India and Pakistan, especially in the Subathu Group of India, although potential remains of the raoellid Khirtharia have been found in northern China. They are relatively small animals; on average, raoellids were the size of a red fox. However, Khirtharia major, at about twice the size of an average-sized raoellid, would have been approximately the size of a coyote. Meanwhile, the smallest raoellid, Metkatius, was roughly the size of a house cat.

Raoellidae is of particular importance to the understanding of cetacean evolution due to representing a transitional form between fully-terrestrial artiodactyls and the semi-aquatic pakicetids. The close relation to Cetacea was found when one of the technicians working for Hans Thewissen accidentally broke a fossilized skull of Indohyus. Thewissen noticed the prominent auditory bulla which were similar to cetaceans. Then, he took the cross section of one of the bones of Indohyus and noticed it was much thicker than terrestrial mammals, hinting at an aquatic lifestyle and confirming their close relation to whales.