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Temporal range: Ypresian, 49 Ma
Indohyus BW.jpg
Indohyus major
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Raoellidae
Genus: Indohyus
Rao 1971

I. indirae
I. major

Indohyus is a genus of extinct digitigrade artiodactyl known from Eocene fossils in Asia. This small chevrotain-like animal found in the Himalayas is a close relative of whales.[1]


A model of Indohyus

The fossils were discovered among rocks that had been collected more than 30 years ago in Kashmir by the Indian geologist A. Ranga Rao who found a few teeth and parts of a jawbone,[2] but when he died many rocks had yet to be broken open. Ranga Rao's widow gave the rocks to Professor Thewissen, who was working on them when his technician accidentally broke one of the skulls they had found and Thewissen recognised the ear structure of the auditory bulla, formed from the ectotympanic bone in a shape which is highly distinctive, found only in the skulls of cetaceans both living and extinct, including Pakicetus.[3]

About the size of a raccoon or domestic cat, this omnivorous pig-like creature shared some of the traits of whales, and showed signs of adaptations to aquatic life, including a thick and heavy outer coating. Their bones were similar to the bones of modern creatures such as the hippopotamus, and helped reduce buoyancy so that they could stay underwater.[4] This suggests a survival strategy similar to that of the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.[3][4][5]


A December 2007 article in Nature by Thewissen et al. used an exceptionally complete skeleton of Indohyus from Kashmir to indicate that raoellids may be the "missing link" sister group to whales (Cetacea).[6][7] All other Artiodactyla are "cousins" of these two groups. 18O values and osteosclerotic bones indicate that the raccoon-like or chevrotain-like Indohyus was habitually aquatic, but 13C values suggest that it rarely fed in the water. The authors suggest this documents an intermediate step in the transition back to water completed by the whales, and suggests a new understanding of the evolution of cetaceans.

However, not all paleontologists are firmly persuaded that Indohyus is the transitional fossil that cetacean-origin experts were looking for. O'Leary & Gatesy 2008[8] postulates an extinct group of carnivorous mammals called "mesonychids" as more closely related to cetaceans. Additionally, ScienceNOW, a daily news feature of the journal Science, notes that "cetaceans are so different from any other creature that researchers haven’t been able to agree which fossil relatives best represent their nearest ancestors."[9]