Fossil range: Middle Eocene
Indohyus ("India's pig") is a genus of extinct artiodactyl known from Eocene fossils in Asia, purported to be approximately 48 million years old. A December 2007 article in Nature by Thewissen et al. using an exceptionally complete skeleton of Indohyus from Kashmir indicates that raoellids may be the "missing link" sister group to whales (Cetacea). All other Artiodactyla are "cousins" of these two groups. δ18O values and osteosclerotic bones indicate that the raccoon-like or chevrotain-like Indohyus was habitually aquatic, but δ13C 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.
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, 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 unusual and only resembles the skulls of whales and the earlier land creature Pakicetus.
However, not all paleontologists are firmly persuaded that Indohyus is the transitional fossil that cetacean-origin experts were looking for. ScienceNOW, a daily news feature of the journal Science, notes that a team set to publish in the journal Cladistics postulates an extinct group of carnivorous mammals called "mesonychids" as more closely related to cetaceans. Additionally, the ScienceNOW article 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."
About the size of a raccoon or domestic cat, this herbivorous deer-like creature shared some of the traits of whales, and showed signs of adaptations to aquatic life, including a thick and heavy outer coating to bones which is similar to the bones of modern creatures such the hippopotamus, and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to 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.
- ^ Thewissen, J. G. M. (2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature 450 (7173): 1190–1194. doi:10.1038/nature06343.
- ^ Minkel, JR (2007-12-19). "Closest Whale Cousin—A Fox-Size Deer? Researchers split on closest evolutionary kin to whales and dolphins". Scientific American. http://www.sciam.com/article.cfm?id=closest-whale-cousin.
- ^ a b Ian Sample (December 19, 2007). "Whales may be descended from a small deer-like animal". Guardian Unlimited. http://www.guardian.co.uk/science/2007/dec/19/whale.deer?gusrc=rss&feed=networkfront. Retrieved on 2007-12-21.
- ^ Stokstad, Erik (2007-12-19). "Long-Lost Relative of Whales Found?". 'ScienceNOW Daily News. http://sciencenow.sciencemag.org/cgi/content/full/2007/1219/2. Retrieved on 2007-12-27.
- ^ Carl Zimmer (December 19, 2007). "The Loom : Whales: From So Humble A Beginning...". ScienceBlogs. http://scienceblogs.com/loom/2007/12/19/whales_from_so_humble_a_beginn.php. Retrieved on 2007-12-21.
- ^ PZ Myers (December 19, 2007). "Pharyngula: Indohyus". Pharyngula. ScienceBlogs. http://scienceblogs.com/pharyngula/2007/12/indohyus.php. Retrieved on 2007-12-21.