Indohyus is a small deer-like creature, which lived about 48 million years ago in Kashmir. It belongs to the artiodactyls family Raoellidae, and is believed to be the closest sister group of Cetacea. About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales, most notably the involucrum, a bone growth pattern which is the diagnostic characteristic of any cetacean, and is not found in any other species. It also showed signs of adaptations to aquatic life, including a thick and heavy outer coating and dense limb bones which is similar to modern creatures such as 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.
Read more about this topic: Evolution Of Cetaceans