Indohyus (meaning "Indian pig", from Greek ινδός, indos, meaning 'Indian' and ὗς, hus, meaning 'pig' or 'swine') is an extinct genus of raoellid artiodactyl known from Eocene fossils in Asia. The fossils were discovered among rocks that had been collected in 1970 in Kashmir by the Indian geologist A. Ranga Rao, who found a few teeth and parts of a jawbone. He named the type and only species, Indohyus indirae, one year later. In 2007, Hans Thewissen recognised an auditory bulla, a highly distinctive ear structure found only in cetaceans, in a broken skull collected from these rocks and given to him by Ranga Rao's widow. Later analysis of oxygen-18 values and the presence of osteosclerotic bones indicate that the chevrotain-like Indohyus was habitually aquatic. About the size of a fox, this omnivorous pig-like creature shared some of the traits of whales, and showed signs of adaptations to aquatic life. Their bones were similar to the bones of modern creatures such as the hippopotamus, and helped reduce buoyancy so that they could stay underwater. 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. Indohyus was possibly sexually dimorphic, as significant intraspecific variation in the size of its maxillary canine teeth has been interpreted as evidence of such.
Discovery In 1970, Ranga Rao, the Indian Directorate of Geology in the Oil & Natural Gas Corporation, discovered the fossils of Indohyus in the northern Indian province of Jammu and Kashmir. Ranga Rao subsequently published his findings in 1971, naming Indohyus indirae in the Journal of the Geological Society of India. The type species was named after Indira Gandhi. Ranga Rao also named another species: Indohyus kalakotensis.
Rao found the fossils in the Upper Subathu Formation, which is composed primarily of green shale, mudstone and limestone. The formation has many different types of rocks, which indicates that the environment was transitioning from being marine to terrestrial. The Upper Subathu formation has been dated to be early-to-middle Lutetian in age (48.7–45.4 ma). The fossils were initially believed to be from the fully terrestrial Murree formation, which is above the Subathu formation and has more recent fossils. The holotype specimen, ONG/K/I, is a damaged left mandible with the third and fourth premolars and all of the molars. The alveolus of the second premolar is preserved, although the mandible forward of the alveolus is missing. The paratype specimen, ONG/K/2, consists of a left mandible with the first to third premolars. All other specimens initially found by Ranga Rao were small mandibular fragments with teeth.
Relationship to Cetacea In 2007, when a student of Hans Thewissen accidentally broke a skull of an Indohyus, Thewissen noticed an inner ear bone with a thickened involcrum, a characteristic of cetaceans. He then investigated further, finding that Indohyus and cetaceans share many distinctive features not found in any other group of mammals: a thickened involcrum, osteosclerotic (thicker) bones, arrangement of incisors, and high crowns in posterior premolars. Following this, he published a study showing that Indohyus and Raoellidae (the family it belongs to) are the closest relatives of Cetacea. Thewissen chose to place Indohyus out of Cetacea because Indohyus was herbivorous while cetaceans are carnivorous. This specimen had been recovered from an Eocene-aged mudstone by Ranga Rao and given to Thewissen by Rao's widow.
Classification
Internal classification Indohyus was described in 1971 with two species: I. indirae and I. kalakotensis. In 1985, Kumar and Sahni moved I. kalakotensis to the genus Kunmunella, creating the new combination Kunmunella kalakotensis. Kumar and Sahni additionally synonymized Raoella with Indohyus indirae because the upper teeth of Raoella fit together with the lower teeth of Indohyus. In 1987, two years later, Thewissen, Gingerich and Russell proposed that Kunmunella was also synonymous with Indohyus, including K. kalakotensis. They believed Kunmunella was the same as Indohyus because many of the differences used to distinguish the genera were either caused by deformation while in the ground, or, in their opinion, too minor to make a distinction between the two genera. Thewissen, Gingerich and Russell also coined the new species Indohyus major, based on fossils recovered from Pakistan. In 2001, Thewissen, Williams and Hussain once again supported a distinct Kunmunella, due to differences in the shape of the second and third molars. In 2020, Kunmunella, in addition to Metkatius and Haqueina were synonymized with Indohyus by Thewissen, Nanda and Bajpai because of a lack of distinguishing features between the different raoellid genera. This was contested only a year later by Rana and colleagues; who separated all raoellid genera aside from Raoella from Indohyus after running a phylogenetic analysis in their description of the raoellid Rajouria. This result was supported by an earlier study, in which Indohyus major was more closely related to Khirtharia based on fossil material found in China. After many new remains of Metkatius were found in 2024, Metkatius was definitively separated from Indohyus due to newly documented differences in their teeth.
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