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.
External classification When Indohyus was named, it was tentatively placed within Choeropotamidae, although it is now considered a member of Raoellidae. Raoellidae was named after Ranga Rao, and published by Ashok Sahni in 1981. At the time, the family included the genera Khirtharia, Kunmunella and Raoella. In 1985, Kumar and Sahni found Raoella to be synonymous with Indohyus, placing the latter within Raoellidae. Raoellidae was first defined as a group of long-snouted artiodactyls with bunolophodont teeth (bumpy with some ridges) and upper molars with a small paraconule and posterior lobe. Though raoellids were already thought to be artiodactyls, this was definitively proven in 1990 when a raoellid astragalus, an ankle bone, was found with a "double-pulley" mechanism, which is found only in artiodactyls.
Historically, Raoellidae was placed in the superfamily Dichobunoidea, a paraphyletic (including a common ancestor but not all descendants) group of basal artiodactyls. In 2004, Raoellidae was moved to Suiformes, and thought to be a group of stem-pigs. Then, in 2007, remains of Indohyus were reexamined. The fossils had features characteristic of whales, leading Indohyus, and thus Raoellidae, to be reclassified as early Cetaceamorphs. Their likely herbivorous to omnivorous diet differentiated them form the more derived pakicetids, and prevented a classification as 'true' cetaceans. Raoellids are known primarily from northern India and Pakistan, which was a freshwater swamp at the time. However, a fossil tooth from China was reexamined in 2011 and found to belong to Khirtharia, a raoellid. Therefore, while Raoellidae is primarily from the Indian subcontinent, some members of the group migrated to Eastern Asia.
Below is a cladogram of the internal relationships of Raoellidae based on the phylogenetic analysis conducted by Rana and colleagues in 2021, which returned similar results to a previous phylogenetic analysis conducted by Orliac and Ducrocq in 2012. Also depicted is a cladogram that shows the relationship of Indohyus to extinct and extant cetacean groups.
Description While no comprehensive size estimate has been completed, Indohyus is generally agreed to be around the size of a fox or a large cat. Indohyus indirae is an average-sized raoellid, although the species Khirtharia major, which may be a member of Indohyus, is twice the size of I. indirae. Indohyus, and likely all other raoellids, have thickened bones. Denser bones are characteristic of aquatic and semiaquatic animals, such as hippopotamuses and whales, because it counteracts the buoyancy of their lungs and thus enables the organism to maintain a neutral buoyancy more efficient for underwater locomotion. Indohyus also has a thickened involcrum, a part of the inner ear, which is a diagnostic feature of cetaceans and also an aquatic adaptation. In terms of dentition, Indohyus is closest to Kunmunella, which has repeatedly been synonymized with Indohyus. They are both distinguished by bunolophodont dentition (bumpy with some ridges), as opposed to the other raoellids, which have a hyperbunodont dentition (very bumpy).
Skull While a nearly complete skull of Indohyus has been found, it is highly deformed from geological pressure; the skull is squished dorsoventrally (top to bottom). Therefore, the exact height, and thus volume, of the brain case is unknown. The vermis of the cerebellum is the highest point of the brain. The olfactory bulb and peduncles compose approximately a third of the 7.4-centimeter-long brain. This trait is similar to early cetaceans. Generally, the brain of Indohyus was relatively basal, and was much more similar to dichobunids (a group of related, basal artiodactyls) than cetaceans. The snout of Indohyus is unusually long for an artiodactyl, which is similar to Khirtharia and other raoellids. The total size of the skull is about twice that of the skull of K. inflata. Although the premaxilla (the bone of the tip of the skull) is not fully preserved, combining measurements from different specimens shows that the length from the tip of the snout to the back of the canine teeth is about a quarter of the total skull length, which is significantly larger than in dichobunids. The nasal bone is likewise long and slender, with the forward most portion missing. This proportionately long snout more closely resembles early cetaceans. The long premaxilla may be related to feeding, as in modern artiodactyls.
While Indohyus was an herbivore, its premolars and tooth wear patterns resemble those of cetaceans, especially Pakicetus. Unlike the dorsally facing eyes of pakicetids, Indohyus has laterally (sideways) facing eyes, similar to modern terrestrial artiodactyls. Indohyus has a thickened medial bullar wall, also called an involcrum, which is a diagnostic feature of cetaceans. The purpose of the thickened bullar wall is better bone-conducted hearing, which is usually found in aquatic species. The maxilla makes up most of the palate, and is narrowest over the second premolar. On the palate, the junction between the maxilla and palatine bones extends forwards from the palatal edge. The junctures on either side of the palate converge at the third premolar. The lacrimal bone is square, although the bone is generally poorly preserved. The frontal bone, which is concave, slightly overhangs the lacrimal. The jugal bone, also called the zygomatic arch, forms most of the bottom side of the orbital ring. It narrows going backwards, and terminates just forward of the mandibular fossa (the joint between the jaw and skull). The maximum width between the two jugal arches is 7.2 centimeters. Some of the bones of the skull were osteosclerotic (thickened), similar to its limbs, which is likely an adaptation for being semi-aquatic.
Ribs and vertabrae