''Indohyus'' is an extinct genus of

digitigrade In terrestrial vertebrates, digitigrade () locomotion is walking or running on the toes (from the Latin ''digitus'', 'finger', and ''gradior'', 'walk'). A digitigrade animal is one that stands or walks with its toes (metatarsals) touching the groun ...

artiodactyls known from Eocene fossils in Asia. This small chevrotain-like animal found in the Himalayas is one of the earliest known non-

cetacean Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...

ancestors of whales.

Discovery

The fossils were discovered among rocks that had been collected more than 30 years ago in

Kashmir Kashmir () is the northernmost geographical region of the Indian subcontinent. Until the mid-19th century, the term "Kashmir" denoted only the Kashmir Valley between the Great Himalayas and the Pir Panjal Range. Today, the term encompas ...

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 Hans 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''. Indohyus and raccoon

Paleobiology

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. 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 Chevrotains, or mouse-deer, are small even-toed ungulates that make up the family (biology), family Tragulidae, the only extant members of the infraorder Tragulina. The 10 Extant taxon, extant species are placed in three genera, but sever ...

water chevrotain The water chevrotain (''Hyemoschus aquaticus''), also known as the fanged deer, is a small ruminant found in tropical Africa. This is the only species in the genus ''Hyemoschus''. It is the largest of the 10 species of chevrotains, basal even-t ...

which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes. From isotopes and the structure of the bones in the fossils ''Indohyus'' had heavy bones. Heavy bones help reduce the buoyancy of living aquatic mammals so that they do not float up to the surface of the water.

Classification

Raoellids may be the " missing link" sister group to whales (

Cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...

). All other Artiodactyla are "cousins" of these two groups. ¹⁸O values and

osteosclerotic Osteosclerosis is a disorder that is characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and ...

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.

References

{{Taxonbar, from=Q133094 Eocene even-toed ungulates Eocene mammals of Asia Transitional fossils Fossil taxa described in 1971