Indohyus

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. These fossils were discovered in 1970 in Kashmir by Indian geologist A. Ranga Rao, who found a few teeth and parts of a jawbone. He named the type and only species, Indohyus indirae, in 1971.

In 2007, Hans Thewissen found an auditory bulla, a special ear structure only found in cetaceans, in a broken skull from these rocks. Later studies of oxygen-18 values and osteosclerotic bones showed that Indohyus lived in water, much like a small fox.

About the size of a fox, this omnivorous pig-like creature had traits shared by whales and was adapted to aquatic life. Its bones were similar to those of the hippopotamus, helping it stay underwater. This suggests Indohyus may have stayed underwater to escape danger, like the African mousedeer or water chevrotain.

Discovery

In 1970, a scientist named Ranga Rao discovered fossils of Indohyus in the northern Indian province of Jammu and Kashmir. He shared his findings in 1971, naming the species Indohyus indirae after Indira Gandhi. The fossils were found in rocks made mostly of green shale, mudstone, and limestone.

Later, scientists studied these fossils more closely. They noticed that Indohyus shared special features with whales, like a special bone in the inner ear. This helped scientists understand that Indohyus was closely related to whales. Over the years, some scientists grouped different species together, while others kept them separate based on small differences in their teeth.

Classification

When Indohyus was first named, scientists thought it belonged to a group called Choeropotamidae. But now, it is known to be part of a family called Raoellidae. This family was named after the geologist Ranga Rao and was described by Ashok Sahni in 1981. It included three groups: Khirtharia, Kunmunella, and Raoella. In 1985, scientists found that Raoella was actually the same as Indohyus, so they placed Indohyus in Raoellidae.

Raoellidae was originally thought to be part of a group called Dichobunoidea. But in 2004, it was moved to another group called Suiformes. Then, in 2007, scientists looked again at fossils of Indohyus and found features that suggested a connection to whales. This led to Indohyus and Raoellidae being grouped with early whale-like animals called Cetaceamorphs. Unlike later whale ancestors, raoellids likely ate plants and sometimes other things, which means they were not fully developed whales.

Raoellids are mostly found in northern India and Pakistan, where there were swamps long ago. But in 2011, a fossil tooth from China was found to belong to Khirtharia, showing that some raoellids lived in Eastern Asia too.

Description

Indohyus was about the size of a fox or a large cat. Its bones were thick, which is a feature seen in animals that spend time in water, like hippos and whales. This suggests that Indohyus might have lived partly in water. It also had a special part in its inner ear that is usually found in animals that live in water.

We know a lot about the skull of Indohyus, though it is squished from pressure over time. Its snout was longer than usual for animals like it, and its teeth were bumpy with some ridges. Unlike some related animals, Indohyus had eyes on the sides of its head. Its bones were thick in places, which helps with hearing underwater.

Indohyus had legs that could support its weight on land, unlike modern whales. Its limb bones were thick, another sign it might have been semi-aquatic. The feet and hands had certain features that were similar to early whales but also showed it could move on land.

Paleobiology

The oxygen-18 isotopes in Indohyus's teeth suggest that Indohyus likely spent a lot of its time in water, possibly for safety or food. Even though it was in the water often, it was not a strong swimmer because its limbs were thin and its bones were heavy. Instead, Indohyus used its heavy bones to stay sunk in water, where it could walk on the bottom. It could walk and run well on land, though. Indohyus might have eaten plants, but we are not sure if these were in water or on land. Another possibility is that it ate small water animals, using its teeth to grab them. Its inner ear was similar to some whales, which might mean it could hear underwater, but this is not certain because its ear was mostly like that of land mammals. It might have had slightly different sizes of teeth between males and females, but some smaller teeth might belong to a related animal called Rajouria.

Diet

Indohyus was probably an animal that ate plants, similar to other early artiodactyls. Its teeth were shaped in a way that suggested it ate plants. It had a long snout, which is common in plant-eating animals. Its jaw was positioned in a way that also suggests it ate plants, unlike meat-eating animals. The wear on its teeth was light, and its teeth were not built for strong biting. Studies of carbon-13 isotopes show that Indohyus did not eat sea animals; its levels were similar to other artiodactyls. This means Indohyus likely ate plants, but we do not know if these were in water or on land. However, its teeth show wear that is different from other artiodactyls, so it might have eaten plants that were tough or hard in a different way.

Another possibility is that Indohyus also ate small water animals that lived in riverbeds. Even though its teeth were very different, the wear on its teeth was similar to some early whales. Indohyus had a weak bite and thin tooth covering, and the wear on its teeth was not heavy. This suggests that Indohyus might have used its teeth to grab small water animals with quick, shallow bites. Its teeth were shaped in a way that would help it hold onto small prey. The fourth premolar tooth was large, similar to some early whales. Overall, we are not sure what Indohyus ate; it might have eaten plants (either in water or on land) or it might have eaten small water animals using its teeth to grab them.

Locomotion

Indohyus, like some early whales but unlike later ones, probably was not a strong swimmer. Indohyus and Pakicetus walked on just their toes, like cats and dogs. Because their hand bones were not fused together, Indohyus could stretch its hands out a little to help on soft ground, like mud, or maybe to move in water by using its hips to paddle. However, hip paddling was unlikely because Indohyus did not have long fingers or webbing between its toes, its limbs were small and thin, and its heavy bones would have made it slow in the water. Instead, it is more likely that Indohyus used its thick bones to stay heavy in the water, like modern hippos. Indohyus also had low levels of oxygen-18, which suggests it spent a lot of time in water. Its heavy bones matched animals that live mostly in water, unlike animals that only go into water to escape danger. Indohyus had bones that were heavier than land animals but not as heavy as some early whales, making it a step between land animals and early whales.

Indohyus could walk and run well on land. It had a stiff back and long, thin legs that were good for moving on land. Indohyus's joints moved in ways that were good for land walking; its feet could not turn sideways like some whales. The long legs would have helped Indohyus run fast and efficiently. Unlike some early whales, Indohyus had thinner back bones, which means it had smaller muscles and its back could not move much. This means its back was built for walking, not for swimming. Indohyus, like most mammals, held its legs straight under its body, which is an adaptation for land movement that later whales lost. However, the heavy bones of Indohyus would have made it a little slower on land.

Paleoecology

Indohyus lived about 48 million years ago during a time when the Indian subcontinent was moving toward Asia. This was a warm period with no ice at the poles, and the climate helped tropical rainforests grow across the region. The area where Indohyus was found had both sea and land, with some parts covered in shallow water.

Fossils of Indohyus were discovered in places called Sindkhatuti in India and Chorlakki in Pakistan. In these areas, Indohyus lived alongside many other ancient animals, including early rodents, primates, and early relatives of whales like Pakicetus. The rocks where these fossils were found tell us about the environment, which included shallow seas, mud, and limestone.