"It was only the size of a raccoon, but if you saw it in a zoo, you would think it was a miniaturized deer," he added.
Despite this, the structure of its bones suggests that the animal spent much of its time in the water.
The limb bones had a thick, dense outer layer—a feature often seen in modern aquatic mammals, such as hippos, that require extra ballast, the study team noted.
"It allows them to walk on the bottom of the river without their buoyancy pushing them up and making them float," Thewissen said. "So [Indohyus] was also a wader in water."
In addition, the ratios of chemical clues called isotopes in the creature's teeth are characteristic of animals that ingest water while feeding, the team said.
The fossil teeth's chemistry also indicates the creature was an herbivore, even though it's widely thought that the earliest whales took to the water in pursuit of fish.
"Clearly, this is not the case," Thewissen commented.
"Indohyus is a plant-eater and already aquatic. Apparently the dietary shift to hunting animals [as modern whales do] came later than the habitat shift to water."
The study proposes that Indohyus may have been drawn to water to escape land predators.
The team says similar behavior is seen today in the African mouse deer, a tropical forest dweller that feeds on land but flees into rivers when in danger.
But this latest attempt to solve the long-running conundrum of how whales first evolved doesn't satisfy University of Michigan paleontologist Philip D. Gingerich, a past grantee of the National Geographic Society's Committee for Research and Exploration. (National Geographic News is part of the National Geographic Society.)
More convincing information is needed than is provided in the Nature paper, he said.
The crucial piece of evidence that would link Indohyus to whales is a full analysis of the ear bone covering the animal's middle ear, Gingerich commented.
"[Thewissen and colleagues] say one side is much thicker than the other, and that that's a whale characteristic, which it is," he said.
"But it's so surprising to see that in an animal that otherwise looks completely terrestrial."
Since this ear bone is "absolutely the key feature," Gingerich said, "I cannot understand why they wouldn't show us some kind of cross-section, computerized tomography scan, or anything that would convince a person that they hadn't just measured [a fossil ear bone] that was broken."
Various scientists have speculated that Indohyus-type mammals might have been the artiodactyls' closest relatives to whales, Gingerich said.
But other researchers favor another, hippolike candidate: anthracotheres, or "coal beasts," which are fossil animals first discovered in swampy coal-peat deposits in Europe.
Anthracotheres are known to have to been at least partially aquatic and are believed to be the relatives of modern-day hippos, Gingerich said.
DNA studies "tell us that hippopotami have some special relationship with whales, exclusive of all other artiodactyls," he said.
The fossil bones of anthracotheres are also consistent with them being the sister group of whales, Gingerich added.
Kenneth D. Rose, from the Center for Functional Anatomy and Evolution at the Johns Hopkins University School of Medicine in Baltimore, also said he has initial reservations about the new research.
"Studies in recent years have shown that whales and artiodactyls are closely related, and some evidence suggests that whales are nested within artiodactyls," Rose said.
Given this previous work, Indohyus could indeed be the ancestor of whales, he noted.
But "I do not believe the evidence presented here demonstrates that with confidence," he said in an email.
"It is an interesting hypothesis to be tested as more complete [Indohyus] fossils are discovered."
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