INDOHYUS SKULL shares similarities with whales and dolphins, particularly in the structure of its middle ear. Researchers say this fox-size, 48-million-year-old wading mammal may be the closest known relative to seafaring mammals. Image: JEANETTE KILLIUS / NEOUCOM
A group of researchers says that the closest known evolutionary cousin of whales, dolphins and porpoises is not the hippopotamus, as conventional wisdom has it, but an extinct deer-like animal roughly the size of a fox or raccoon. In a new study, the team finds that a fossilized specimen of the extinct, 48-million-year-old mammal Indohyus bears several telling similarities to whales, including dense limb bones for ballast and a middle ear structure found only in the cetaceans, or sea-dwelling mammals, which is thought to help them hear underwater.
"What we think happened is that the ancestors of both Indohyus and whales were animals that looked like a tiny deer," says Hans Thewissen, professor of anatomy at Northeastern Ohio Universities Colleges of Medicine and Pharmacy, who led the study, published in Nature. The modern creature that most resembles Indohyus, however, is the African mousedeer (or chevrotain), which lives on the forest floor but scurries into the water to take cover from predators [see video of eagle pursuing chevrotain]. Similarly, Thewissen says, the common ancestor of whales and Indohyus may have been a herbivore (plant-eater) that took to water to hide out, but eventually switched to a swimming, meat-eating lifestyle, which it passed down to modern cetaceans.
Other experts, however, caution that although the scenario is possible, the ancestry analysis is based on incomplete data. Researchers "really thought the book was closed on this," says Annalisa Berta, an evolutionary biologist at San Diego State University. "To suggest that this fossil somehow is closer than hippos, that's a big deal—I'm just not convinced."
Whatever its relationship with whales, Indohyus was probably not a direct predecessor of them, Thewissen says, because the specimen, unearthed 30 years ago in Kashmir, dates to roughly two million years after the earliest known cetacean fossils.
Over the past 15 years, researchers have uncovered a series of fossils intermediate between whales and land animals, but were still missing a link to landlubbing beasts, which Thewissen says Indohyus now provides.
The disagreement reflects the fact that researchers weren't around to watch successive populations of animals budding off from those that came before, resulting in a bushy pattern of new species over the millennia. Instead they have to infer the bush's branch points. They do this by categorizing extinct and living species together into the evolutionary bush that has the fewest cases of unrelated species evolving the same trait, which is in general less likely than one species passing down a unique trait to another species.
Indohyus, for example, has a half-walnut-shaped tympanic (a bony casing around the middle ear bones) with thicker walls on the sides than in front. The same thickening is found in all fossil and modern cetaceans but not in other mammals, the group notes. The fossil also has dense limb bones that would have weighed it down in water. But in contrast to the pointy molars of dolphins or killer whales, however, its equivalent nchompers are squarish like those of hippos, possibly for grinding plant matter, the group suggests.
The researchers used a computer program to test the possible evolutionary bushes that could have yielded cetaceans along with artiodactyls—the mammalian order made up of two-toed, hoofed animals—which include Indohyus and hippos.
The new analysis does not yet unseat the hippo as cetaceans' kissing cousin, because it only takes into account anatomical features, not molecular ones, says Maureen O'Leary, a professor in the department of anatomical sciences at Stony Brook University on Long Island, N.Y. She says that her own categorization of artiodactyls supports the hippo as the closest relative to cetaceans, but notes that it did not include the features uncovered by the Ohio team.