A group of paleontologists has identified what they believe is the closest relative of whales, dolphins, and porpoises--an extinct, raccoon-sized creature that sloshed along river bottoms and could have eaten like a landlubber. The find promises to give scientists a better idea of where whales and their ilk came from.
Ever since whales first appeared, more than 50 million years ago, their origin has remained murky. Whales and their cetacean cousins, the dolphins and porpoises, are thought to have evolved from some sort of hoofed mammal (ScienceNOW, 30 July 1998). But cetaceans are so different from any other creature that researchers haven't been able to agree which fossil relatives best represent their nearest ancestors.
One candidate is a group of mammals called raoellids, which are known from little more than their teeth (but these place them among the hoofed mammals). Paleontologists led by Hans Thewissen of Northeastern Ohio Universities College of Medicine in Rootstown studied fossils that had been collected in Kashmir more than 20 years ago from 48-million-year-old river deposits and that were recently chiseled out of the rock.
The bones belong to a raoellid known as Indohyus, and several newly discovered features now link Indohyus closely to cetaceans. For example, Thewissen points to a bony feature, called the involucrum, that covers the inner ear. The relative thickness of various parts of the involucrum is characteristic of all modern and fossil cetaceans. "I got this shock--I said, “This must be it’ "--the closest relative to cetaceans, Thewissen recalls. A comparison of the features of Indohyus with those of other fossil mammals reinforced that impression. The result makes sense to Mark Uhen of the Smithsonian Institution's National Museum of Natural History in Washington, D.C. The raoellids, he says, are in the right place--Asia--at the right time, some 50 million years ago.
Indohyus seems to have lived mainly in water, as a wader. The limb bones have thick outer layers, which make them dense, like those of manatees, hippos, and early whales, the team reports 20 November in Nature. The chemistry of the teeth--relatively depleted of stable-oxygen isotopes compared to contemporaneous terrestrial fossils found elsewhere in India--also suggests that Indohyus spent considerable time in water. Like the water chevrotain, an 80-centimeter-long herbivore that lives in Africa, Indohyus may have used the water as a way to escape predators. The carbon isotopes don't reveal Indohyus's diet with certainty, but they show it differed from that of early whales.
"It's significant work," says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. "This much improves the picture of incremental evolution toward whales and their aquatic life." Not everyone is convinced that Indohyus is the closest cetacean relative, however. Another analysis, in press at Cladistics, suggests that an extinct group of carnivorous mammals, called mesonychids, were more closely related to cetaceans.