New Fossils Resolve Whale’s Origin

September 19, 2001 at 5:13 pm

Recent fossil finds from Pakistan overturn the picture of whale evolution long championed by paleontologists, bringing them closer to agreement with an alternative view proposed by molecular biologists. The discoveries establish a close evolutionary link between cetaceans, which include whales, dolphins, and porpoises, and a group of mammals known as artiodactyls. These hoofed animals with an even number of toes include cows, sheep, goats, pigs, deer, and hippopotamuses.

Approximately 50 million years ago, this wolf-size pakicetid–an ancestor of the whale–lived on land but may have waded into streams to feed on fish. C. Buell/NEOUCOM

An artist’s rendering of a largely aquatic early whale of the genus Rodhocetus. This mammal lived near the shores of the Tethys Sea, between Asia and the South Asian subcontinent, about 47 million years ago. J. Klausmeyer/Science

Four-legged, terrestrial ancestors of whales and other cetaceans waded into the sea about 55 million years ago and gradually developed skeletal adaptations for aquatic life, paleontologists hold. But precisely what variety of land mammal first got its feet wet has been a source of spirited debate (SN: 11/6/99, p. 296).

“We’ve thought since the ‘ 60s that whales evolved from hoofed, carnivorous mammals” known as mesonychians, says Philip D. Gingerich of the University of Michigan in Ann Arbor. Like most paleontologists, he says, he “considered it pretty well established” on the basis of dental similarities that cetaceans were surviving descendants or close relatives of this otherwise extinct group.

However, Gingerich’s latest report, in the Sept. 21 Science, removes whales from the mesonychians and places them in the same evolutionary lineage as artiodactyls–just as researchers analyzing genetic and immunological data have maintained.

Gingerich and his colleagues reversed their position after analyzing the skeletons of two early aquatic whales, about the size of sea lions, that they unearthed last year in central Pakistan. The fossils, which they gave species names of Artiocetus clavis and Rodhocetus balochistanensis, are the first of cetaceans ever discovered with intact ankle bones.

To the researchers’ surprise, the ankle had a unique form that’s found only in artiodactyls. Since living cetaceans have no vestige of these bones, the discovery of early whales with distinctly artiodactyl ankles provides a “Rosetta stone” linking modern marine mammals to living artiodactyls, says Gingerich.

“The new [fossil] data are much more in agreement with the molecular data than what we thought before,” says J.G.M. Thewissen, a paleontologist at the Northeastern Ohio Universities College of Medicine in Rootstown. Working independently in fossil beds in northern Pakistan, he and his colleagues have concluded that pakicetids, a group ancestral to modern cetaceans and predating the transition to marine life, also had the artiodactyl ankle form. They report their work in the Sept. 20 Nature.

The new finds are “very exciting for those of us working on molecular data,” says John Gatesy, an evolutionary biologist at the University of California, Riverside, whose work in nuclear DNA sequencing supports a close whale-artiodactyl relationship.

Gatesy’s data and other lines of molecular evidence also suggest that whales have a closer evolutionary relationship with hippos than with other artiodactyls. Paleontologists, however, aren’t ready to group whales with any specific artiodactyl.

“None of the fossil evidence supports hippos and whales as being sister taxa,” says Kenneth D. Rose, a paleontologist at the Johns Hopkins Medical Institutions in Baltimore.

Nevertheless, Gingerich predicts that renewed focus on the fossil record of early artiodactyls will help researchers determine just where the whales fit in. And that, he acknowledges, could be alongside the hippos.