Whales Linked to Land
Fossils suggest sea mammals' ancestors may have been amphibious
HANS THEWISSEN, a Duke University paleontologist, was cleaning rock off the fossilized bone of a Pakicetus, a 50-million-year-old whale, when a tiny bone fell out of a cavity.
The bone was an anvil, the part of the ear that allows land mammals to hear. The find makes it a virtual certainty that the early whales could hear outside of the water - much the same way seals and sea lions do today. And it adds to the body of evidence that the Pakicetus - originally discovered in 1979 - spent at least a part of its life on land.
"Everyone knows whales had ancestors who lived on land, but we have never found them," says Dr. Thewissen.
Modern whales receive sound through their lower jawbone and rely on fat deposits to transmit the sounds to the lower ear. Although they have the three major bones involved in hearing, the hammer, anvil, and stirrup, the bones are greatly modified.
Thewissen says he believes his discovery goes a long way to proving that the Pakicetus was not just an aquatic animal.
"This might be a swimmer, might be a land animal. These bones are different [from those of] all swimming whales, so this might be the intermediate, this might be the missing link," Thewissen says.
Philip Gingerich, professor of paleontology at the University of Michigan in Ann Arbor, Mich., and the original discoverer of the Pakicetus, says Thewissen's discovery "is new and exciting independent evidence that whales at that stage were not able to hear in the water as they do today." The ear bones, he agrees, show that the early whale had the morphology - the form and structure - of land animals, not water animals.
It is not surprising that researchers had never found the tiny ear bones before. The bone Thewissen discovered is so small and fragile that even he was surprised to find it.
"In most animals, [such bones] are not connected to the skull by a bony connection, so if an animal dies, the soft stuff it is attached to in the skull will just rot away and the bones will run out," Thewissen says. The bone is about the size of a match head.
The Pakicetus bones were discovered in a fossil-strewn geologic formation in Pakistan by Thewissen and Taseer Hussain, a researcher at Howard University in Washington. The two scientists began working at the site in January 1991. After six days of field work, the Gulf war forced them to leave Pakistan. But in that short period of time, Thewissen and Dr. Hussain had found the critical fossil.
Thewissen says the jaw and ear bones, as well as other bones found in the dig, give a much clearer picture of the whale's early ancestor. "It looked like a hyena or a big wolf," he says.
Holding up the jawbone of a modern dog and the early whale, Thewissen says, "Someone who knows jaws would say those are pretty similar animals." The early whale, for example, had most of the type of teeth of a modern mammal: canines, molars, incisors. Thewissen says the fossils indicate that the whales are related to such other mammals as the pig, camel, deer, giraffe, and hippopotamus.
Whale teeth did not change until much later. Today, some whales have pegged teeth, while others, such as baleen whales, have sieves to filter out krill and other food.
The teeth in the fossil Thewissen found indicate that the mammal was a juvenile. Some of the teeth are still pushing through what would have been the gum area. This illustrates another change in modern whales - they generally have only one generation of teeth. Thewissen is also interested that the researchers have found mainly juvenile Pakicetus. "Where were the adults?" he asks.
Thewissen is now studying the chemical composition of the teeth to determine if they were used in sea water or fresh water. In addition, he is taking magnified pictures of the tooth enamel to compare with other land mammals.
Other Pakicetus fossils have been found in river beds. This was true of Thewissen's find. The researchers also found fossils relating to rodents and tapirs. "The assemblage of animals suggests it was a dry area," he says.
A sedimentologist, someone who studies rocks laid down by rivers and lakes, has decided that the river bed was filled with water only intermittently. Judging from the size and direction of ripples, he says the water was about 10 inches deep. This probably indicates that the juvenile whale was too large to swim in the water.
Thewissen returned to the site a year after the discovery and recovered other whale fossils that are geologically younger. He is now in the time-consuming process of removing rock from bone with repeated acid baths. Since he has found more Pakicetus bones, including those related to the animal's limbs, he expects that these new fossils will lead to a clearer idea of what the early whale looked like.