ANN ARBOR --- Partial skeletons of ancient whales found in Pakistan last year resolve a longstanding controversy over the origin of whales, confirming that the giant sea creatures evolved from early ancestors of sheep, deer and hippopotami and suggesting that hippos may be the closest living relatives of whales. The new finds, reported in the Sept. 21 issue of the journal Science, are the first and only specimens known that combine sheep-like ankle bones and archaic whale skull bones in the very same skeletons.
The 47-million-year-old fossils, found by University of Michigan paleontology Prof. Philip D. Gingerich, graduate student Iyad Zalmout, and colleagues from the Geological Survey of Pakistan and the University of New Hampshire, also show how early whales swam.
"Whales are warmblooded animals like we are---that has been known for a long time," Gingerich explains. "Yet they're so different from other warmblooded, furry things that it's been a mystery, both how they came to live in the sea and what ancestors they might have come from on land."
Some clues have come from studies using immunological, molecular, and genetic techniques to explore relationships among groups of animals. As long ago as 1950, scientists using immunological methods on material from living animals came to the surprising conclusion that whales are most closely related to artiodactyls, a group of hoofed animals that includes cows, sheep, goats, deer, and hippos. But paleontologists, who piece together evolutionary stories from fossils, found the artiodactyl explanation hard to accept. Based on similarities in teeth, they maintained that whales probably evolved from mesonychid condylarths, meat-eating animals that resembled hyenas with hooves.
Gingerich has been searching since the late 1970s for evidence that would clear up the confusion. Working in Egypt in the early 1990s, he found Basilosaurus, a fossil whale with leg, foot and toe bones. Artiodactyls have ankle bones unlike those of any other living or extinct animals, so comparing fossil whale feet with those of artiodactyls should provide the crucial clues to their relationship. But Basilosaurus, which was some 5 million years younger than the whales Gingerich found last year, had evolved to the point that its feet were useless for walking, so its ankle bones were too rudimentary for meaningful comparisons.
Over the next 10 years, Gingerich searched in Pakistan, where he had found older whale fossils in the late 1970s. To his frustration, the fossils he found were nearly complete but lacked hand and foot bones.
"Over and over again, we'd get the same backbones, but the parts we needed were gone," says Gingerich, who speculates that ancient sharks scavenged those parts before the bones became fossilized. Finally last October, on their first morning at a new field site in Pakistan, Gingerich's team found a whale ankle bone that would answer the artiodactyl question.
The bone was so clearly like that of an artiodactyl that Gingerich---who previously had embraced the view that whales evolved from mesonychid condylarths---struggled for months to make sense of it, finally concluding without question that whales had artiodactyl ankles.
"Now I even admit the possibility that hippos are a side line of artiodactyls that might be closer to the whales than any other living animals," he says.
Gingerich had expected that the feet and hands of the older whales would be more like those of land animals than they turned out to be. While they are more fully developed than those of Basilosaurus, they still are not designed for walking long distances.
"It's clear that these animals could hitch their way out of water and back in like sea lions do today, but they were more aquatic than I realized," says Gingerich. The size and shape of their bones suggest that they had webbed hands and feet and probably also used their tails to propel themselves through the water.
Gingerich hopes to return to Pakistan soon to continue looking for whale fossils, and he expects that other researchers will take a closer look at fossils of hippo-like animals to better understand the hippopotamus family tree.
In addition to clearing up confusion about whale evolution, the recent discoveries should also lend more credibility to molecular, genetic and immunological approaches to understanding evolutionary relationships, says Gingerich. Paleontologists have been skeptical of DNA analysis, for example, because the genetic code is continually overwritten during the course of evolution. That makes it difficult to tell whether DNA evidence reveals true relationships or coincidental resemblances.
"In the last few years, 15 or 20 DNA studies have come out supporting this artiodactyl connection," says Gingerich. "Those weren't taken very seriously, but this finding shows that they need to be. If the studies are done well, the DNA that animals carry in their bodies today gives us a better picture of the past than we might have thought it did. If we can make reliable inferences from animals that are living today, we can learn a lot about the past much faster. These techniques will never replace paleontology, but they will complement it and expand on what we can competently infer."
Materials provided by University Of Michigan. Note: Content may be edited for style and length.
Cite This Page: