J. John Sepkoski Jr. (right) and Mike Foote examine the skeleton of a capybara, a large rodent indigenous to South America.

Fossil research may end feud

Humans can trace the origins of many of their mammalian relatives back either 65 million years or 130 million years, depending on which group of scientists they choose to trust. Now a research team led by Chicago paleontologist Mike Foote has developed a mathematical model that could resolve this scientific family feud.

The clearest fossil evidence indicates that modern placental mammals––a diverse group that includes humans, bats and moles––first evolved about 65 million years ago, around the time the dinosaurs became extinct at the end of the Cretaceous Period. But according to genetic data, this same group of mammals should have appeared 130 million years ago, early in the Cretaceous Period.

To test molecular biologists’ assumptions that the fossil record is incomplete and leaves a 65-million-year gap, Foote and his colleagues, J. John Sepkoski Jr., Professor in Geophysical Sciences at Chicago, John Hunter of the New York College of Osteopathic Medicine, and Christine Janis of Brown University, used their mathematical model to estimate the quality of the mammalian fossil record. Their findings were reported in the Feb. 26 issue of the journal Science.

“We find that the quality of the fossil record is something like 10 to 100 times greater than the quality that would be required by this hypothesis of missing species diversity,” said Foote, Associate Professor in Geophysical Sciences at Chicago. “It’s such a large discrepancy, we ended up concluding that it’s difficult, if not impossible, to maintain that there are 65 million years of fossils missing from the history of modern placental mammals. This result calls into question the use of a strict molecular clock to date the origins of major biologic groups.”

The trigger for the study was a paper published in the April 30, 1998, issue of the journal Nature, written by Sudhir Kumar and S. Blair Hedges of Pennsylvania State University. Kumar, Hedges and other scientists using molecular techniques have claimed that various groups of plants and animals––not just placental mammals––must have had a long history that is simply not found in the fossil record.

These claims are based on the assumption that the molecular clock, the rate of genetic change any given species undergoes, is relatively constant over time. Thus, a measurement of the genetic difference between any pair of species would reveal how long ago their ancestral lineages went their separate ways.

The fossil record shows species continually give rise to new species and continually die off, with a few happening to survive any given span of time. Evolutionary biologists have known this for more than a century, since the days of Charles Darwin.

If that is the case, Foote said, “there are a lot more species that have to escape preservation in order to make this hypothesis of missing time plausible.” And it is not just the number of species that matters but also their duration on Earth. “The longer the duration, the more opportunities there would be for fossilization,” Foote said.

In the study’s next step, Hunter and Janis compiled all the available data worldwide on fossil occurrences of Cretaceous mammals, except modern placental mammals, which are missing from the Cretaceous fossil record. These data allowed the researchers to measure the quality of the mammalian fossil record and the duration of mammalian species.

Hunter and Janis found between 225 and 460 known species of nonplacental mammals from the Cretaceous Period, depending on how they counted forms that are difficult to identify. But the numbers are significant no matter how you count them, said Chicago paleontologist Sepkoski. And the numbers say the fossil record is much better than some molecular biologists and paleontologists have assumed.

“We’re just trying to make the whole question more explicit and more testable,” Foote said. Until now, some molecular biologists and paleontologists have tended to go back and forth, blaming each other for bad data.

“We thought the right approach was to ask, what would the fossil data have to say in order to favor one hypothesis or the other, and what do the data actually say?” Foote said. “If this had come out the other way, if our conclusion had been, yes, that it’s very easy to believe there’s 65 million years of missing mammal history, I would have been perfectly happy.” But, Sepkoski added, “I would have been skeptical.”