Bones of Contention
When he's note digging for fossils in the Gobi Desert and other faraway lands, U of L's Guillermo Rougier wades into a scientific debate on the origin of the Middle East, a key feature of mammals.
Nearly everyone can look at a land animal and know if it is a mammal, a bird or an amphibian. Simply put, if it has feathers it's a bird; if it has hair, it's a mammal; and if it has scales, it's a lizard. But these and other traits that help identify animal groups rarely are preserved in fossil form, says paleontologist Guillermo Rougier.
Because fossilized remains usually are bones, paleontologists have to find osteological (bone-related) features that distinguish the animal groups from each other. A key distinguishing feature for mammals, Rougier says, is the middle ear.
"The presence of three middle ear bones, although it seems to be a trivial detail, is a unique feature in mammals today, so that is the most prominent and most clear-cut feature that we have in osteology to distinguish mammals from any other group," he says.
Rougier, one of the world's foremost experts on Mesozoic-era mammals and U of L professor in anatomical sciences and neurobiology, is involved in a friendly debate with colleagues on the origin of the modern mammal middle ear.
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Guillermo Rougier hunts for early mammal fossil in Mongolia's Gobi Desert.
How did the middle ear develop? The answer resides "sometimes in the very distant past," Rougier says--and if it is the distant past to a paleontologist, the timeframe probably is unfathomable for everyone else.
On the evolutionary timeline, modern mammals appeared 65 million years ago. An animal that lived 10 million years ago is too modern for a paleontologist who is studying a very old event such as when mammals developed their unique middle-ear structure, he says.
Described by colleagues as the "go-to" guy for information about ancient mammals, Rougier may be more familiar with long-extinct animals that co-existed with dinosaurs or flourished right after the dinosaurs' demise than most people are with their pet cat or dog.
Rougier specializes in the evolutionary history of mammals from about 230 million years ago to 65 million years ago. He wants to explain how modern mammals developed and answer questions about animal diversity, specifically: How related to one another are today's animals?
Rougier's quest has been lifelong, starting when, as a young boy, he hunted fossils on his grandfather's farm in Argentina.
Today he is still looking for fossils in Argentina as well as in Mongolia's Gobi Desert. The two spots are teeming with potential answers to his questions.
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"I'm looking at the cretaceous period of the Gobi Desert in Asia and the Jurassic and Cretaceous periods of South America," he says. "I'm trying to approach the same questions (of animal relationships and origins) from several different angles."
At the heart of the debate about the origin of the middle ear, which is being played out in the pages of Science magazine, is whether or not egg-laying mammals (monotremes) and placental mammals (therians) had a common ancestor with a typical mammalian middle ear.
According to an article in the Feb. 11, 2005, issue of Science, written by a group that includes James Hopson, Rougier's mentor, modern monotremes and therians developed almost identical middle-ear configurations along two independent, parallel tracks--the three-bone middle ear was not present as such in their common ancestor.
Rougier and colleagues from the University of Texas have countered that hypothesis in two independent technical notes that appeared in the Sept. 2, 2005 issue of Science.
They argue that the middle ear in the two types of modern mammals developed along one path. Basing their hypothesis on fossil evidence, they say that monotremes and therians share an ancestor that the characteristic features of the middle-ear region that are in modern mammals.
"You can never have absolute proof that something appears only once," he says. "You can have proof that it appears more than once," and right now, existing evidence does not unequivocally support the case that mammal ears in the two types of modern mammals developed independently of each other.
The debate goes to the root of the mammal group, Rougier says. If the distinguishing middle ear appeared independently more than once in history, it loses its power to identify members of the group.
Rougier recently returned from an American Museum of Natural History-led expedition to the Gobi. The museum began expeditions there in 1990, and Rougier has been a regular member since he began his postdoctoral work at the museum in 1996.
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Rougier and a colleague catalog fossil data on their laptops inside a ger, a traditional dwelling of the area.
This summer the group went to Ukhaa Tolgod to a basin known as the Camel's Hump. They had found the area, which measures about 100 meters by 40 meters, to be rich in fossils on previous expeditions--so rich they decided to leave it for several years to explore other areas of the Gobi and to give the rain and wind time to uncover more bones.
"The rocks are relatively soft and have the consistency of loose sand," Rougier says. "The erosion was actually quite substantial"--and quite revealing."
On its first day, the team discovered 10 skulls of early mammals and lizards--a number that any team would be ecstatic to claim for an entire expedition, let alone one day, he says.
Rougier credits the Gobi project with helping to make his career, and although there is still some work to be done there, he says he is ready to switch his focus to Argentina's Patagonia region.
"In the five years that I have been going there, I have found five times more and better quality specimens than were collected before, so it is a radical improvement of the record," he says.
Most of what he has found has yet to be examined because it takes at least a year to prepare specimens and he has been finishing other projects. Rougier says he has seen enough to know that the fauna in South America developed differently because of its isolation from the rest of the world, and that a lot of it is "peculiar" when compared to fauna from the same time period in other locations.
"Pretty much anything I can say about South America is new," he says.
But do South American fossils hold the answers to the origin of modern mammal ears? If proof exists, he says, it is out there somewhere waiting to be discovered.
"Actually I'm happy when (debates) like this happen because they certainly crystallize the different positions," Rougier says. "Now we have a test case. This is what we expect to happen; now let's go see what actually happens."
