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Rodent jaw lights up dark age of evolution.

Scrambling about the slopes of the Tinguiririca River valley in the Chilean Andes, Andre R. Wyss hoped to find dinosaur bones. Wyss, a paleontologist at the University of California, Santa Barbara, reasoned he stood a good chance because a nearby tourist attraction featured exposed footprints of the ancient beasts.

Bones he found, but of a different sort--a beautifully preserved jaw of the earliest rodent ever encountered in South America lay embedded in rock, along with hundreds of other mammal fossils.

The discovery of this jaw pushes back the arrival of rodents in South America some 10 million years. Furthermore, the fossil's morphology supports the long-contested theory that ancestral rodents from Africa somehow crossed the Atlantic Ocean to populate South America, Wyss and his co-workers report in the Sept. 30 NATURE. This collection of mamal remnants defines a new interval in South American mammalian evolution, helping to fill a huge gap in current understanding of that process, Wyss says.

South America was an island continent for most of the past 80 million years, and for some 40 million years its mammals evolved undisturbed by outside competitors. What happened in the next 20 million vears remains a mystery, even though the study of South America's mammal evolution began before Charles Darwin's visit.

"At some unknown point during this gap, rodents and primates must have arrived from other parts of the world and quickly thrived in South America. The Tinguiririca fauna falls nicely within that hole," explains Wyss. Given that scientists know neither rodents nor primates evolved in South America, "the big questions is when, whence, and how did they get there?" observes paleontologist Bob Savage of the University of Bristol in England. Some scientists hold that ancestral rodents invaded South America from North America, while others argue that they came from Africa.

The when and whence of the rodents' voyage are now clearer: "Our rodent sets at least a minimum age of between 31 and 37 million years," says Wyss, so they must have arrived sometime earlier. The molars of the ancient rodent appear to have had five crests, Wyss says. Such teeth establish a phylogenetic relationship with early rodents found in Egypt, strengthening the African connection. In contrast, the molars of primitive North American rodents show only four crests.

Scientists obviously need more than this jaw to be sure," says savage, "but it already makes a pretty strong case for the African origin." Wyss is optimistic that the will find more fossils, including primate bones, now that the Tinguiririca River valley has become a treassure trove for paleontologists. "We've already discovered a second site of an even older period," he says. "I think we may find many different slices of evoluntionary times represented in that area."

One advantage of the fossil-containing volcanic deposits is that they can be dated precisely, using a technique based on the gradual decay of the trace gas argon. The more than 400 fossils Wyss and his colleagues have excavated in five field seasons since 1988 are tightly and wiched between layers of sediment 31 and 37 million years old. These measurements provide the first definite dating in a period stretching from roughly 28 to 58 million years ago, Wyss says.

Fossils unearthed earlier, in other parts of South America, could only be dated relative to each other. The new Chilean site lies in volcanic rock and thus may improve dating of other evolutionary phases in that still-fuzzy period 37 to 58 million years ago, says Wyss.

Although these volcanic deposits preserve animal bones extremely well, the rock is so hard that it takes months to get a fossil out, notes Wyss. That's why it took him three years to realize that he had found a paleological gem. "When we broke the rock encasing that specimen, we saw only the cross section of a jaw and thought it was just another marsupial--of which we had found so may that we were tired of them. Given our preparation backlog, this jaw then sat around until someone finally got around to preparing it."
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Title Annotation:dating change on origin of rats in South America
Author:Strobel, Gabrielle
Publication:Science News
Date:Oct 9, 1993
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