From DISCOVER Magazine, February 1994, p. 17: Pioneering Rodent Between 80 million years a

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From DISCOVER Magazine, February 1994, p. 17: Pioneering Rodent Between 80 million years ago, when South America split apart from Africa, and 3.5 million years ago, when the Isthmus of Panama emerged from the sea, South America was an island unto itself. This period of quarantine enabled the continent to evolve a bizarre menagerie of marsupials, sloths, anteaters, and hoofed mammals that were unlike mammals found anywhere else in the world. (A similar evolutionary freak show occurred in Australia.) But South America was not completely isolate: it was invaded by rodents and monkeys sometime before 25 million years ago, when those animals first appear in the South American fossil record. The invaders came from either Africa or North America, both of which were already populated by rodents and primates. But which continent they came from has been the subject of a contentious debate among paleontologists. A recent discovery by a team of American and Chilean researchers led by paleontologist Andre' Wyss of the University of California at Santa Barbara may help resolve the issue. Fueling the debate has been a huge gap in South America's fossil record. The 25-million-year-old rodent and monkey fossils that have been found in South America are clearly not those of the first pioneers, because they represetnt a great diversity of species. Yet no land fossils at all had been found from the crucial period between 40 and 25 million years ago, when rodents and monkeys are thought to have arrived. Wyss's team has begun to fill this gap. In a treasure trove of mammal fossils in the Tinguiririca River valley of the Chilean Andes, the researchers discovered the lower lawbone of a 34-million-year-old rodent. Wyss thinks this unnamed rodent was about six to eight inches long; it may have resembled a cross between a guinea pig and a porcupine. More important, judging from the shape of its lower teeth, the upper molars of this oldest of South American rodents apparently had five distinct crests. The African rodents of the period also had five distinct crests on their upper molars, whereas North American species had only four. "The dentition in mammals is very distinctive," says Wyss. "This strongly points to an African origin for South American rodents." Wyss says animals could have crossed the Atlantic about 40 million years ago, when it was 800 miles across at its narrowest point--half its present width--by drifting on floating trees or other vegetation. Volcanic islands that have since become submerged may have served as evolutionary way stations. And although he has no direct evidence, Wyss thinks monkeys probably came from Africa, too. "Whatever circumstances allowed rodents to get in," he says, "it seems likely that monkeys took advantage of the same mechanism."


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