Rodents & Monkeys in S. America

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."