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

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."