New fossil
finds are challenging the idea that six disparate orders of African mammals
— elephants, sea cows, aardvarks, elephant shrews, hyraxes, and golden
moles and tenrecs — all evolved from a single common ancestor isolated
on the continent of Africa by the breakup of Gondwana about 100 million years
ago.
The endangered golden-rumped sengi or
elephant shrew, about the size of a rabbit, is one of 15 species of elephant
shrews, which live only in Africa. Elephant shrews were originally named for
their trunk-like snout, not because zoologists thought that they were related
to elephants. Ironically, a recent hypothesis based on molecular evidence now
groups them into a “superorder” called Afrotheria that includes elephants.
Photo courtesy of Galen Rathbun, California Academy of Sciences.
The hypothesis, put forth in 1998, classifies the six mammalian orders as a
“superorder” called Afrotheria, based on molecular biology studies
that found many DNA sequences in common among modern members of the group. Most
living “afrotheres” are endemic to Africa.
However, the discovery in North America of a 54-million-year-old fossil of an
extinct relative of modern elephant shrews, the oldest found to date, is casting
doubt on the group’s origin. Such a discovery, which predates any African
record, suggests that the common ancestor did not arise in Africa.
This paper has “very appropriately raised the possibility that Afrotheria
is not African,” says Robert Asher, curator of mammals at the Museum of
Natural History at Humboldt University in Berlin, Germany, who specializes in
the study of afrotheres and is not involved with the recent study. The new finding,
the researchers say, also supports other fossil evidence that suggests the ancestors
of modern afrotheres could have migrated later to Africa from Asia, or from
North America through Beringia and Asia.
The fossil, encased in hematite, was actually unearthed decades ago from the
early-Eocene Willwood Formation in the Bighorn Basin of Wyoming, says Shawn
Zack, a graduate student in functional anatomy at the Johns Hopkins University
School of Medicine in Baltimore, Md. But the bones were only recently rediscovered
and identified by Zack and colleagues as belonging to Apheliscus chydaeus,
a member of a group called condylarths, early hoofed mammals that were present
in North America.
Working with Jonathan Bloch, assistant curator of vertebrate paleontology at
the Florida Museum of Natural History at the University of Florida in Gainesville,
Zack’s team realized that some fossils in the Florida museum’s collection
might be related as well. Comparing the leg and foot bones to those of living
elephant shrews, they concluded that condylarths are closely related to modern
elephant shrews, which they described in the March 24 Nature.
Other studies, based on jaws and teeth, had previously suggested a possible
link between condylarths and modern elephant shrews, Zack says. But some are
not convinced that the new fossil is actually an afrothere, saying that similarities
could have evolved independently, a point the authors acknowledge.
Their conclusion “leads to a very complicated biogeographic story,”
says Erik Seiffert, an afrotherian specialist and curator of geological collections
at the Oxford University Museum of Natural History in the United Kingdom. “It
implies that each of these afrotherian lineages independently crossed the Tethys
Sea and made it into Africa, and at the same time they all happen to be, coincidentally,
more closely related to each other than to any other mammals,” he says.
“To me it’s a real stretch.”
The new discovery also highlights an ongoing debate in paleontology over how
best to determine evolutionary relationships. “Molecular phylogeny”
determines relationships based on genetic similarities in DNA or proteins of
modern descendants, while “morphological phylogeny” determines relationships
by studying the anatomy of living and extinct animals.
Asher says that the new finding does not discredit the molecular data; it just
contests the geographic component of the theory. “In terms of the relationships,
Afrotheria is correct,” he says; “however the name might be wrong,
or at least a little misleading.”
Some morphologists say that while some group members are indeed closely related,
such as sea cows and elephants, those orders are too different from the insect-eating
aardvarks and golden moles to be classified together. However, Seiffert says,
some modern taxa, such as horses, tapirs and rhinos, have ancestors so similar
to hyraxes that, had they gone extinct and left no DNA, “we might come
to the [incorrect] conclusion that they were afrotherians” based on morphology,
he says.
Asher says that the new study points out that both methods have value. “This
paper is very, very exciting because it suggests that understanding mammalian
diversity with only living animals is never going to give you the whole picture,”
he adds. “You have to have fossils.”
Sara Pratt
Geotimes contributing writer
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |