Gondwana Split Sorts Out Mammalian Evolution

[Richard W Travsky <rtravsky@uwyo.edu>]

Science Daily recently had this (to me) very interesting article on
sorting out mammalian evolution based on the Gondwana split.

(from a u of calif riverside press release)

 http://www.sciencedaily.com/releases/2002/01/020121090546.htm

...
Although independent studies have resolved placental mammals into four
major groups, it is not clear what the hierarchical relationships within
the groups are, thus hampering the understanding of the early
biogeographic history of placentals.
...
In the 14 December 2001 issue of Science, a team of scientists discuss
alternative positions for the root of the placental tree. They report
results based on Bayesian and other statistical methods and use a data set
that comprises approximately 16,400 base pairs for each of 44 mammals and
that includes segments from 22 different genes. "We have resolved the
interordinal relationships almost entirely," says Mark S. Springer of the
University of California, Riverside, a member of the team. "Based on
molecular clocks, we found that the deepest split occurs between
Afrotheria and other placentals at ~103 million years, a date that
coincides with a major plate tectonic separation."

The result is controversial. Some researchers cite fossil evidence that
suggests that mammals diversified only ~65 million years ago. But Springer
and colleagues argue that the separation of South America and Africa
around 100 million years ago during the Cretaceous in Gondwana (the
southern hemisphere super-continent that incorporated Africa, Antarctica,
Australia, India, Madagascar and South America before it broke apart)
explains the split. "We suggest that the common ancestor of living
placental mammals occurred not in the northern hemisphere, as is commonly
believed, but in the southern hemisphere instead, in Gondwana," says
Springer. "Furthermore, our study provides the first convincing molecular
evidence that flying lemurs and tree shrews are the closest relatives to
primates."

The scientists find that Afrotheria is the oldest group, with some of its
orders never having left Africa. The Xenartha, which populate South
America, constitute the next group. Because these oldest groups are
southern groups, the placental mammals originated in the south, the
scientists contend. They also determine that Laurasiatheria and
Euarchontoglires are sister taxa (i.e., taxa derived from a common
ancestral node), and together constitute a clade (i.e., an organism and
all of its descendants) named Boreoeutheria, with a northern hemispheric
origin. Deeper in the placental tree, the authors find that Xenartha and
Boreoeutheria are sister taxa.