In contrast, marsupials are commonly thought of as pouched mammals. While the latter also give live birth, they do not have long gestation times; the early development is completed instead in the pouch.

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. The four major groups are:

(1) Afrotheria [elephants, hyraxes, manatees and dugongs, aardvarks, golden moles, tenrecs, and elephant shrews],

(2) Xenartha [armadillos, anteaters, and sloths],

(3) Laurasiatheria [carnivores (e.g., bears, cats, dogs), pangolins, whales and dolphins, even-toed ungulates (e.g., hippos, cows, pigs), odd-toed ungulates (e.g., horses, rhinos), bats, and insectivores (e.g., shrews, moles, hedgehogs)], and

(4) Euarchontoglires [rodents, rabbits, tree shrews, flying lemurs, and primates (e.g., humans, monkeys, lemurs)].

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.

Such deciphering of higher level relationships among mammalian orders is important because of its ramifications for evolutionary biology and genomics. "A well resolved phylogeny offers a good framework for performing other studies," notes Springer.

"It allows for better predictions on what fossils may look like and where they might be found in different parts of the world. Our findings are also likely to assist genomicists in determining which organisms they should sequence genomes for in future."

Related Links
University of California - Riverside
ODSN Plate Tectonic Reconstruction Service
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