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Tiny Skeleton May Be From the World's Oldest Marsupial

By Guy Gugliotta
Washington Post Staff Writer
Friday, December 12, 2003; Page A16

Scientists searching an ancient volcanic deposit in China have unearthed the skeleton of the oldest
marsupial yet found, a furry, long-tailed, mouse-size creature that ate insects and probably climbed
trees to escape hungry dinosaurs.

The discovery of Sinodelphys szalayi in the 125 million-year-old fossil beds of China's northeastern
Liaoning province has deepened the mystery surrounding the origins, dispersion and fate of marsupials
-- pouched mammals represented today mostly by opossums and Australian species such as
kangaroos, koala bears and wombats.

"More and more it looks like marsupials originated in a northern continent -- either Asia or North
America," said John R. Wible, an associate curator of the Carnegie Museum of Natural History and a
member of the international team that reported its discovery today in the journal Science. "Yet there
are no marsupials in Asia today."

The find also suggests that the divergence between marsupials, whose females nurture their babies
inside a pouch, and placental mammals, whose young mature inside the mother, occurred earlier than
once thought. Placentals include mammals from squirrels and elephants to humans.

"We're dealing here with a major event in mammalian history," said Michael Novacek, curator of
paleontology at the American Museum of Natural History.

Novacek noted that until relatively recently, researchers studying ancient marsupials were lucky to
recover a partial jawbone and a few teeth. But Liaoning -- where an entire ecosystem appears to have
been asphyxiated in a volcanic eruption and "flash-frozen" in ash -- has produced spectacular fossils of
all kinds, ranging from Sinodelphys to feathered dinosaurs.

"In the past, you worked with good [marsupial] dentitions -- teeth and jaws -- but with Liaoning, you
get almost everything, including hands and feet," Novacek said. "Fairly complete skeletons like this are
critical for identification, and they tell us so much more."

The Sinodelphys team, led by Zhe-Xi Luo, also of Carnegie, and including Wible and Chinese paleontologists Qiang Ji and
Chong-Xi Yuan, had been working in the Liaoning fossil beds for some time. In 2002, they reported finding the oldest placental
creature yet discovered, a tiny mammal known as Eomaia.

"That wasn't much of a surprise, given the variety of placentals in Asia," said Richard L. Cifelli, curator of vertebrate paleontology
at the Sam Noble Oklahoma Museum of Natural History and co-author of a commentary on the discovery. "But this marsupial was
a tremendous surprise. We never expected to see something of this age in Asia."

Wible said the team found Sinodelphys "in the same general area" as Eomaia, but in a different fossil bed. He described it as a
tiny, fur-covered, four-footed animal with a long tail and weighing about an ounce.

Wible said paleontologists customarily differentiated marsupials from placental fossils by the fact that marsupials have more
molars. Liaoning, however, allows comparisons of "wrists" and "ankles," he added, which are also different.

"But what's interesting is that Sinodelphys had tree-climbing ability," he added, noting that earlier fossils were not complete enough
for them to make such a judgment. "They appear to be terrestrial, but they were invading the trees."

Wible suggested that Sinodelphys may have used climbing to "exploit a new food source" -- berries or tree-dwelling insects -- but
acknowledged that trees also could present "a better place to hide." For one of Liaoning's famed feathered raptors, Sinodelphys
"would have been an appetizer," Wible said.

Cifelli said the discovery could change the way paleontologists think about marsupials. Until Sinodelphys, the oldest marsupial
remains were 110 million years old and found in North America. "In fact, marsupials are so dominant in North America, it seemed
that North America was the likely place of origin," Cifelli said. "Now we have this."

But it goes no further. Asia and Europe have no marsupials, and North America mysteriously lost all of its marsupials when the
dinosaurs disappeared 65 million years ago, probably in a catastrophic geological event.

They reappeared in South America, Australia and even briefly in Antarctica, which were all part of the same landmass. Today,
besides a few rodents, the only native mammals in Australia are marsupials, and South America also has several species. North
America, however, has only one -- the Virginia opossum -- a relatively recent immigrant from South America.

"Because of the age of our fossil, Asia for now is the continent of origin," Wible said. "But it's still an open question."