Extinct Species of "Mosaic" Mammal Found in China

James Owen
for National Geographic News

January 11, 2006

Scientists in China (map) have discovered a fossilized small, furry animal that walked like a platypus but looked like a shrew. The unusual find provides important new clues to the evolution of early mammals, the researchers say.

Found in the province of Liaoning in northeast China, the well-preserved fossil shows a previously unknown species of insect-eating mammal that lived alongside dinosaurs some 125 million years ago.

Measuring 4 inches (10 centimeters) long and weighing 15 to 20 grams (0.5 to 0.7 ounce), the shrewlike creature had a thick coat of fur.

Writing in tomorrow's issue of the journal Nature, paleontologists Gang Li and Zhe-Xi Luo say the animal reveals some unexpectedly primitive features, and that it strengthens Asia's claim as the site where the main mammal groups originated.

Named Akidolestes, the extinct animal had jaws, teeth, and forelimbs that identify it as a close relative of modern placental and marsupial mammals. Placental mammals give birth to fully developed young, while marsupials bear premature young that continue to develop outside the mother's body.

But the researchers noted a highly unusual back-half to its skeleton—similar to that of primitive, egg-laying mammals known as monotremes.

The only living descendants of monotremes are the strange duck-billed platypus of Australia and two species of spiny anteaters, or echidnas.

"This new fossil is a chimera of body structures of different kinds of mammals," said Zhe-Xi Luo, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

"Its front half resembles those of more derived marsupials and placentals, but its back half is unmistakably monotreme-like."

Different Mammals

Luo says the animal had a front posture and gait similar to that of a squirrel, with elbows tucked under its body, but its hind legs had the sprawling appearance of a lizard's.

"The walking and running movement in Akidolestes would be similar to the platypus," he said.

But, Luo added, unlike the water-dwelling platypus, Akidolestes was a land mammal that preyed on insects using its sharp teeth.

Such a mix of modern and primitive features hasn't been seen before in a mammal, the authors say.

They add that the fossil challenges conventional wisdom about how placental mammals split from earlier egg-layers.

The split may not have been as clear-cut as previously thought, they say. Some placental mammals have have readopted some of the physical characteristics of monotremes.

"It is quite unusual that this mammal reacquired some primitive hind-limb feature," Luo said.

Evolutionary Throwback

Thomas Martin, head of mammalogy at the Senckenberg Research Institute in Frankfurt, Germany, agrees that Akidolestes may represent some kind of evolutionary throwback.

The animal's curious combination of traits could be caused "by developmental genes that sporadically become active in widely separated mammalian [groups]," he said.

"Akidolestes impressively shows that the evolution of the mammalian skeleton followed a mosaic pattern," Martin added.

The fluffy animal also offers further clues to the origins of the large group of mammals that arose following the demise of the dinosaurs some 65 million years ago.

The researchers say Akidolestes and its immediate fossil relatives all belong to the same extinct family of mammals, whose older species all lived in Asia while the younger species were found in North America.

"Akidolestes strengthens the case for Asia being the place where the main mammal groups first originated," Martin said.

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