New fossil may rewrite history of mammal evolution

From: 2006-02-22 14:25:51

NANJING, Feb. 22 (Xinhuanet) -- With its forelegs upright like a dog and hind legs stretching out like a lizard, experts say a newly discovered fossil may rewrite the history of mammalian evolution.

In their joint research on mammal fossils in the western part of Northeast China's Liaoning Province, two distinct bone characteristics in the fossil of a sharp-mouthed mammal were collected by Chen Piji, a Nanjing-based researcher of the Chinese Academy of Sciences (CAS).

The British research journal, Nature, reported the unprecedented discovery, which is believed by some as holding the potential to change the traditional theory on the evolution of mammals.

Li Gang, one of the co-authors of the paper, said existing mammals are classified into two groups -- viviparous therians which have fully evolved bones, such as kangaroos and elephants; and oviparous monotremes, which have rather primitive bones, like lizards.

The newly discovered fossil, however, has the characteristics of both, a trait that was never observed before, said the researcher with CAS Nanjing Institute of Geology and Paleontology.

An analysis of the fossil shows the mammal, which lived some 120 million years ago in the early Cretaceous period, was about 12 centimeters long and weighed between 15 and 20 grams. It looks very much like a gecko, but with the help of a microscope, experts saw it had straight front legs like a rat or dog, while its hind legs resemble those of a lizard, with a 90-degree angle between the thigh and calf.

Further observation also found many evolutionary discrepancies.For example, it has the teeth of a therian but shares the lumbar ribs found only in primitive mammals. It resembled a viviparous therian in the skull, upper limbs and shoulder bones but had the lumbar, pelvis and lower limbs of an ovipara.

"It's really unique," said Li. "We had never observed such traits in modern animals or any fossil we studied in the past."

Li and his colleagues have not been able to explain this peculiar phenomenon, though he guessed the mammal might have finished evolution to become a therian, but its hind legs revertedinto a more primitive form in order to survive under certain circumstances. Enditem