Leigh Kish, Media Relations Manager
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For Immediate Release
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January 11, 2006
BY NATURE UNTIL 1800 London time /
Chinese and American paleontologists discovered
Placentals and marsupials are characterized by giving live birth, and are known as therians. Because therians (marsupials and placentals) represent the vast majority of mammals living today, scientists have always been interested in studying their Mesozoic fossil relatives as a way to find out the conditions of the ancestor from which these modern therian mammals evolved.
The newly discovered mammal, named as Akidolestes after its pointed rostrum, is about 4 inches (or 12 cm) long and estimated to weigh about 15 to 20 grams; it has triangular teeth cusps and shearing crests for feeding on insects and worms. The skull, fore-limb, and shoulder of Akidolestes show many advanced features of therians that the scientists conclude that is an extinct relative to the more modern therians including marsupials and placentals.
Akidolestes, however, is very unusual in the vertebral column, the pelvis and the hind-limb. The back-half of its skeleton is primitive and very similar to egg-laying monotreme mammals. Carnegie Museum of Natural History Curator of Vertebrate Paleontology Zhe-Xi Luo, PhD, noted that "This new fossil is a chimera of body structures of different kinds of mammals. Its front half resembles those of more derived marsupials and placentals, but its back half is unmistakably monotreme-like." Its skeleton is a mosaic of structures that are found either in modern monotremes, or in modern therians, but never together in one animal," said Luo.
Luo notes that the new fossil of Akidolestes is a challenge to the conventional wisdom about how therians split from monotremes in the early mammalian evolutionary history. Its unusual combination of advanced therian features and the primitive monotreme features can be the consequence of convergence, in which the hind-limb of Akidolestes has re-developed the monotreme-like features.
"Metaphorically, this newly discovered fossil mammal has a forelimb posture and gaits like those of a squirrel, with elbows tucked under its body, but its hind-limb would be sprawling with a posture that is similar to a lizard," said Luo. "It is quite unusual that this mammal re-acquired some primitive hind-limb feature."
After the extinction of dinosaurs and in the Cenozoic (65 million years to Present) or Ages of Mammals, a large number of mammal groups initially evolved in Asia and then migrated to North America, a broad geographic pattern discovered by Carnegie Museum of Natural History Curator of Vertebrate Paleontology Chris Beard, PhD. Akidolestes and its immediate fossil relatives all belong to an extinct mammalian family of spalacotheroids, in which the more primitive and older species were all Asiatic and the advanced and younger species are all North American. Zhe-Xi Luo notes: "Now we have some tantalizing evidence that the prevalent dispersal of mammals from Asia to North America during the Early Cenozoic may have had a longer history going back to Cretaceous."
The discovery of the new fossil of Akidolestes its scientific study was published in the 12 Jan 2006 in the prestigious British scientific journal Nature by Dr. Gang Li of Nanjing Institute of Geology and Palaeontology of Chinese Academy of Sciences (Nanjing, China) and Zhe-Xi Luo of Carnegie Museum of Natural History, with support from National Science Foundation (USA), National Natural Science Foundation (China) and National Geographic Society.
Dr. Gang Li
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