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6月21日听抄的一些背景资料
1.关于placental mammals的共同祖先
Two Carnegie scientists report discovery of human ancestor
One of mankind's earliest ancestors was a 5-inch-long, shrew-like creature that survived on insects and was fodder for dinosaurs, according to two scientists at the Carnegie Museum of Natural History.
Zhe-Xi Luo and John Wible are part of a team of American and Chinese researchers that have identified a fossil of the earliest-known placental mammal, called the eomaia scansoria. Eomaia is Greek for "ancient mother," and scansoria is Latin for "climber."
The scientists described their find in today's edition of the scientific journal Nature. The unusually well-preserved fossil is about 125 million years old and was discovered two years ago in northeastern China's Liaoning province. That region has yielded the fossils of birds, feathered dinosaurs and early mammals.
The fossil was found by a resident of the area and turned over to the Chinese Academy of Geological Sciences, which shared it with Luo and Wible. They said the eomaia fossil (pronounced "ay-oh-my-a") will provide scientists with important clues about the evolution of placental mammals, which is the largest group of mammals and includes humans.
"One of the things that we're concerned with doing is building genealogies. We want to put together family trees for living animals. … This is a wonderful piece of evidence," said Wible, the associate curator of the Oakland museum's mammal section.
The eomaia fossil is about 15 million years older than the next oldest-known placental mammal, said Luo, the associate curator of vertebrate paleontology at the Carnegie. The eomaia also is significant because it is has almost all of its bones in place. Many fossils consist only of a few teeth and bones, Wible and Luo said, leaving scientists to speculate about its size, eating habits and habitat.
Luo said the eomaia was furry, lived in lakeside bushes and climbed trees. Among the traits that identify the eomaia as placental — which it shares with humans--are three molars on each of its jaws, and a jawbone that forms a right angle just below the ear, Wible said.
The fetuses of placental mammals are nourished and protected in their mother's uterus until fully developed. That has given placental mammals an evolutionary edge over other species and accounts for their great diversity, Luo said. About 5,000 species of placental mammals currently exist, including apes, cats, dogs, rats and whales.
"Now we are getting to the very root of all mammalian groups. It helps to illustrate the earlier history of our evolution," Luo said.
The first mammals appeared about 170 million to 225 million years ago, and the modern group of mammals appeared on the evolutionary stage about 65 million years ago, after the extinction of the dinosaurs, according to Wible and Luo.
Wible said the eomaia was likely a "transitional" species, meaning it possessed traits of modern placental mammals and the mammals from which it evolved. In this case, the eomaia has pelvic bones characteristic of marsupials.
Modern marsupials include kangaroos and opossums. Scientists believe marsupials and placentals share a common ancestor, and the eomaia, sharing characteristics of both, may provide evidence of that, Wible said.
"It's extremely exciting, and for me, my life could not be brighter for this discovery, and I'm very happy we got to be a part of it," Luo said.
The research of the eomaia is funded by the Carnegie, the National Science Foundation, the National Geographic Society, the Natural Science Foundation of China and Ministry of Land Resources of China.
2. 关于placental mammals
placental mammals(有胎盘哺乳动物)即真兽类哺乳动物,是哺乳动物中十分重要的类群,现代的有胎盘哺乳动物是真兽类哺乳动物类群中的一部分,是当今世界上繁荣兴旺,数量最多的哺乳动物,广为人们所熟悉的有:猿、猴、老鼠、兔子、大象、虎、熊猫、海豚和鲸等,包括人在内的灵长类动物也是有胎盘类哺乳动物。
哺乳类动物的分类 :
原兽类 特徵 卵生,卵有壳. (例:鸭咀兽)
后兽类 特徵 不具真正的胎盘,幼儿在育儿袋中发. (例:袋鼠)
真兽类 特徵 有胎盘,胎儿发育完善后才产出,占哺乳类的绝大部份.并分为以下十四个类别:
食虫类(例:鼹鼠) 鳞甲类(例:穿山甲) 翼手类(例:蝙蝠) 兔形类(例:兔)
啮齿类(例:鼠,箭猪) 贫齿类(例:食蚁兽) 食肉类(例:狮,犬,熊猫)
鳍足类(例:海狮,海豹,海象) 海牛类(例:海牛) 鲸类 (例:海豚,鲸) 长鼻类(例:象)
奇蹄类(例:斑马,犀牛) 偶蹄类(例:河马,牛,猪,鹿,骆驼) 灵长类(例:猩猩,猴,人)
[ 本帖最后由 woodman 于 2007-6-21 18:18 编辑 ] |
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