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谁产下了地球上的第一颗卵?南中国发现的六亿年
Blacksburg, Va. ?In 1998, Shuhai Xiao and colleagues reported finding thousands of 600 million year old embryo microfossils in the Neoproterozoic Doushantuo Formation, a fossil site near Weng'an, South China, (Xiao, S., Zhang, Y., and Knoll, A.H., 1998, "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite," Nature, v. 391). Within the egg cases they examined at that time, they discovered animals in the first stages of development ?from a single cell to only a few dozen cells. "The cellular preservation is amazing," says Xiao, assistant professor of geosciences at Virginia Tech
But what kind of adult would these ancient embryos have hatched into?
In 2000, Xiao's team reported the discovery of a coral-like animal that might be a candidate for parenthood (Xiao, S., Yuan, X., and Knoll, A.H., 2000, "Eumetazoan fossils in terminal Proterozoic phosphorites?" Proceedings of the National Academy of Sciences, USA, v. 97). "It was tubular, not spherical. But in some of the best specimens, we could see that the tube branches and has cross-walls," Xiao said. "But can it be linked to the embryos?"
Upon examination of more embryos collected from the original site, Xiao's research team has discovered some embryos that may be at the hatching stage. He will report on this latest finding at the Geological Society of America meeting in Denver Nov. 7-10.
"Looking now at these egg cases, we can see clockwise spiral grooves, as if a knife sliced the egg open," Xiao said. "The embryo was beginning to hatch. When we removed the egg case, we found that the post blastula but pre-hatching embryo at this stage is beginning to transform into a spiral animal. Each such animal had three clockwise spires. After hatching, the spiral organism began to uncoil slightly," Xiao said.
"They look as if they can unwind to a tube structure. We are looking for more evidence, but if that is true, it might link the embryo fossils to the tubular coral-like animal."
The researchers will slice open a hatched embryo specimen that appears to be unwinding to see if it has cross-walls. "That would indicate it is related to the tubes," Xiao said
"These organisms lived 600 million years ago ?before big animals. This the very first moment of animal evolution preserved in the fossil record." [标签:content1][标签:content2]
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作者:admin@医学,生命科学 2011-09-07 05:14
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