发现早于恐龙的八足生物化石

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Eight-armed animal preceded dinosaurs

                               
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Scientistshave discovered what they believe is an eight-armed creature, whichcolonised a large section of the world's oceans over 300 million yearsbefore the first dinosaurs emerged.

The findings represent thefirst comparable animal fossils from the Ediacaran Period, 635 to 541million years ago, which appear in two drastically differentpreservation environments - black shale of South China and quartz rockof South Australia.

"According to palaeogeographicreconstructions, South China and South Australia were close to eachother at the time, belonging to a supercontinent called Gondwana," sayslead author Dr Maoyan Zhu.

Zhu, a scientist at the Nanjing Institute of Geology and Paleontologyat the Chinese Academy of Sciences, first helped to make theChina-Australia connection two years ago during a Beijing conference.He showed a photo of the unusual eight-armed creature, called Eoandromeda octobrachiata, to co-author Dr James Gehling of the South Australia Museum.

"Hewas so surprised and immediately opened his laptop and showed me imagesof new fossils uncovered from a new locality at the Flinders Ranges ofSouth Australia," says Zhu. "We wondered if these were the samefossils."

Zhu, Gehling and their colleagues collected eight compressions of the animals from the Doushantuo Formation at Wenghui, China.

Theythen traveled to Flinders Ranges, Australia, and collected sevenspecimens, leaving 31 others on two excavated and reassembled beds.

The findings are published in the November issue of Geology.

There is no question the creature, believed to represent one type of animal, had a lot of arms.

Simple and symmetrical

Theeight arms are clearly preserved in our specimens," says Zhu, addingthat the arms were tubular and in close contact with each other, butnot joined.

He and his colleagues believe the animal was asoft-bodied, dome-shaped organism that lived on seabeds and fed byabsorbing dissolved nutrients from the ambient environment.

Beforethe latest fossils were found, some researchers identified thecreatures as lichens or fungus-like organisms, but Zhu and his teamsuspect that at least some Ediacara fossils represent now-extinctdiploblastic animals, or creatures that possess only two cellularlayers separated by a jelly-type substance.

"Diploblasticanimals are common creatures on present day earth," he said, mentioningthat jellyfish, corals and sea anemones belong to the group.

"These animals (display) radial symmetry but lack complex organs, as shown by E. octobrachiata," he adds.

Themulti-armed creature, and several other early life forms, went extinctaround 542 million years ago, which Zhu says, "left empty niches forthe subsequent Cambrian explosion of complex animals."

Representativesof nearly all existent animals emerged at this time, when a rapidincrease in oxygen made respiration and metabolism possible.

Bridging the gap

Ina separate paper, Professor Shuhai Xiao, a researcher in the Departmentof Geosciences at the [/url=http://www.vt.edu/]Virginia PolytechnicInstitute and State University[/url], and colleague Dr Marc Laflammeprovide an overview of Ediacara fossils.

In the paper, which has been accepted for publication in the journal Trends in Ecology & Evolution,Xiao and Laflamme agree that, "Ediacara biota bridges the crypticevolution of multicellular life in the early Ediacaran and theextraordinary radiation of animals in the Cambrian period."

Inaddition to the eight-armed creature, they describe other early livingthings that looked like leaves, shells, stars and something almost akinto a peace symbol.

Xiao and Laflamme hope that as the Ediacarafossil database grows ever larger, more mysteries about these veryearly organisms will be solved.