原文網址：http://www.geologypage.com/2017/05/worlds-oldest-animal-fossils.html
有關世上最古老動物化石的新觀點
甕安動物群是位於中國南方，年代約為6億年的化石富集帶。分子時鐘預估當時為動物演化出來的時間點，甕安動物群提供了獨一無二的縮影，記錄了其中一段時期的海洋生物樣貌。
然而，從這個年代出土的所有化石都有爭議存在。
布里斯托大學地球科學院的John
Cunningham博士表示：「這些岩石的定年結果為6億年左右，此年代完美地使其保存的微體化石群集，成為動物出現的最古老證據的候選者之一。」
「科學家不認為這些化石是完全成型的動物留下的遺骸，它們有些跟胚胎十分相似，從一個單獨的細胞到由數千個細胞組成的團塊都有。」
「它們的保存情況十分良好，甚至可以辨識出次細胞結構，包括可能是細胞核的構造。」
論文共同作者之一，布里斯托大學的博士後研究員Kelly Vargas博士表示：「但是古生物學家缺乏可以指出它們真實身分的成體化石，因此他們必須仰賴細胞解剖學來判斷這些微小化石是否屬於動物，或是屬於其他種類。」
科學家審視了所有將甕安化石的身分指認為動物的證據。
他們的發現顯示之前用來將化石定義為動物的特徵，沒有一項是動物獨有的，這開啟了它們的真實身分另有所屬的可能性。
另一位布里斯托大學的共同作者，Philip
Donoghue教授補充：「許多支持它們跟動物關係密切的學者主張，在化石中，細胞彼此之間的ㄚ型連接是動物的重要特徵。但此特色在許多多細胞群體中，包括跟動物親緣關係甚遠的藻類身上都能見到。」
Cunningham博士接著說：「這些化石可能隸屬於其他族群，比方說藻類。此可能性未來需要仔細研究才行。」
儘管已經有了諸多結果，古生物學家仍持續從甕安生物群中得到新的發現，而幫助我們更加瞭解埃迪卡拉紀(Ediacaran)發生的演化事件。
Cunningham博士總結：「或許我們終將在陡山沱地層組中找到肯定為動物的化石，但這就像是在大海撈針，或者應該說是要在一座超級超級大的採石場中，找到一個胚胎一樣。」
New light shed on 'world's oldest animal
fossils'
A
team of researchers, led by the University of Bristol, has uncovered that
ancient fossils, thought to be some of the world’s earliest examples of animal
remains, could in fact belong to other groups such as algae.
The Weng’an
Biota is a fossil Konservat-Lagerstätte in South China that is around 600
million-years-old and provides an unparalleled snapshot of marine life during
the interval in which molecular clocks estimate that animal groups had evolved.
However, all
fossil evidence from this time has met with controversy.
Dr John
Cunningham from the University of Bristol’s School of Earth Sciences, said:
“Dated at around 600 million years old, these rocks preserve an assemblage of
microscopic fossils, perfectly-aged to be candidates for the oldest evidence of
animal life.
“These fossils
aren’t recognisable as remains of fully grown animals, but some resemble
embryos, ranging from single cells to clusters of thousands.
“The
preservation is so exquisite, that even sub-cellular structures can be
identified, including possible nuclei.
Dr Kelly
Vargas, a postdoctoral researcher from the University of Bristol and one of the
paper’s co-authors, said: “But with the lack of adult forms that could indicate
their identity, paleontologists have to rely on information from cellular
anatomy to determine whether these tiny fossils belong to animals or to a
different group.”
Now scientists
have reviewed all the evidence pointing towards an animal identity of the
Weng’an fossils.
Their findings
have revealed that none of the characteristics previously used to define the
fossils as animals are actually unique to animals alone, opening up the
possibility for alternative identifications.
Professor
Philip Donoghue, another Bristol co-author, added: “Many proponents of animal
affinity have argued that the Y-shaped junctions between the cells in the
fossils are an important animal character, but this a feature common to many
multicellular groups, including algae, that are very distant relatives of
animals.”
Dr Cunningham
added: “It could be that the fossils belong to other groups, such as algae, and
these possibilities need to be investigated carefully.”
Despite these
results, paleontologists are continuing to make new discoveries from the
Weng’an Biota, and these are helping to refine our knowledge of evolution
during the Ediacaran.
Dr Cunningham
concluded: “It might be possible that we’ll find definite animals in the
Doushantuo Formation, but it’ll be like finding a needle in a haystack, or
should we say an embryo in a really, really big quarry.”
原始論文：John A. Cunningham, Kelly Vargas, Zongjun Yin,
Stefan Bengtson, Philip C. J. Donoghue. The Weng’an Biota (Doushantuo
Formation): an Ediacaran window on soft-bodied and multicellular
microorganisms. Journal of the Geological Society, 2017; jgs2016-142 DOI: 10.1144/jgs2016-142
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