五億年前的化石揭曉了櫛水母的起源
科學家發現一種鮮為人知的海洋掠食者，跟另外一種海床上的生物有著極為相似的外型。這項發現使科學家重新定義這種海洋掠食者在演化樹上的位置。
三穹傣花蟲的正模式標本。圖片來源：趙陽
櫛水母在動物演化史上具有相當重要的地位，因為有些科學家主張牠們是最早演化出來的動物之一。最近一組國際古生物學家團隊發現的化石證據，指出某些生活在海床、擁有類似水螅觸手的古老生物跟櫛水母有親緣關係。
3月21日發表於《當代生物學》(Current Biology)的論文中，英國布里斯托大學、倫敦自然史博物館和中國雲南大學的研究人員，比對一具5.2億年前的化石跟其他骨骼結構類似的化石之後，發現牠們都是演化自同一個祖先。
研究共同作者侯先光教授發現了這具鑲嵌在黃綠色的泥岩中、貌似花朵的化石。地點是中國南部雲南省，昆明南部的一個岩石露頭。
雲南省位在熱帶中國。過去30年來，古生物學家在零星分布於稻田和農地的露頭中，挖出了許多保存情況極為良好的化石。
團隊以雲南傣族和它花朵般的外型將其命名為「傣花蟲」(Daihua )。傣花蟲的外型像是杯子，口部位在正中間，周圍有18條觸手散發出去。這些觸手有如羽毛一般分岔，其上大量成排的纖毛被保存了下來。
「我第一次看到這具化石，就立刻注意到它有一些在櫛水母身上可以看到的特徵。」布里斯托大學地球科學院的分子古生物學家Jakob Vinther博士表示。「你可以看到每條觸手上面都有深色斑點重複出現，這跟櫛水母的化石非常相似。此外，也能看到成排的纖毛因為數量龐大而保存了下來。縱觀生命樹上，這種由大量纖毛組成的構造，只能在櫛水母身上見到。」
櫛水母是悠遊在海洋中的掠食者，其中某些種類是有害的入侵種。牠們利用身體側邊縱向排列、像是霓虹燈閃閃發光的櫛板來游泳。櫛板是由一種從細胞伸出的構造――纖毛緊密排列而組成。生命樹上由纖毛組成的最大型構造，就是櫛水母身上的櫛板。
一種現生的櫛水母Euplokamis。沿身體縱向排列的櫛板散發出像彩虹一般的光芒。圖片來源：Alexander Semenov
研究人員注意到傣花蟲跟另外一種化石――高足杯蟲(Dinomischus)有相似之處。這種著名的奇怪生物出土於伯吉斯頁岩(5.08億年前)。牠們的長柄上方一樣有18根觸手，而且骨骼也是由有機質組成。之前的研究人員把高足杯蟲分類至內肛動物。
共同作者叢培允說：「我們也發現之前古生物學家一直認為是海葵的先光海葵(Xianguangia)化石，實際上是櫛水母家族的一根分枝。」
研究人員觀察到的全新關聯，完美展示出這些化石如何一路演變成現今的櫛水母。
「這大概是我生命中最開心的時刻。」Vinther博士說：「之前我們翻閱動物學的教科書，試著釐清許多化石之間的異同關係。就在此時，突然出現了一個新的化石，完美補齊缺失的部分。」
研究顯示了櫛水母的演化過程。櫛水母的祖先具有有機質組成的骨骼，寒武紀的某些種類仍然保有這些骨骼，並且可以用來游泳。而固著在海床上、類似水螅的祖先的觸手則演化成了櫛板。之後牠們的口部逐漸像氣球一樣膨脹，本來的身體則漸漸縮小，造成口部周圍的觸手變成由櫛水母的後端延伸出來。
共同作者Luke Parry博士補充：「櫛水母的身世是個相當難解的問題。我認為身體構造經歷了如此大的轉變，便是原因之一。這可以解釋為什麼櫛水母的基因大部分都消失了，而且外型跟別的動物十分相似。」
將近150年來，動物學家一直認為櫛水母跟腔腸動物有親緣關係。近期新的基因資訊才挑戰了這項理論：分析顯示在外觀非常簡單的海棉之下，櫛水母這條分枝可能跟所有的現生動物都距離相當遙遠。
研究作者認為他們的發現提供了強烈證據，顯示櫛水母應該重新擺在演化樹上遠早於珊瑚、海葵跟水母的位置。
Half a
billion-year-old fossil reveals the origins of comb jellies
One of the ocean’s little
known carnivores has been allocated a new place in the evolutionary tree of
life after scientists discovered its unmistakable resemblance with other
sea-floor dwelling creatures.
Comb jellies occupy a pivotal place in the history of animal
evolution with some arguing that they were among the first animals to evolve.
Now an international team of palaeontologists have found fossil evidence that
proves comb jellies are related to ancestors that sat on the sea floor with
polyp-like tentacles.
As reported today in Current
Biology, researchers from the University of Bristol, Yunnan
University in China and London’s Natural History Museum, compared a 520
million-year-old fossil with fossils of a similar skeletal structure and found
that all evolved from the same ancestors.
The fossil, set in a yellow and olive coloured mudstone and
resembling a flower, was found in outcrops south of Kunming in the Yunnan
Province, South China by Professor Hou Xianguang, co-author of the study.
Several amazingly preserved fossils have been unearthed from
outcrops scattered among rice fields and farmlands in this part of tropical
China in the last three decades.
It has been named Daihua after
the Dai tribe in Yunnan and the Mandarin word for flower ‘Hua’, a cup-shaped
organism with 18 tentacles surrounding its mouth. On the tentacles are fine
feather-like branches with rows of large ciliary hairs preserved.
“When I first saw the fossil, I immediately noticed some
features I had seen in comb jellies,” said Dr
Jakob Vinther, a molecular palaeobiologist from the
University of Bristol's School of Earth Sciences. “You could see these repeated dark stains along each
tentacle that resembles how comb jelly combs fossilise. The fossil also
preserves rows of cilia, which can be seen because they are huge. Across the
Tree of Life, such large ciliary structures are only found in comb jellies.”
In today's oceans, comb jellies are swimming carnivores. Some of
them have become invasive pests. They swim using bands of iridescent, rainbow
coloured comb rows along their body composed of densely packed cellular protrusions,
known as cilia. Their hair-like structures are the largest seen anywhere in the
tree of life.
The researchers noticed that Daihua resembled another fossil, a
famous weird wonder from the Burgess Shale (508 million years old) called Dinomischus. This
stalked creature also had 18 tentacles and an organic skeleton and was
previously assigned to a group called entoprocts.
“We also realised that a fossil, Xianguangia, that we
always thought was a sea anemone is actually part of the comb jelly branch,”
said co-author Prof Cong Peiyun.
This emerging pattern led researchers to see a perfect transition
from their fossils all the way up to comb jellies.
“It was probably one of the most exhilarating moments of my
life,” said Dr Vinther. “We pulled out a zoology textbook and tried to wrap our
head around the various differences and similarities, and then, bam!—here is
another fossil that fills this gap.”
The study shows how comb jellies evolved from ancestors with an
organic skeleton, which some still possessed and swam with during the Cambrian.
Their combs evolved from tentacles in polyp-like ancestors that were attached
to the seafloor. Their mouths then expanded into balloon-like spheres
while their original body reduced in size so that the tentacles that used to
surround the mouth now emerges from the back-end of the animal.
“With such body
transformations, I think we have some of the answers to understand why comb
jellies are so hard to figure out. It explains why they have lost so many genes
and possess a morphology that we see in other animals,” added co-author Dr Luke
Parry.
Until around 150 years ago, zoologists had considered comb
jellies and cnidarians to be related. This theory was challenged more recently
by new genetic information suggesting comb jellies could be a distant relative
to all living animals below the very simple looking sponges.
The authors of this new study believe their findings make a
strong case for repositioning the comb jelly back alongside corals, sea
anemones and jellyfish.
原始論文：Yang Zhao,
Jakob Vinther, Luke A. Parry, Fan Wei, Emily Green, Davide Pisani, Xianguang
Hou, Gregory D. Edgecombe, Peiyun Cong. Cambrian Sessile, Suspension
Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan. Current
Biology, 2019; DOI: 10.1016/j.cub.2019.02.036
引用自：University of Bristol.
"Half-a-billion-year-old fossil reveals the origins of comb jellies."
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