2020年11月5日 星期四

一具五億兩千萬年前、擁有五顆眼睛的化石揭露了節肢動物的起源

 原文網址:http://english.cas.cn/newsroom/research_news/earth/202011/t20201105_248674.shtml

一具五億兩千萬年前、擁有五顆眼睛的化石揭露了節肢動物的起源

從五億兩千萬年前左右的寒武紀開始,節肢動物一直都是地球上最成功的動物。牠們極為常見且無所不在,現今的動物種類將近百分之八十都是節肢動物,遠遠超過其他動物。

Kylinxia化石的正型標本。圖片來源:曾晗

但是節肢動物如何演化出來?牠們的祖先樣貌為何?這些有關動物演化的重大難題,一個多世紀以來難倒了一代又一代的科學家。

中國科學院南京地質古生物研究所(NIGPAS)的研究人員,最近發現了一副擁有五顆眼睛、外型類似蝦子的化石,他們從中得到了關於節肢動物演化史早期的重要見解。研究於114日發表於《自然》(Nature)

這具被命名為Kylinxia的化石採集自中國西南方雲南省的澄江動物群,此動物群擁有寒武紀早期最為完整的動物化石記錄。

NIGPAS的黃迪穎教授是這篇研究的通訊作者,他說:Kylinxia(麒麟蝦,暫譯)是非常稀有的嵌合體物種。牠們結合了不同動物的外型特徵,就像中國傳統神話裡,結合不同生物外型的『麒麟』一樣。」

共同通訊作者趙方臣表示:「由於埋藏時的環境條件相當特別,使得Kylinxia的化石展現出極為細緻的解剖構造,比方說神經組織、眼睛、消化系統這些柔軟的身體部位,它們在一般的化石上面通常是無法看到的。

Kylinxia顯示了真正的節肢動物獨有的特徵,像是硬化的角質層、分段的軀幹以及具有關節的肢體。不過,牠也同時帶有非常原始的動物類型才有的型態特徵,包括被稱為寒武紀「神奇怪物」的歐巴賓海蠍(Opabinia)五顆怪異的眼睛,以及寒武紀海中巨大的頂尖掠食者――奇蝦(Anomalocaris)標誌性的掠食用附肢。

Kylinxia的解剖重建結果。圖片來源:黃迪穎

在澄江動物群當中,可以到達兩公尺長的奇蝦是最頂尖的掠食者。牠們也被認為是節肢動物的祖先型,但是奇蝦和真正的節肢動物之間有巨大的型態差異,這道在演化上的巨大鴻溝使得科學家難以建立牠們兩者之間的關係。因此,在研究節肢動物的起源時,這道鴻溝也成為了重要的「失落環節」。

研究團隊對Kylinxia的化石進行了詳細的解剖分析。結果證實奇蝦的第一對附肢和真正的節肢動物系出同源。親緣關係的分析結果指出Kylinxia的前肢、螯肢動物亞門(包括蜘蛛和蠍子的分類群)嘴巴前方用來捕食的小附肢、有顎亞門(節肢動物的一支,包括螞蟻和蜜蜂)的觸角,三者之間有密切關係。

共同作者朱茂炎博士表示:「我們的結果顯示Kylinxia的演化地位就在奇蝦和真正的節肢動物之間。因此,我們的發現觸及了真正的節肢動物在演化樹上的根源。

研究第一作者曾晗表示:「達爾文的演化論預測了過渡化石的存在,Kylinxia即為一個重要例子。牠連結了奇蝦和真正的節肢動物在演化過程中的缺失,因此在節肢動物的起源中扮演了關鍵的『失落環節』。這對生命是由演化而來的理論來說,是種相當強力的化石證據。」

研究經費來自中國科學院和中國國家自然科學基金委員會。

 

A 520-million-year-old five-eyed fossil reveals arthropod origin

The arthropods have been among the most successful animals on Earth since the Cambrian Period, about 520 million years ago. They are the most familiar and ubiquitous, and constitute nearly 80 percent of all animal species today, far more than any other animals.

But how did arthropods evolve and what did their ancestors look like? These have been a major conundrum in animal evolution puzzling generations of scientists for more than a century.

Now researchers from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS) have discovered a shrimp-like fossil with five eyes, which has provided important insights into the early evolutionary history of arthropods. The study was published in Nature on Nov. 4.

The fossil species, Kylinxia, was collected from the Chengjiang fauna in southwest China's Yunnan Province. The fauna documents the most complete early animal fossils in the Cambrian time.

Prof. HUANG Diying, corresponding author for the study from NIGPAS, said, "Kylinxia is a very rare chimeric species. It combines morphological features from different animals, which is analogous to 'kylin,' a chimeric creature in traditional Chinese mythology."

"Owing to very special taphonomic conditions, the Kylinxia fossils exhibit exquisite anatomical structures. For example, nervous tissue, eyes and digestive system - these are soft body parts we usually cannot see in conventional fossils," said Prof. ZHAO Fangchen, co-corresponding author of the study.

Kylinxia shows distinctive features of true arthropods, such as a hardened cuticle, a segmented trunk and jointed legs. However, it also integrates the morphological characteristics present in very ancestral forms, including the bizarre five eyes of Opabinia, known as the Cambrian "weird wonder," as well as the iconic raptorial appendages of Anomalocaris, the giant apex predator in the Cambrian ocean.

Among the Chengjiang fauna, Anomalocaris is a top predator that can reach two meters in body length, and has been regarded as an ancestral form of arthropod. But huge morphological differences exist between Anomalocaris and true arthropods. There is a great evolutionary gap between the two that can hardly be bridged. This gap has become a crucial "missing link" in the origin of arthropods.

The research team conducted detailed anatomical examinations of the fossils of Kylinxia. They demonstrated that the first appendages in Anomalocaris and true arthropods were homologous. The phylogenetic analyses suggested that there was affinity between the front appendages of Kylinxia, small predatory appendages in front of the mouth of Chelicerata (a group that includes spiders and scorpions) and the antennae of Mandibulata (a subdivision of arthropods including insects such as ants and bees).

"Our results indicate that the evolutionary placement of Kylinxia is right between Anomalocaris and the true arthropods. Therefore, our finding reached the evolutionary root of the true arthropods," said Prof. ZHU Maoyan, a co-author of the study.

"Kylinxia represents a crucial transitional fossil predicted by Darwin's evolutionary theory. It bridges the evolutionary gap from Anomalocaris to true arthropods and forms a key "missing link" in the origin of arthropods, contributing strong fossil evidence for the evolutionary theory of life," said Dr. ZENG Han, first author of the study.

The research was funded by the Chinese Academy of Sciences and the National Natural Science Foundation of China.

原始論文:Han Zeng, Fangchen Zhao, Kecheng Niu, Maoyan Zhu & Diying Huang. An early Cambrian euarthropod with radiodont-like raptorial appendages. Nature, 2020. DOI: 10.1038/s41586-020-2883-7

引用自:Chinese Academy of Sciences. “A 520-million-year-old Five-eyed Fossil Reveals Arthropod Origin.”

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