2022年10月21日 星期五

魚類起源:揭開至留紀早期有頜脊椎動物的全身樣貌

 原文網址:https://english.cas.cn/newsroom/research_news/life/202209/t20220928_320888.shtml

科學家新發現了一座年代為43600萬年前,大約是至留紀早期的化石「寶庫」,其中首度揭曉了最初幾種有頜魚的完整形體與樣貌。

包含奇蹟秀山魚與蠕紋沈氏棘魚正模標本的石板。圖片來源:朱敏等人

由中國科學院古脊椎動物與古人類研究所(IVPP)的教授朱敏,以及烏普薩拉大學的教授Per E. Ahlberg主持的國際團隊928日在《自然》(Nature)發表了這項發現,是當期封面以及四篇系列論文之一。

有頜類(Gnathostomata,有頜的脊椎動物)不只包括了幾乎所有在動物園和水族館能看到的脊椎動物,就連人類也是其中一員。但牠們的起源卻是一道謎題:透過計算兩種動物DNA的差異來推導兩者最後的共同祖先生活在什麼年代的方法——「分子時鐘」透露出所有現生的有頜脊椎動物,最後的共同祖先生活在45000萬年前的奧陶紀,因此上下頜的起源不可能比這更晚。

然而,有頜脊椎動物的化石紀錄在泥盆紀早期(大約為41900萬年前),也就是「魚類時代」開始之時才變得相當豐富。直到過去10年科學家才找到幾具來自志留紀晚期(大約是42500萬年前)的完整有頜魚化石。即便如此,這些化石紀錄仍然比上下頜應該要出現的年代晚了2500萬年以上。有頜脊椎動物初期的化石相當稀少,意味牠們在志留紀早期是所謂的「幽靈譜系」(ghost lineage)

這篇志留紀早期完整的有頜魚化石是相當驚人的發現。作者20年來在中國志留紀所有可能的岩層中,持續不懈地尋找魚類化石。他們最終在2020年末有所斬獲,於重慶秀山縣找到了完整的志留紀早期魚類化石。

身為共同作者與野外調查主持人的李強和陳陽回憶起他們的研究過程:「還記得當天下著雨,我們沿著一條山路前進。然後在第38個彎道處,我們發現了一條來自志留紀的完整魚類——它開啟了後續兩年在這個地區得到的爆炸性發現。」

作者發表的魚類有兩種。第一種也是數量最多的被命名為奇蹟秀山魚(Xiushanosteus mirabilis),是一種3公分長的小型盾皮魚(placoderm,有盔甲的有頜魚)。牠扁平的半圓形頭部、覆有盔甲的身軀都讓人聯想到牠的無頜魚祖先,但是秀山魚成對的鰭以及有力的尾巴都指出牠更擅長游泳。

第一作者朱幼安是IVPP的副研究員以及烏普撒拉大學的校友,他說:「身為一個研究盾皮魚的專家,秀山魚的年代之早以及完整程度都讓我目眩神迷,猶如夢想成真一般。牠的許多解剖特徵都讓我感到如此完美,當時就像是『哇,我知道這就是我研究生涯的關鍵時刻。』」

奇蹟秀山魚的復原圖。由Zhang Heming所繪。

他們發表的第二種魚類命名為蠕紋沈氏棘魚(Shenacanthus vermiformis),體型同樣很小的牠是一種早期的鯊魚近親。雖然我們所知的鯊魚身上覆蓋的鱗片都很細小,頂多只有零星分布的小骨板,然而沈氏棘魚卻擁有相當明顯、由數片大骨板組成而環繞身體整整一圈的「肩甲」。這種之前認為只有盾皮魚才有的特徵,強烈暗示了最早的軟骨魚就像盾皮魚一樣身上也覆有鎧甲。

「不過在20年前我們還認為鯊魚是原始的魚類,其他的有頜魚都是從狀似鯊魚的原型演化而成。現在有了沈氏棘魚這項發現之後,我們終於可以確定反過來說才是對的,」朱幼安表示。

蠕紋沈氏棘魚的復原圖。由Zhang Heming所繪。

「保存如此良好的遠古化石對於先前的我們來說只是個夢想,」通訊作者Ahlberg教授表示。「然而它們不只滿足了我們的好奇心,也是前所未聞且極為重要的資料來驗證關於人類的譜系如何崛起這方面,長久以來我們的假說是否正確——不論它們是加以支持或是帶來更多疑問。」

「我們仍在挖掘出令人驚訝的化石,」主持這項計畫的中國科學院院士朱敏教授表示。「重慶化石庫將會跟澄江與熱河動物群一樣,成為舉世聞名的古生物學遺址,並且提供關鍵證據來讓我們了解今日所見的有頜脊椎動物,為何能夠發展出如此非凡的多樣性。」

Dawn of Fishes—Early Silurian jawed vertebrates revealed head to tail

A newly discovered fossil "treasure hoard" dating back some 436 million years to the early Silurian period reveals, for the first time, the complete body shape and form of some of the first jawed fishes.

The discovery was published in Nature on Sept. 28 by an international team led by Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and Prof. Per E. Ahlberg from Uppsala University, as the cover story and one in a series of four papers in the same issue.

The Gnathostomata or jawed vertebrates, which include not only almost all the backboned animals you see in zoos and aquariums but humankind as well, have a mysterious origin. The so-called molecular clock, which deduces the age of the most recent common ancestor of two animals by evaluating the difference between the two sets of DNA, suggests that the most recent common ancestor of all modern jawed vertebrates lived 450 million years ago during the Ordovician period. As a result, the origin of jaws cannot be later than that.

However, the fossil record of jawed vertebrates only becomes abundant from the Early Devonian (~419 million years ago), i.e., the beginning of the "Age of Fishes." Only in the past 10 years have scientists found several complete jawed fishes from the Late Silurian (~425 million years ago). Even so, these records are still more than 25 million years later than when jaws should have originated. The dearth of earlier fossils means that jawed vertebrates are a " ghost lineage " in the early Silurian.

The remarkable discovery of complete early Silurian jawed fishes is the result of 20 years of continuous effort by the authors searching for fossil fishes in all possible Silurian rock strata in China. The breakthrough was finally made in late 2020, when complete early Silurian fishes were found in Xiushan County, Chongqing.

LI Qiang and CHEN Yang, both co-authors and leaders of the fieldtrips, recalled their research: "We remember it was a rainy day. We climbed a mountain ghat. At the 38th turn we found a complete Silurian fish, which initiated an explosion of discoveries in this area in the next two years."

The authors reported two species. The first one and the most abundant species was named Xiushanosteus mirabilis. It is a tiny, 3-cm-long placoderm or armored jawed fish. The flat and semicircular head, along with the trunk armor, are reminiscent of its jawless ancestors, but its paired fins and powerful tail made Xiushanosteus a much more capable swimmer.

First author ZHU You'an, associate research professor at IVPP and also an Uppsala University alumnus, said, "As a placoderm expert, I am dazzled by the early age and completeness of Xiushanosteus. It is like a dream. A lot of the anatomical features make perfect sense; it was an 'Oh, now I know' moment in my career."

The second fish reported is named Shenacanthus vermiformis. Also very small, it is an early shark relative. However, all the sharks we know are covered in tiny scales, or at most small mosaic plates. Shenacanthus instead has prominent "shoulder armor" made of several large plates that completely encircle its body. This feature, thought to be exclusive in placoderms, provides a strong hint that the first cartilaginous fishes were armored, similar to placoderms.

"Only 20 years ago it was still believed that sharks are primitive and other jawed fish evolved from a shark-like archetype. Now with the discovery of Shenacanthus, we can finally make certain that the opposite is true," said Prof. ZHU You'an.

"Previously we could only dream of such exceptional and early fossils," said corresponding author Prof. Ahlberg. "However, they are more than curiosities; they are first and foremost crucial data to test—and either support or confound—our long-held hypotheses regarding the rise of our lineage."

"The excavation continues to yield remarkable materials," said Prof. ZHU Min, who led the project and is also a CAS academician. "The Chongqing Lagerstätte, like the Chengjiang and Jehol biotas, will become a world-famous paleontological heritage and will provide key evidence for how the extraordinary diversity of the jawed vertebrates we see today arose."

原始論文:You-an Zhu, Qiang Li, Jing Lu, Yang Chen, Jianhua Wang, Zhikun Gai, Wenjin Zhao, Guangbiao Wei, Yilun Yu, Per E. Ahlberg & Min Zhu. The oldest complete jawed vertebrates from the early Silurian of China. Nature, 2022 DOI: 10.1038/s41586-022-05136-8

引用自:Chinese Academy of Sciences. “Dawn of Fishes—Early Silurian jawed vertebrates revealed head to tail.”

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