現代的動物可能起源自三角洲

 原文網址：https://www.exeter.ac.uk/research/news/articles/modernanimallifecouldhave.html

新研究提出今日存活的許多動物的祖先，可能曾經生活在中國的一座三角洲。

節肢動物(娜羅蟲)的化石。圖片來源：Xiaoya Ma博士

在五億多年前的寒武紀大爆發可以看到兩側對稱動物迅速地擴展，這類生物的身體構造沿著一條中心線而左右對稱，就像今日大部分的動物(包括人類在內)。

位在中國西南方的雲南省，年代為5億1800萬年的澄江生物群是科學家目前所知最古老的動物化石群之一，也是寒武紀大爆發的關鍵紀錄。

此處發現的化石超過250種，包括各種蠕蟲、節肢動物(現生蝦蟹、昆蟲、蜘蛛、蠍子的祖先)，甚至還有最早的脊椎動物(魚類、兩棲類、爬蟲類、鳥類和哺乳類的祖先)。

新研究首次發現當時這裡的環境是座位在淺海、養分充足的三角洲，並且會受到風暴帶來的洪水影響。

雖然此區目前位於群山遍布的雲南省內陸，但是團隊研究岩芯樣品之後發現的證據，顯示了這裡的環境過往會受到海流的影響。

「現在普遍認定寒武紀大爆發毫無疑問地是場劇烈的演化事件，但它的成因長久以來仍未定論，相關的假說包括環境、基因或者生態因素所引發，」資深作者Ma Xiaoya博士表示。他是艾希特大學與雲南大學的古生物學家。

「發現澄江生物群所處的環境是三角洲，讓我們能從全新的角度來理解由兩側對稱動物主導的海洋生物群集為什麼會在寒武紀崛起，以及它們的軟組織為什麼可以保存得這麼完整。」

「不穩定的環境帶來的壓力，或許也造成早期動物發生輻射適應的現象。」

共同主要作者，雲南大學的沉積學家與埋藏學家Farid Saleh表示：「我們從為數眾多的沉積物流之間的關係，看出澄江生物群所處的環境十分複雜。過往文獻曾提出類似的動物群集居住在什麼深度，但我們很肯定此處要來得更淺。」

另一位共同主要作者，雲南大學的地球化學家Changshi Qi也說：「我們的研究顯示澄江生物群居住的主要環境是氧氣充足、水深較淺的三角洲。」

「風暴造成的洪水把這些生物往下帶到附近較深且缺氧的環境，造成我們現在看到它們保存得十分完好。」

共同作者，薩斯喀徹溫大學的古生物學家暨沉積學家Luis Buatois表示：「如同類似動物群在別處描述的情況，澄江生物群也是保存在細顆粒的沉積物當中。」

「關於這些泥質沉積物是如何形成，我們的理解在過去15年來發生了劇烈的轉變。」

「未來在研究含有保存相當良好的化石的沉積物時，應用這份最新取得的知識，可以大幅改變我們對於這類沉積物的形成過程與地點的認知。」

新研究的結果重要之處在於證實了最早的動物可以忍受嚴苛的環境條件，像是變動的鹽分以及大量堆積的沉積物。

這和先前的研究不同――它們提出類似的動物是居住在水深較深，較為穩定的海洋環境。

「難以相信這些動物可以處理如此嚴苛的環境條件，」薩斯喀徹溫大學的古生物學家M. Gabriela Mángano表示。她曾經研究位在加拿大、摩洛哥和格陵蘭，其他化石保存狀況良好的著名場址。

薩斯喀徹溫大學的博士後研究員Maximiliano Paz是細顆粒系統的專家，他補充：「由於澄江地區的岩石露頭有受到風化，因此通常難以觀察到岩石的細節，但是取得沉積物岩芯讓我們可以辦到這件事。」

這項研究是國際合作的成果，成員包括雲南大學、艾希特大學、薩斯喀徹溫大學、中國國家科學院、洛桑大學與萊斯特大學。

研究經費來自中國博士後科學基金會、中國國家科學基金會、古生物學和地層學國家重點實驗室、加拿大自然科學暨工程研究委員會、George J. McLeod地質學推動計畫。

此論文發表於期刊《自然通訊》(Nature Communications)，標題為「澄江生物群居住的環境為三角洲」(The Chengjiang Biota inhabited a deltaic environment)。

 

Modern animal life could have origins in delta

The ancestors of many animal species alive today may have lived in a delta in what is now China, new research suggests.

The Cambrian Explosion, more than 500 million years ago, saw the rapid spread of bilaterian species – symmetrical along a central line, like most of today's animals (including humans).

The 518-million-year-old Chengjiang Biota – in Yunnan, south-west China – is one of the oldest groups of animal fossils currently known to science, and a key record of the Cambrian Explosion.

Fossils of more than 250 species have been found there, including various worms, arthropods (ancestors of living shrimps, insects, spiders, scorpions) and even the earliest vertebrates (ancestors of fish, amphibians, reptiles, birds and mammals).

The new study finds for the first time that this environment was a shallow-marine, nutrient-rich delta affected by storm-floods.

The area is now on land in the mountainous Yunnan Province, but the team studied rock core samples that show evidence of marine currents in the past environment.

"The Cambrian Explosion is now universally accepted as a genuine rapid evolutionary event, but the causal factors for this event have been long debated, with hypotheses on environmental, genetic, or ecological triggers," said senior author Dr Xiaoya Ma, a palaeobiologist at the University of Exeter and Yunnan University.

"The discovery of a deltaic environment shed new light on understanding the possible causal factors for the flourishing of these Cambrian bilaterian animal-dominated marine communities and their exceptional soft-tissue preservation.

"The unstable environmental stressors might also contribute to the adaptive radiation of these early animals."

Co-lead author Farid Saleh, a sedimentologist and taphonomist at Yunnan University, said: "We can see from the association of numerous sedimentary flows that the environment hosting the Chengjiang Biota was complex and certainly shallower than what has been previously suggested in the literature for similar animal communities."

Changshi Qi, the other co-lead author and a geochemist at the Yunnan University, added: "Our research shows that the Chengjiang Biota mainly lived in a well-oxygenated shallow-water deltaic environment.

"Storm floods transported these organisms down to the adjacent deep oxygen-deficient settings, leading to the exceptional preservation we see today."

Co-author Luis Buatois, a paleontologist and sedimentologist at the University of Saskatchewan, said: "The Chengjiang Biota, as is the case of similar faunas described elsewhere, is preserved in fine-grained deposits.

"Our understanding of how these muddy sediments were deposited has changed dramatically during the last 15 years.

"Application of this recently acquired knowledge to the study of fossiliferous deposits of exceptional preservation will change dramatically our understanding of how and where these sediments accumulated."

The results of this study are important because they show that most early animals tolerated stressful conditions, such as salinity (salt) fluctuations, and high amounts of sediment deposition.

This contrasts with earlier research suggesting that similar animals colonised deeper-water, more stable marine environments.

"It is hard to believe that these animals were able to cope with such a stressful environmental setting," said M. Gabriela Mángano, a palaeontologist at the University of Saskatchewan, who has studied other well-known sites of exceptional preservation in Canada, Morocco, and Greenland.

Maximiliano Paz, a postdoctoral fellow at the University of Saskatchewan who specializes in fine-grained systems, added: "Access to sediment cores allowed us to see details in the rock which are commonly difficult to appreciate in the weathered outcrops of the Chengjiang area."

This work is an international collaboration between Yunnan University, University of Exeter, the University of Saskatchewan, the Chinese Academy of Sciences, the University of Lausanne, and the University of Leicester.

The research was funded by the Chinese Postdoctoral Science Foundation, the Natural Science Foundation of China, the State Key Laboratory of Palaeobiology and Stratigraphy, the Natural Sciences and Engineering Research Council of Canada, and by the George J. McLeod Enhancement Chair in Geology.

The paper, published in the journal Nature Communications, is entitled: "The Chengjiang Biota inhabited a deltaic environment."

原始論文：Farid Saleh, Changshi Qi, Luis A. Buatois, M. Gabriela Mángano, Maximiliano Paz, Romain Vaucher, Quanfeng Zheng, Xian-Guang Hou, Sarah E. Gabbott, Xiaoya Ma. The Chengjiang Biota inhabited a deltaic environment. Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-29246-z

引用自：University of Exeter. "Modern animal life could have origins in delta."