在史上最大滅絕事件中，魚的體型大小並非存活關鍵

原文網址：http://www.bris.ac.uk/news/2017/june/fish-extinction-.html

在史上最大滅絕事件中，魚的體型大小並非存活關鍵

瞭解現今的生物多樣性和生物面臨的存亡威脅是相當重要的。科學家一般認為在滅絕危機發生時，較大的體型會讓生物的處境變得更加危險。

然而，布里斯托大學和中國地質調查局成都地質調查中心的研究人員，卻發現在史上最嚴重的滅絕事件中，魚類的存亡與否跟體型大小沒有任何關係。

研究聚焦在2.52億年前二疊紀―三疊紀大滅絕事件中硬骨魚的演化歷程。在此危機期間，俄羅斯發生的巨型火山噴發導致了氣候劇烈變遷，地球上所有物種多達90%因而滅亡。

火山噴發釋放的氣體讓全世界皆下起了酸雨，並讓大氣的溫度升高將近20℃。這使得陸地上的植物死亡，土壤因此受到雨水沖刷而大量流入海中。同時海洋也大幅暖化，使得生物逃離赤道。

科學家預期滅絕事件的一項重要特徵是體型：大型動物會率先因高熱與飢荒帶來的壓力所苦。然而，在今日刊登於《古生物學》(Palaeontology)的新論文中，卻顯示出大型魚類滅亡的可能性並不會比小型魚類來得高。

研究詳細彙整了從滅絕事件發生許久之前到結束很久之後，長達1億年間跟魚類化石有關的任何資訊。他們從中辨識出超過750種魚類的體型資訊，並且利用多種計算方式來將演化樹結構以及物種定年結果的變異性列入考量。結果相當清楚――在危機發生時，體型並沒有提供給魚類任何優勢或劣勢。

主要作者，英國倫敦自然史博物館和布里斯托大學地球科學院的博士Mark Puttick如此解釋：「這些結果延續了近期研究的趨勢，它們皆提出在決定哪些物種得以倖存或走向毀滅時，身體大小無關緊要。雖然此結果跟人們預期的恰好相反，但卻更加支持了前人研究顯示的，在滅絕選擇性中體型沒有任何作用。」

團隊檢索了在進行此類分析時數據最豐富的資料庫，同時運用多種演化計算模型來瞭解許久以前魚類的演化模式。這些模型也將化石資料的品質以及建構演化樹時產生的不確定性考慮進去，分析出來的結果相當明確。

同為布里斯托大學的教授Michael Benton補充：「成果令人感到相當興奮。同樣重要的一點是，我們可以利用新方法來進行有眾多不確定因素的研究。」

「建構出相當詳盡的演化樹是這些新方法的運作基礎。因此，不同於以往的研究大多數是在野外進行，我們將心力著重在列入考量的所有物種彼此之間的關係上。」

中國地質調查局的負責人胡世學博士表示：「很高興看到這些新的分析成果。我們成功納入了從中國保存格外良好的生物群中出土的眾多新化石，使得我們可以完整看出滅絕事件造成的衝擊，以及後續三疊紀時生物的回復過程。」

Size not important for fish in the largest mass extinction of all time

Understanding modern biodiversity and extinction threats is important. It is commonly assumed that being large contributes to vulnerability during extinction crises.

However, researchers from the University of Bristol and the Chengdu Center of the China Geological Survey, have found that size played no role in the extinction of fish during the largest mass extinction of all time.

The study focused on the evolution of bony fishes during the Permian-Triassic mass extinction 252 million years ago. During this crisis, as many as 90 percent of all species on Earth were killed by massive climate change triggered by huge volcanic eruptions in Russia.

The erupted gases led to worldwide acid rain and atmospheric warming of as much as 20 degrees centigrade. This killed plants, and soil was stripped by rainfall and washed into the sea. Oceans were also heated and life fled from the tropics.

It was expected that a key feature in extinction would have been body size: the large animals would suffer heat and starvation stress first. However, in the new paper, published today in Palaeontology, it is shown that larger fish were no more likely to go extinct than small fish.

The study used a detailed summary of all information on fossil fish through a span of over 100 million years, from well before to well after the disaster. Body size information was identified for over 750 of these fishes, and multiple calculations were carried out to allow for variations in the shape of the evolutionary tree and the exact dating of all the species. The result was clear – body size did not provide any advantages or disadvantages to fish during the crisis.

Lead researcher Dr Mark Puttick from the Natural History Museum and University of Bristol’s School of Earth Sciences, explained: "These results continue the trend of recent studies that suggest body size played no role in determining which species survive or go extinct. This is the opposite result we would expect, but provides increasing support for previous studies that show body size plays no role in extinction selectivity."

The team explored the largest dataset used in an analysis of this type and applied a range of computational evolutionary models to understand these patterns in deep time. The models take account of uncertainties in the quality of the fossil data and the reconstructed evolutionary tree, and the result was clear.

Professor Michael Benton, also from the University of Bristol, added: "These are exciting results. What is important also is that we were able to deploy new methods in the study that take greater account of uncertainties.

"The methods are based around a detailed evolutionary tree so, unlike most previous work in the field, we paid attention to the relationships of all the species under consideration."

Professor Shixue Hu, leader of the China Geological Survey: "It's great to see this new analytical work. We were able to include many new fossils from our exceptional biotas in China, and we can see the full impact of the extinction and the subsequent recovery of life during the Triassic."

原始論文：Mark N. Puttick, Jürgen Kriwet, Wen Wen, Shixue Hu, Gavin H. Thomas, Michael J. Benton. Body length of bony fishes was not a selective factor during the biggest mass extinction of all time. Palaeontology, 2017; DOI: 10.1111/pala.12309

引用自：University of Bristol. "Size not important for fish in the largest mass extinction of all time."