地球最嚴重的大滅絕事件中，陸地生物開始相繼死亡的時間遠比海洋生物的更迭還早發生
By Robert Sanders
2億5200萬年前二疊紀結束時的大滅絕事件，是生物改朝換代的事件最劇烈的其中之一。最近重新定年南非和澳洲的化石地層之後，顯示這起事件的發生時間與過程在陸地和海洋有明顯的差異。
研究人員定年這座山丘(南非語稱作koppie)裡的火山灰沉積物。Loskop山丘的下部露出的地層為二疊紀末大滅絕事件前行成的(巴爾福層的Palingkloof段)，而上部的岩層則是在滅絕事件後沉積的(卡特堡層)。圖片來源：John
Geissman
對二疊紀晚期主要的動物相消失不久之後便出現的脊椎動物重新定年之後，顯示出陸地生態系的變化比海洋還早數十萬年發生，最後導致將近70%的陸生脊椎動物死亡。稍後才發生的海洋滅絕事件則可能歷經了數萬年，造成幾乎95%的海洋生物消失。
雖然大多數科學家認為二疊紀結束時的大滅絕事件，主要原因是現為西伯利亞的區域在數百萬年當中接連發生大型火山爆發事件。但是南半球的陸地生物和北半球的海洋生物是在不同的時間滅亡，顯示出兩者的直接原因並不相同。
「大部分的人都以為陸地和海洋生態系是在同一時間崩潰，而且南北半球的發生時間也一樣。」加州大學柏克萊分校整合生物學系的副教授，古植物學家Cindy
Looy表示。「南北半球並非同時發生巨變的這項事實，對於滅絕原因的假說具有重大影響。海洋生物的滅絕事件本身並沒有非得要和陸地生物的滅絕事件有同樣的機制或成因。」
滅絕事件是由臭氧層變薄導致？
Looy實驗室的成員之前以現生植物來進行實驗，探討保護地球的臭氧層大幅減薄是否會使植物暴露於輻射之下而滅絕。其他二疊紀結束至三疊紀開始之際全球發生的變化，像是氣候暖化、大氣裡的二氧化碳濃度上升、海洋酸化……等，也是可能的原因。
在陸上，最詳實紀錄二疊紀末脊椎動物滅絕的地點為盤古超大陸的南半部——岡瓦那大陸，我們現在所知的南極洲、澳洲、南美洲、澳洲都是由它分裂而成。南非的卡魯盆地位在岡瓦那大陸，此處顯示出當時大型草食動物族群的化石組合從二齒獸轉變成水龍獸(這些動物都已經滅絕)。
在海裡，最詳實記錄滅絕事件的地點位在北半球，尤其是中國的化石。或許最能代表二疊紀末滅絕事件的指標即為三葉蟲的全數消失。
為了更加精確地定出陸上滅絕事件發生的時間點，Looy參與的國際科學團隊從卡魯盆地保存良好的火山灰堆積物中取出鋯石晶體，然後用鈾鉛定年法得出它們的年代。Looy同時也是加州大學柏克萊分校古生物博物館的古植物研究員，以及該校大學與Jepson標本館的裸子植物研究員。她確認了用來定年的地層上方數公尺的沉積物中缺少了舌羊齒的花粉，證明二疊紀末岡瓦那大陸最主要的植物——種子蕨，在當時就已經滅絕了。
鋯石是一種微小的矽酸鹽類晶體，形成於火山內部上湧的岩漿當中，並隨著火山爆發而噴到大氣裡頭。他們的定年結果顯示這些鋯石的年代為2億5224萬年，比位於中國公認的二疊紀—三疊紀邊界的年代還早了三十萬年。代表南非這處被認為擁有二疊紀—三疊紀邊界的沉積岩，至少比公認的邊界還要早三十萬年形成。
內布拉斯加大學林肯分校的Christopher
Fielding和同僚一月發表的研究中，他們在澳洲記錄植物開始滅絕的岩層上方不遠處採集了火山灰堆積物，經過定年之後也得到了類似的年代，比公認的還要早了將近四十萬年。
「卡魯盆地是二疊紀末脊椎動物更迭過程的代表地點，但一直以來都沒有對它進行良好的定年。」Looy表示。「我們最新的鋯石定年結果顯示水獸龍化石帶的下界比海洋滅絕事件還早了數十萬年，而澳洲的研究也得出了類似的模式。這代表岡瓦那大陸動植物的更迭和北半球海洋生物經歷的危機是不同步的。」
「我們這些年來已經知道跟發生在海裡的大滅絕事件不同，陸地生物一直到三疊紀開始很久之後還是有出現幾波劇烈的變動。但令人驚訝的是，陸地生物演替的開始時間竟然比海洋的滅絕事件還要提早許多。」
Looy和國際科學團隊在論文最後總結：「我們應該更加深入地探討長興期(二疊紀最後一個時期)，甚至是三疊紀早期陸地生態系發生了哪些漸進、複雜且細微的轉變。」
Looy和同僚的研究成果3月19日發表在開放取用期刊《自然通訊》(Nature
Communications)。共同作者包括美國緬因州科爾比學院的Robert
Gastaldo、加拿大安大略省多倫多大學的Sandra
Kamo、南非普利托利亞地球科學理事會的Johann
Neveling、美國德州大學達拉斯分校的John
Geissman、美國麻州安默斯特學院的Anna
Martini。研究經費來自美國國家科學基金會。
In Earth’s largest extinction, land
die-offs began long before ocean turnover
The mass extinction at the end of the
Permian Period 252 million years ago — one of the great turnovers of life on
Earth — appears to have played out differently and at different times on land
and in the sea, according to newly redated fossils beds from South Africa and
Australia.
New ages for fossilized vertebrates that lived just
after the demise of the fauna that dominated the late Permian show that the
ecosystem changes began hundreds of thousands of years earlier on land than in
the sea, eventually resulting in the demise of up to 70% of terrestrial
vertebrate species. The later marine extinction, in which nearly 95% of ocean
species disappeared, may have occurred over the time span of tens of thousands
of years.
Though most scientists believe that a series of
volcanic eruptions, occurring in large pulses over a period of a million years
in what is now Siberia, were the primary cause of the end-Permian extinction,
the lag between the land extinction in the Southern Hemisphere and the marine
extinction in the Northern Hemisphere suggests different immediate causes.
“Most people thought that the terrestrial collapse
started at the same time as the marine collapse, and that it happened at the
same time in the Southern Hemisphere and in the Northern Hemisphere,” said
paleobotanist Cindy Looy, University of California, Berkeley, associate
professor of integrative biology. “The fact that the big changes were not
synchronous in the Northern and Southern hemispheres has a big effect on
hypotheses for what caused the extinction. An extinction in the ocean does not,
per se, have to have the same cause or mechanism as an extinction that happened
on land.”
Did loss of ozone
layer contribute to extinction?
Members of Looy’s lab have conducted experiments on
living plants to determine whether a collapse of Earth’s protective ozone layer
may have irradiated and wiped out plant species. Other global changes — a
warming climate, a rise in carbon dioxide in the atmosphere and an increase in
ocean acidification — also occurred around the end of the Permian period and
the beginning of the Triassic and likely contributed.
On land, the end-Permian extinction of vertebrates is
best documented in Gondwana, the southern half of the supercontinent known as
Pangea that eventually separated into the continents we know today as
Antarctica, Africa, South America and Australia. There, in the South African
Karoo Basin, populations of large herbivores, or plant eaters, shifted from the
Daptocephalus assemblage to the Lystrosaurus assemblage. These groups are now
extinct.
In the ocean, the extinction is best documented in
the Northern Hemisphere, in particular by Chinese fossils. The end-Permian
extinction is perhaps best associated with the demise of trilobites.
To improve on previous dates for the land extinction,
an international team of scientists, including Looy, conducted uranium-lead
dating of zircon crystals in a well-preserved volcanic ash deposit from the
Karoo Basin. Looy, who is also a curator of paleobotany at the campus’s Museum
of Paleontology and curator of gymnosperms at the University and Jepson Herbaria,
confirmed that sediments from several meters above the dated layer were devoid
of Glossopteris pollen, evidence that these seed ferns, which used to dominate
late Permian Gondwanan floras, became extinct around that time.
At 252.24 million years old, the zircons —
microscopic silicate crystals that form in rising magma inside volcanoes and
are spewed into the atmosphere during eruptions — are 300,000 years older than
dates obtained for the confirmed Permian-Triassic (P-T) boundary in China. This
means that the sediment layer assumed to contain the P-T boundary in South
Africa was actually at least 300,000 years too old.
Dates for an ash deposit in Australia, just above the
layers that document the initial plant extinction, similarly came in almost
400,000 years older than thought. That work was published in January by
Christopher Fielding and colleagues at the University of Nebraska in Lincoln.
“The Karoo Basin is the poster child for the
end-Permian vertebrate turnover, but until recently, it was not well-dated,”
Looy said. “Our new zircon date shows that the base of the Lystrosaurus zone
predates the marine extinction with several hundred thousand years, similar to
the pattern in Australia. This means that both the floral and faunal turnover
in Gondwana is out of sync with the Northern Hemisphere marine biotic crisis.
“For some years now, we have known that — in contrast
to the marine mass extinction — the pulses of disturbance of life on land
continued deep into the Triassic Period. But that the start of the terrestrial
turnover happened so long before the marine extinction was a surprise.”
In their paper, Looy and an international team of
colleagues concluded “that greater consideration should be given to a more
gradual, complex, and nuanced transition of terrestrial ecosystems during the
Changhsingian (the last part of the Permian) and, possibly, the early
Triassic.”
Looy and colleagues published their findings March 19
in the open access journal Nature
Communications. Her co-authors are Robert Gastaldo of Colby College in
Maine; Sandra Kamo of the University of Toronto in Ontario; Johann Neveling of
the Council for Geosciences in Pretoria, South Africa; John Geissman of the
University of Texas in Dallas and Anna Martini of Amherst College in
Massachusetts. The research was funded by the National Science Foundation.
原始論文：Robert A. Gastaldo, Sandra L. Kamo, Johann
Neveling, John W. Geissman, Cindy V. Looy, Anna M. Martini. The base of the Lystrosaurus
Assemblage Zone, Karoo Basin, predates the end-Permian marine extinction. Nature Communications,
2020; 11 (1) DOI: 10.1038/s41467-020-15243-7
引用自：University of California - Berkeley. "In
Earth's largest extinction, land animal die-offs began long before marine
extinction: New dates for fossils indicate land animal turnover extended for
hundreds of thousands of years."
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