在小行星撞擊而滅絕之前，恐龍仍然相當繁榮
在小行星撞擊而突然消失之前，恐龍仍然相當繁榮，並未受到長期氣候變遷的影響。
重建麥斯特里希特期(6600萬年前)北美大陸的古環境。生活這片氾濫平原上的恐龍有暴龍、埃德蒙頓龍、三角龍等。圖片來源：Davide Bonadonna
恐龍在6600萬年前白堊紀結束時全數消失。科學家大都同意原因是一顆小行星撞上地球，或許還伴隨著強烈的火山活動。
然而，有爭議的地方是恐龍在此之前仍然相當強盛嗎？數百萬年來的長期氣候變遷是否讓牠們逐漸減少？
之前的研究利用化石紀錄和數學模型的預測結果，認為小行星撞擊前恐龍的數量以及物種多樣性都正在減少。
不過，英國的倫敦帝國學院、倫敦大學學院和布里斯托大學的研究人員最近運用新的模型，分析當時北美洲的環境變遷以及恐龍種類的分布情形之後，認為小行星撞擊前恐龍可能並未衰減。
註定走向滅絕？
帝國學院地球科學與工程系的博士生Alessandro Chiarenza是研究主要作者，他說：「恐龍本來可能不會走上滅絕一途，但白堊紀結束時卻有一顆小行星撞上地球，終結了恐龍的統治，使地球的主宰變成其他動物，像是哺乳類、蜥蜴和一小群倖存下來的恐龍：鳥類。」
「我們的研究結果指出恐龍整體來說是適應力相當強的動物，足以應付白堊紀晚期最後數百萬年的環境變遷與氣候變化。當時的氣候變遷是在很長的時間尺度下發生，並未讓恐龍處於長期衰退。」
團隊的研究重點擺在北美大陸。這裡保存了許多白堊紀晚期的恐龍，像是暴龍與三角龍。當時北美大陸被一座廣大的內海分成兩半。
內海西半部有新形成的洛磯山脈穩定提供沉積物，使得環境十分有利於恐龍死後變成化石；反之，東半部的環境則不利於化石形成。
這代表從西半部找到的恐龍化石會比東半部多出不少。之前的研究利用這些化石紀錄，提出恐龍在小行星撞擊前的數百萬年已經處在衰退的狀態。
共同作者，倫敦大學學院的Philip Mannion博士評論：「之前我們對白堊紀晚期北美大陸恐龍的了解，多半是來自於不到這座大陸三分之一的地區。但現在我們知道從阿拉斯加到紐澤西，再往南到墨西哥，恐龍的足跡曾經踏遍了整座北美大陸。」
繪製生態棲位的分布
除了已知的化石紀錄之外，團隊還運用了「生態棲位模擬」來進行研究。這個方法模擬了每個物種生存所需的環境條件，像是溫度和雨量。
團隊接著繪製出隨著時間經過，北美大陸各個區域擁有什麼樣的環境條件。這可以讓研究人員了解隨著環境變化，不同種類的恐龍族群會生活在哪裡，而不是只依靠化石出土的地點來判斷。
團隊發現白堊紀末期可以支持許多種恐龍的棲地，實際分布範圍比過去認為得更加廣泛，但它們所處的地區比較難形成化石。
此外，這些可能擁有許多恐龍的棲地在任何地方的面積都不大，也會降低找到恐龍化石的可能性。
Dinosaurs
were thriving before asteroid strike that wiped them out
Dinosaurs were unaffected by long-term
climate changes and flourished before their sudden demise by asteroid strike.
Scientists largely
agree that an asteroid impact, possibly coupled with intense volcanic activity,
wiped out the dinosaurs at the end of the Cretaceous period 66 million years
ago.
However, there is
debate about whether dinosaurs were flourishing before this, or whether they
had been in decline due to long-term changes in climate over millions of years.
Previously,
researchers used the fossil record and some mathematical predictions to suggest
dinosaurs may have already been in decline, with the number and diversity of
species falling before the asteroid impact.
Now, in a new
analysis that models the changing environment and dinosaur species distribution
in North America, researchers from Imperial College London, University College London
and University of Bristol have shown that dinosaurs were likely not in decline
before the meteorite.
Not doomed to extinction
Lead
researcher Alessandro Chiarenza, a PhD student in the Department of Earth Science and
Engineering at Imperial, said: “Dinosaurs were likely not doomed to extinction
until the end of the Cretaceous, when the asteroid hit, declaring the end of
their reign and leaving the planet to animals like mammals, lizards and a minor
group of surviving dinosaurs: birds.
“The results of our
study suggest that dinosaurs as a whole were adaptable animals, capable of
coping with the environmental changes and climatic fluctuations that happened
during the last few million years of the Late Cretaceous. Climate change over
prolonged time scales did not cause a long-term decline of dinosaurs through
the last stages of this period.”
The study, published
today in Nature
Communications,
shows how the changing conditions for fossilisation means previous analyses
have underestimated the number of species at the end of the Cretaceous.
The team focused
their study on North America, where many Late Cretaceous dinosaurs are
preserved, such as Tyrannosaurus rex and Triceratops. During this period, the
continent was split in two by a large inland sea.
In the western half
there was a steady supply of sediment from the newly forming Rocky Mountains,
which created perfect conditions for fossilising dinosaurs once they died. The
eastern half of the continent was instead characterised by conditions far less
suitable for fossilisation.
This means that far
more dinosaur fossils are found in the western half, and it is this fossil
record that is often used to suggest dinosaurs were in decline for the few
million years before the asteroid strike.
Co-author Dr Philip
Mannion, from
University College London, commented: “Most of what we know about Late
Cretaceous North American dinosaurs comes from an area smaller than one-third
of the present-day continent, and yet we know that dinosaurs roamed all across
North America, from Alaska to New Jersey and down to Mexico.”
Mapping
niches
Instead of using
this known record exclusively, the team employed ‘ecological niche modelling’.
This approach models which environmental conditions, such as temperature and
rainfall, each species needs to survive.
The team then mapped
where these conditions would occur both across the continent and over time.
This allowed them to create a picture of where groups of dinosaur species could
survive as conditions changed, rather than just where their fossils had been
found.
The team found
habitats that could support a range of dinosaur groups were actually more
widespread at the end of the Cretaceous, but that these were in areas less
likely to preserve fossils.
Furthermore, these
potentially dinosaur-rich areas were smaller wherever they occurred, again
reducing the likelihood of finding a fossil from each of these areas.
原始論文：Alfio Alessandro Chiarenza, Philip D. Mannion, Daniel J. Lunt,
Alex Farnsworth, Lewis A. Jones, Sarah-Jane Kelland, Peter A. Allison. Ecological
niche modelling does not support climatically-driven dinosaur diversity decline
before the Cretaceous/Paleogene mass extinction. Nature
Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-08997-2
引用自：Imperial College London. "Dinosaurs were
thriving before asteroid strike that wiped them out."
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