恐龍在滅亡之前已邁入衰敗

 原文網址：http://www.bristol.ac.uk/news/2021/june/dinosaurs-in-decline-.html

恐龍在6600萬年滅亡的原因是一顆巨大的小行星撞擊地球。然而，古生物學家之間仍持續爭論著恐龍在撞擊之前是否就已經逐漸衰亡。

圖表顯示在恐龍活著的最後一千萬年，種化速率(藍線)有所下降，滅絕速率(紅線)則快速上升。兩者結合之後可以得出物種數目(黑線)，顯示在6600萬年前的小行星撞擊事件不久之前，恐龍的種類就已經快速減少。圖片來源：Fabien L. Condamine

包括布里斯托大學的科學家在內的國際科學團隊，今日(6/29)發表在期刊《自然通訊》(Nature Communications)的新研究指出，恐龍可能早在最後的致命一擊來到的一千萬年前就已經開始衰亡。

任職於蒙彼利埃演化科學研究所的CNRS(法國國家科學研究中心)研究員Fabien Condamine是主要作者，他說：「我們觀察從1億5000萬年到6600萬年前整個白堊紀期間，六種數量最豐富的恐龍家族，結果發現牠們都在持續演化且變得更多，顯然牠們是相當成功的物種。」

「接著在7600萬年前，牠們的狀況急轉直下。滅亡的速率開始增加，在某些家族當中新物種的產生速率也下降許多。」

團隊利用貝氏模擬法來考量幾種不確定因素，像是化石紀錄的缺失、化石定年的誤差以及演化模型的不確定性。為了顧慮所有可能的誤差來源，每個模型都會執行數百萬次並且找出這些分析是否可以歸納成一個可能性最高的結果。

同為蒙彼利埃演化科學研究所的Guillaume Guinot協助了計算的執行過程，他接著表示：「在所有的情況下，我們發現的證據都顯示在那顆火球撞擊之前恐龍就已經邁入衰亡了。」

「我們也探討了恐龍生態系的運作情形，結果清楚地顯示草食性物種往往是最先消失的，這讓最晚期的恐龍生態系變得不穩定，如果之後環境條件遭到破壞的話，整個生態系就會很容易崩潰。」

艾德蒙頓大學(位於加拿大亞伯達省)的Phil Currie是研究共同作者之一，他說：「我們運用了白堊紀期間超過1600筆經過仔細審核的恐龍紀錄。」

「我在北美洲、蒙古、中國以及其他地方蒐集恐龍化石已經有段時日，期間對於含有恐龍的岩層屬於什麼年代，我們在這方面的知識有了大幅進展。」

「這代表數據的品質正在與時漸進。恐龍在牠們的最後一千萬年逐漸衰亡是合理的推論；事實上，正如我們的研究呈現的，該段期間也是恐龍的化石紀錄中樣本最佳的部分。」

另外一位共同作者，布里斯托大學地球科學院的教授Mike Benton也說：「我們在分析的時候探討了可能導致恐龍衰減的不同因素。」

「結果清楚顯示主要原因有二：第一個是地球整體的氣候逐漸變冷，這讓需要較高溫度的恐龍越來越難生存。」

「再來是草食動物的消失使得生態系變得不穩定，造成物種容易接連滅絕。我們也發現壽命較長的恐龍比較容易滅亡，或許反映出牠們難以適應新的環境條件。」

Fabien Condamine補充：「這是生命演化史上的一個關鍵時刻。當時恐龍已經統治世界超過1億6000萬年，隨著牠們逐漸消失，其他的族群便開始崛起並掌管地球，當中也包括了哺乳類。」

「恐龍的體型是如此龐大，也許牠們從未曉得樹叢當中有身形矮小、毛茸茸的哺乳類。但是在恐龍消失之前哺乳類的種類就已經開始越變越多，使得牠們能在撞擊之後乘機建立起我們今日所見的新型生態系。」

 

Dinosaurs were in decline before the end

The death of the dinosaurs 66 million years ago was caused by the impact of a huge asteroid on the Earth. However, palaeontologists have continued to debate whether they were already in decline or not before the impact.

In a new study, published today in the journal Nature Communications, an international team of scientists, which includes the University of Bristol, show that they were already in decline for as much as ten million years before the final death blow.

Lead author, Fabien Condamine, a CNRS researcher from the Institut des Sciences de l’Evolution de Montpellier (France), said: “We looked at the six most abundant dinosaur families through the whole of the Cretaceous, spanning from 150 to 66 million years ago, and found that they were all evolving and expanding and clearly being successful.

“Then, 76 million years ago, they show a sudden downturn. Their rates of extinction rose and in some cases the rate of origin of new species dropped off.”

The team used Bayesian modelling techniques to account for several kinds of uncertainties such as incomplete fossil records, uncertainties over age-dating the fossils, and uncertainties about the evolutionary models. The models were each run millions of times to consider all these possible sources of error and to find whether the analyses would converge on an agreed most probable result.

Guillaume Guinot, also of the Institut des Sciences de l’Evolution de Montpellier, who helped run the calculations, added: “In all cases, we found evidence for the decline prior to the bolide impact.

“We also looked at how these dinosaur ecosystems functioned, and it became clear that the plant-eating species tended to disappear first, and this made the latest dinosaur ecosystems unstable and liable to collapse if environmental conditions became damaging.”

Phil Currie, a co-author of the study, from the University of Edmonton (Alberta, Canada), said: “We used over 1,600 carefully checked records of dinosaurs through the Cretaceous.

“I have been collecting dinosaurs in North America, Mongolia, China, and other areas for some time, and I have seen huge improvements in our knowledge of the ages of the dinosaur-bearing rock formations.

“This means that the data are getting better all the time. The decline in dinosaurs in their last ten million years makes sense, and indeed this is the best-sampled part of their fossil record as our study shows.”

Professor Mike Benton from the University of Bristol’s School of Earth Sciences, another co-author, added: “In the analyses, we explored different kinds of possible causes of the dinosaur decline.

 “It became clear that there were two main factors, first that overall climates were becoming cooler, and this made life harder for the dinosaurs which likely relied on warm temperatures.

“Then, the loss of herbivores made the ecosystems unstable and prone to extinction cascade. We also found that the longer-lived dinosaur species were more liable to extinction, perhaps reflecting that they could not adapt to the new conditions on Earth.”

Fabien Condamine added: “This was a key moment in the evolution of life. The world had been dominated by dinosaurs for over 160 million years, and as they declined other groups began their rise to dominance, including the mammals.

“The dinosaurs were mostly so huge they probably hardly knew that the furry little mammals were there in the undergrowth. But the mammals began to increase in numbers of species before the dinosaurs had gone, and then after the impact they had their chance to build new kinds of ecosystems which we see today.”

原始論文：Fabien L. Condamine, Guillaume Guinot, Michael J. Benton, Philip J. Currie. Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-23754-0

引用自：University of Bristol. "Dinosaurs were in decline before the end, according to new study"