運用統計學分析恐龍多樣性的新結果，顯示6600萬年前小行星撞擊地球，造成恐龍滅亡之前，當時的恐龍其實還是相當強盛。
在6600萬年前白堊紀結束，小行星撞擊地球的時候，泰坦巨龍是當時相當常見的恐龍。圖片來源：AlienCat
巴斯大學和倫敦自然史博物館的研究人員表示如果撞擊事件沒有發生，恐龍或許還能繼許主宰地球。
在白堊紀結束，小行星撞擊地球的時候，全世界都有恐龍的蹤跡。牠們佔據了地球的每一片大陸，同時也是大部分的陸地生態系中最主要的動物種類。
然而，滅絕發生的時候恐龍的多樣性是否早就開始下降，在古生物學家之間仍然沒有定論。
統計模型
研究團隊為了處理這項問題，牠們蒐集了幾種不同恐龍的家族樹，接著運用統計模型來評估每一個當時主要的恐龍族群，是否還能繼續產生新的種類。
他們的研究發表在期刊《皇家學會開放科學》(Royal Society Open Science)。跟一些前人研究相反，他們發現恐龍在小行星撞擊之前並未減少。作者也認為如果沒有發生撞擊事件，恐龍或許仍然是地球的陸地動物中最主要的族群。
Joe Bonsor是這項研究的主要作者，他目前是巴斯大學米爾納演化中心和自然史博物館聯合指導的博士生。
他說：「之前其他人利用不同研究方法得出的結論，都指出恐龍在白堊紀結束時已是強弩之末，無論如何都將走向滅亡一途。」
「然而，我們證明如果將分析時的數據納入更多近期得到的恐龍家族樹，並且涵蓋更多的恐龍類型，結果顯示並非所有恐龍都會邁入這種結局。實際上，大約只有一半的恐龍會是如此。」
取樣偏差
評估恐龍的多樣性並不容易，原因是化石紀錄有所缺失。其中的影響因素包括了哪些骨頭成功成為化石保存下來，化石在岩石裡的保存方式有多容易讓人發現，以及古生物學家選擇在哪裡尋找化石。
研究人員運用統計方法來克服這些取樣偏差。他們探討每一科恐龍的種化速率，而不是只有計算屬於該科的恐龍有多少種。
Joe Bonso說：「我們論文的重點是不能只探討幾棵家族樹就做出結論。化石紀錄中無法避免的重大偏差加上資料不足，時常會讓物種的數量看起來正在減少，但是這可能並未反映出當時真正的情況。」
「我們可能永遠無法知道恐龍真實的演化速率，因為唯一確定可以得到最佳答案的方法是把化石紀錄裡的缺失填補起來，而我們完全不認為這能很快達成。」
「我們目前的數據顯示牠們並未減少，事實上像鴨嘴龍和角龍之類的某些恐龍當時還相當繁盛。沒有證據指出如果6600萬年滅絕事件沒有發生的話，牠們還是會徹底消失。」
雖然小行星撞擊時哺乳類就已經存在，但是唯有恐龍滅絕才能讓牠們佔據的生態棲位空出來，使得哺乳類可以取而代之，繼而主宰地球。
研究經費來自利華休姆信託與自然史博物館。
New analysis
refutes claim that dinosaurs were in decline before asteroid hit
A new statistical analysis of dinosaur
diversity shows they were not in decline at the time of their extinction by an
asteroid hit 66 million years ago.
Researchers from the University of Bath and the
Natural History Museum London say that had the impact not happened, dinosaurs
might have continued to dominate the Earth.
Dinosaurs were widespread globally at the time of the
asteroid impact at the end of the Late Cretaceous period, occupying every
continent on the planet and were the dominant form of animal of most
terrestrial ecosystems.
However, it is still contentious amongst
paleobiologists as to whether dinosaurs were declining in diversity at the time
of their extinction.
Statistical
modelling
In order to address this question, the research team
collected a set of different dinosaur family trees and used statistical
modelling to assess if each of the main dinosaur groups was still able to
produce new species at this time.
Their study, published in the journal Royal Society Open Science, found that
dinosaurs were not in decline before the asteroid hit, contradicting some
previous studies. The authors also suggest that had the impact not occurred,
dinosaurs might have continued to be the dominant group of land animals on the
planet.
Lead author of the study, Joe Bonsor, is undertaking
his PhD jointly at the Milner Centre for Evolution at the University of Bath
and the Natural History Museum.
He said: “Previous studies done by others have used
various methods to draw the conclusion that dinosaurs would have died out
anyway, as they were in decline towards the end of the Cretaceous period.
“However, we show that if you expand the dataset to
include more recent dinosaur family trees and a broader set of dinosaur types,
the results don’t actually all point to this conclusion - in fact only about
half of them do.”
Sampling bias
It is difficult to assess the diversity of dinosaurs
due to gaps in the fossil record. This can be due to factors such as which
bones are preserved as fossils, how accessible the fossils are in the rock to
allow them to be found, and the locations where palaeontologists search for
them.
The researchers used statistical methods to overcome
these sampling biases, looking at the rates of speciation of dinosaur families
rather than simply counting the number of species belonging to the family.
Joe Bonsor said: “The main point of our paper is that
it isn’t as simple as looking at a few trees and making a decision - The large
unavoidable biases in the fossil record and lack of data can often show a
decline in species, but this may not be a reflection of the reality at the
time.
“We may never know the true rates of evolution of the
dinosaurs, since the only way to know for sure is to fill in the gaps in the
record to get the best answer, and we simply don’t think we’re there just yet.
“Our data don’t currently show they were in decline,
in fact some groups such as hadrosaurs and ceratopsians were thriving and
there’s no evidence to suggest they would have died out 66 million years ago
had the extinction event not happened.”
Whilst mammal existed at the time of the asteroid
hit, it was only due to the extinction of the dinosaurs that led to the niches
being vacated, allowing mammals to fill them and later dominate the planet.
The research was funded by the Leverhulme Trust and
Natural History Museum.
原始論文：Joseph A.
Bonsor, Paul M. Barrett, Thomas J. Raven, Natalie Cooper. Dinosaur
diversification rates were not in decline prior to the K-Pg boundary. Royal
Society Open Science, 2020; 7 (11): 201195 DOI: 10.1098/rsos.201195
引用自：University of Bath. "New analysis refutes
claim that dinosaurs were in decline before asteroid hit."
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