恐龍時代的哺乳類彼此打壓

 原文網址：https://oumnh.ox.ac.uk/mammals-time-dinosaurs-held-each-other-back

發表在⟪當代生物學⟫(Current Biology)，由牛津大學自然史博物館、牛津大學和伯明罕大學的研究人員領導的新研究，利用新的方法分析哺乳類化石的多樣性之後得到了出人意料的結果：不論是在恐龍大滅絕之前或之後，現代哺乳類的主要競爭者都不是恐龍，而是其他哺乳類。

圖中為蒙古出土的大型戈壁尖尺獸(Gobiconodon)。在恐龍時代這類哺乳類的早期分支或許比現代哺乳類更具競爭力。圖片來源：Corbin Rainbolt

哺乳類似乎在非鳥類恐龍滅亡之後才開始多樣化，體型變得更大且開始嘗試新的飲食、運動以及生活方式。不同於過往的假說，這項研究對此現象的成因提出了新的見解，它指出了一則不同類群的哺乳類之間彼此競爭的複雜故事，同時也強調出利用最新的統計工具來檢驗現有的演化學說非常重要。

「在恐龍時代有很多令人大開眼界的哺乳類，像是會滑翔、游泳或者挖洞的類型，但它們都不屬於現代的哺乳類，而是來自哺乳類演化樹較初期的分支，」共同作者，牛津大學自然史博物館的研究人員Elsa Panciroli表示。「這些哺乳類大多跟非鳥類恐龍同時滅絕，於此同時現代的哺乳類也變得更大，並且開始探索新的食物與生活方式。我們的研究結果顯示滅絕事件之前，可能是較早發生輻射演化的哺乳類比現代哺乳類更具優勢，使得後者無法佔據這些特殊的生態區位。」

6600萬年前的大滅絕事件消滅了所有的非鳥類恐龍，此時哺乳類的類型也爆炸性地增長，現今活著的哺乳類大多數的起源都可以追溯至此。傳統上，科學家認為滅絕事件之前哺乳類生活在恐龍的陰影之下。過往推測哺乳類無法佔據那些已經被巨大的爬蟲類所佔據的生態區位，使得牠們的體型長期以來都較小，食物和生活方式也比較沒有特化的現象。似乎要在恐龍消失，這些生態區位空出來之後，哺乳類才能繁榮起來。

然而，新的統計方法可以分析不同類型的哺乳類的演化過程，在滅絕事前後受到了什麼樣的限制。這些方法可以辨認出在哪些時間點演化停止產生新的特徵，而開始形成別的譜系已經演化出來的特性。研究人員藉此能找出不同哺乳類演化途徑上的「限制」，得出牠們是在什麼時候與其他動物競爭而退出不同的生態區位。結果顯示現代哺乳類祖先的演化過程中，一直以來最大的限制或許並非來自恐龍，而是牠們關係最近的親戚。

研究觀察了與恐龍同期的所有哺乳類類型的解剖構造，包括了現代哺乳類的祖先therians。研究人員透過測量新的特徵的出現頻率，像是體型、骨頭和牙齒形狀的變化，以及大滅絕前後這些特徵出現的模式和時間，結果得出恐龍時代的現代哺乳類比牠們的近親更受壓抑。這意味著當現代哺乳類的近親正在嘗試更大的體型、不同的食物，以及爬樹或滑行等新的生活方式的時候，也把現代哺乳類給排除在外，使得牠們的體型一直都很小而且無法在棲地中佔據特殊地位。

「如果你預設恐龍是限制therians發展的主因，那你會覺得我們的結果是一派胡言，」這項研究的主持人，牛津大學的Neil Brocklehurst博士表示。「然而恐龍會選擇性地與therians競爭而讓其他哺乳類繁榮發展才是毫無道理。因此therians 是受到其他類型的哺乳類壓制看來才是對的。」

研究人員提出在現代哺乳類成功的道路中，其他哺乳類的滅亡扮演了更重要的腳色。為了得到更多證據支持這項說法，他們探討了不同哺乳類的體型大小，結果發現體型最小與最大的哺乳類在恐龍滅絕事件之後從限制中解放出來的程度都是一樣的，代表體型跟成功與否沒有太大關係。

共同作者，伯明罕大學的Gemma Benevento表示：

「與恐龍同期的哺乳類體重大部分都不到100公克，比所有的非鳥類恐龍都還小。因此這些體型最小的哺乳類應該不會直接和恐龍競爭。然而，在滅絕事件之後這些小型的哺乳類卻和較大的哺乳類一樣，多樣性都有大幅的增長。」

Brocklehurst 博士表示：

「古生物學正在經歷一場革命。我們可用的工具種類經過了大幅擴充，讓我們可以分析龐大的數據並且直接驗證我們的演化理論。過往關於哺乳類輻射演化的研究大部分著重在牠們的演化速率，但是分析牠們演化過程中所受的限制則給了我們全新的觀點。這些利用這些最尖端的分析方法所得出來的結果，使我們必須重新思考許多現有的理論。」

Mammals in the time of dinosaurs held each other back

A new study led by researchers from the Oxford University Museum of Natural History, University of Oxford and the University of Birmingham for Current Biology has used new methods to analyse the variability of mammal fossils, revealing extraordinary results: it was not dinosaurs, but possibly other mammals, that were the main competitors of modern mammals before and after the mass extinction of dinosaurs.

The study challenges old assumptions about why mammals only seemed to diversify, becoming larger and exploring new diets, locomotion and ways of life, after the extinction of the non-bird dinosaurs. It points to a more complex story of competition between distinct mammal groups. The new research also highlights the importance of testing old and established ideas about evolution using the latest statistical tools.

“There were lots of exciting types of mammals in the time of dinosaurs that included gliding, swimming and burrowing species, but none of these mammals belonged to modern groups, they all come from earlier branches in the mammal tree,” said Dr Elsa Panciroli, a researcher from the Oxford University Museum of Natural History and a co-author of the study. “These other kinds of mammals mostly became extinct at the same time as the non-avian dinosaurs, at which point modern mammals start to become larger, explore new diets and ways of life. From our research it looks like before the extinction it was the earlier radiations of mammals that kept the modern mammals out of these exciting ecological roles by outcompeting them”. 

Most of the mammal species alive today trace their origins to groups that expanded explosively 66 million years ago, when a mass extinction killed all non-bird dinosaurs. It was traditionally thought that, before the extinction, mammals lived in the shadow of the dinosaurs. They were supposedly prevented from occupying the niches that were already occupied by the giant reptiles, keeping the mammals relatively small and unspecialised in terms of diet and lifestyle. It appeared that they were only able to flourish after the dinosaurs’ disappearance left these niches vacant. 

However, new statistical methods were used to analyse how constrained different groups of mammals were in their evolution before and after the mass extinction. These methods identified the point where evolution stopped producing new traits and started producing features that had already evolved in other lineages. This allowed the researchers to identify the evolutionary “limits” placed on different groups of mammals, showing where they were being excluded from different niches by competition with other animals. The results suggest that it may not have been the dinosaurs that were placing the biggest constraints on the ancestors of modern mammals, but their closest relatives.

The study looked at the anatomy of all the different kinds of mammals living alongside dinosaurs, including the ancestors of modern groups, also known as therians. By measuring how frequently new features appeared, such as changes in the size and shape of their teeth and bones, and the pattern and timing of their appearance before and after the mass extinction, the researchers determined that the modern mammals were more constrained during the time of the dinosaurs than their close relatives. This meant that while their relatives were exploring larger body sizes, different diets, and novel ways of life such as climbing and gliding, they were excluding modern mammals from these lifestyles, keeping them small and generalist in their habits.

“This result makes very little sense if you assume that it was the dinosaurs constraining the therians,” said Dr Neil Brocklehurst of the University of Oxford, who led the research. “There is no reason why the dinosaurs would be selectively competing with just these mammals and allowing others to prosper. It instead appears that the therians were being held back by these other groups of mammals.”

The researchers suggest the extinction of other mammal groups was more important in paving the way for modern mammal success. As further evidence for this, the researchers looked at body size in different mammal groups. They discovered that both the smallest and largest mammals showed the same release from constraints following the dinosaur extinction, suggesting that size made little difference to their success.

Co-author Dr Gemma Benevento of the University of Birmingham said:

“Most of the mammals that lived alongside the dinosaurs were less than 100g in body mass – that’s smaller than any non-bird dinosaur. Therefore, these smallest mammals would probably not have been directly competing with dinosaurs. Despite this, small mammals show diversity increases after the extinction which are just as profound as those seen in larger mammals.”

Dr Brocklehurst added:

“Palaeontology is undergoing a revolution. We have greatly expanded the toolkit available to analyse large datasets and directly test our ideas about evolution. Most studies of the mammal radiation have focused on how fast they evolved, but analysing what limits there were on the evolution provides new perspectives. We have had to rethink many of our theories using these state-of-the-art approaches.”

原始論文：Neil Brocklehurst, Elsa Panciroli, Gemma Louise Benevento, Roger B.J. Benson. Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals. Current Biology, 2021; DOI: 10.1016/j.cub.2021.04.044

引用自：University of Oxford. "Mammals in the time of dinosaurs held each other back."