恐龍教科書要改寫了嗎？言之甚早！

原文網址：http://www.bristol.ac.uk/news/2017/november/dinosaur-textbooks.html

恐龍教科書要改寫了嗎？言之甚早！

恐龍的分類看似太過艱澀，除了專家以外大概沒人會有興趣。

然而，實情絕非如此。最近劍橋大學的Matthew Baron和其同事對我們所知的恐龍主要支系關係，提出了相當激進的修訂版本，但今日刊出的一則評論文章也提出在我們重寫教科書之前要注意的事項。

每個孩童都學過恐龍可以分成兩大類：鳥臀目(像是劍龍、三角龍、禽龍和牠們的親戚)與蜥臀目(包括身為掠食者的獸腳亞目，像是霸王龍；以及具有長頸的蜥腳亞目，像是為人熟知的梁龍)。

Baron和他的同事提出了一種非常不同的分類。他們將鳥臀目和獸腳亞目合在一起成為新的類群，稱為Ornithoscelida；而蜥腳亞目則獨立成一類。

他們的證據看起來相當有說服力。他們辨識出18項僅由鳥臀目和獸腳亞目共享的特徵，並以此當作牠們擁有相同祖先的證據。

由研究早期恐龍的專家所組成的學會，近日重新評估Baron等人提出的數據能否支持他們的主張。此學會由巴西聖保羅大學的Max Langer領導，成員包括來自阿根廷、巴西、德國、英國和西班牙的專家。

他們今日發表在期刊《自然》(Nature)的結果，顯示要改寫恐龍教科書恐怕仍言之甚早。

作者在他們新的評估結果中，發現仍有證據支持把恐龍分成鳥臀目和蜥臀目的傳統劃分方式，同時指出儘管這些證據十分薄弱，但是以Ornithoscelida作為替代方案的可行性卻還要更低一些。

Max Langer表示：「我們學會花費許多心力親自檢查所有陸塊上許多種類的恐龍，以確定我們對牠們特徵的編纂方式正確無誤。」

「起初我們以為只會對Ornithoscelida這個想法提出些許質疑，但現在我會說必須要非常仔細地重新審視整個問題。」

Baron和其同事認為他們的想法也證明恐龍可能是源自於北半球，但重新分析之後證明了長久以來的看法：恐龍最有可能誕生於南半球，或許是位在南美洲。

布里斯托大學地球科學院的教授Mike Benton是重審學會的一員，他補充：「在科學上，若你想要推翻傳統觀點，就需要相當強力的證據才行。」

「我們在恐龍演化樹底端發現的證據對兩種可能分類方式都各有相當的支持。雖然Baron和其同僚有可能是對的，但我們仍主張應該暫時堅持傳統的鳥臀目―蜥臀目二分法，直到出現更有說服力的證據。」

同為重審學會的一員，ARAID的高級研究員暨曼徹斯特大學的名譽高級研究員Fabien Knoll也表示：「Baron和其同僚建構出的提案雖然驚人；然而，正如所有新的科學假說一樣，都必須要先經過科學家同儕的嚴格審視才行。」

「對這些證據提出的最合理解釋唯有評估支持和反對兩方的論述之後，最終才能決定要採用或者是揚棄這些解釋。我們的分析結果僅是重審過程中的第一步，顯示早期恐龍分類關係的主要理論沒有任何一方顯得特別牢靠。」

愛丁堡大學的Steve Brusatte是重審學會的一員，他說：「我們之前以為今年此時就已經可以得到恐龍演化樹的樣貌。」

「但目前我們還是無法確定這三種主要恐龍類群彼此之間的關係到底為何。某種程度上來說這很令人挫敗，但另一方面卻讓我們感到十分振奮，因為這代表我們需要繼續尋找新的化石來解開這道謎題。」

Time to rewrite the dinosaur textbooks? Not quite yet!

The classification of the dinosaurs might seem to be too obscure to excite anyone but the specialists.

However, this is not at all the case. Recently, Matthew Baron and colleagues from the University of Cambridge proposed a radical revision to our understanding of the major branches of dinosaurs, but in a critique published today some caution is proposed before we rewrite the textbooks.

Every child learns that dinosaurs fall into two major groups, the Ornithischia (bird-hipped dinosaurs; Stegosaurus, Triceratops, Iguanodon and their kin) and the Saurischia (lizard-hipped dinosaurs; the predatory theropods, such as Tyrannosaurus, and the long-necked sauropodomorphs, including such well-known forms as Diplodocus).

Baron and colleagues proposed a very different split, pairing the Ornithischia with the Theropoda, terming the new group the Ornithoscelida, and leaving the Sauropodomorpha on its own.

Their evidence seemed overwhelming, since they identified at least 18 unique characters shared by ornithischians and theropods, and used these as evidence that the two groups had shared a common ancestor.

An international consortium of specialists in early dinosaurs, led by Max Langer from the Universidade de São Paulo, Brazil, and including experts from Argentina, Brazil, Germany, Great Britain, and Spain has now re-evaluated the data provided by Baron et al. in support of their claim.

Their results, presented today in the journal Nature, show that it might still be too early to re-write the textbooks for dinosaurs.

In this new evaluation, the authors found support for the traditional model of an Ornithischia-Saurischia split of Dinosauria, but also noted that this support was very weak, and the alternative idea of Ornithoscelida is only slightly less likely.

Max Langer said: "This took a great deal of work by our consortium, checking many dinosaurs on all continents first-hand to make sure we coded their characters correctly.

"We thought at the start we might only cast some doubt on the idea of Ornithoscelida, but I'd say the whole question now has to be looked at again very carefully."

Baron and colleagues believed their data suggested that dinosaurs might have originated in the northern hemisphere, but the re-analysis confirms the long-held view that the most likely site of origin is the southern hemisphere, and probably South America.

Professor Mike Benton from the University of Bristol’s School of Earth Sciences, a member of the revising consortium, added: "In science, if you wish to overthrow the standard viewpoint, you need strong evidence.

"We found the evidence to be pretty balanced in favour of two possible arrangements at the base of the dinosaurian tree. Baron and colleagues might be correct, but we would argue that we should stick to the orthodox Saurischia–Ornithischia split for the moment until more convincing evidence emerges."

Fabien Knoll, ARAID Senior Researcher and Honorary Senior Research Fellow at the University of Manchester, a member of the revising consortium, added: "Baron and colleagues formulated a stimulating proposition. However, like every new scientific hypothesis, it has to be critically evaluated by the community of scientists.

"It is only by weighing the arguments for and against it that it will eventually be adopted or disregarded as the most reasonable explanation of the evidence. We have just provided the first of these re-examinations, which shows that none of the major hypotheses of early dinosaur relationships is particularly secure."

Steve Brusatte of the University of Edinburgh, a member of the consortium, said: "Up until this year, we thought we had the dinosaur family tree figured out.

"But right now, we just can’t be certain how the three major groups of dinosaurs are related to each other. In one sense it’s frustrating, but in another, it’s exciting because it means that we need to keep finding new fossils to solve this mystery."

原始論文：Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol, Stephen L. Brusatte. Untangling the dinosaur family tree. Nature, 2017; 551 (7678): E1 DOI: 10.1038/nature24011

引用自：University of Bristol. "Time to rewrite the dinosaur textbooks? Not quite yet!."