光溜溜的史前怪獸！證據反駁古代飛行爬蟲類可能具有羽毛的說法

原文網址：https://www.port.ac.uk/news-events-and-blogs/news/naked-prehistoric-monsters

光溜溜的史前怪獸！證據反駁古代飛行爬蟲類可能具有羽毛的說法

關於恐龍的羽毛是在什麼時候演化出來的爭議有了新的轉折。一篇論文反駁了之前在恐龍的近親——稱作翼龍的飛行爬蟲類身上發現羽毛的說法。

翼龍是全身赤裸(左)還是覆有羽毛(右)？圖片來源：University of Portsmouth.

萊斯特大學古生物研究中心的翼龍專家David Unwin博士，以及普茨茅斯大學的David Martill教授檢驗了之前被認為是翼龍羽毛的證據之後，認為這些生物其實沒有毛髮。

他們的回應是針對Zixiao Yang領導的古生物研究團隊提出來的說法：某些翼龍化石裡呈現的證據顯示牠們的皮膚上面有類似羽毛的分岔細毛，也就是原始羽毛。

南京大學的楊博士和同事2018年在期刊《自然—生態學與演化》(Nature Ecology and Evolution)的論文中發表了他們的論點。最近Unwin和Martill也在同一期刊中提出了這些化石裡的證據不用羽毛也可以解釋。

雖然這看起來不過是篇沒什麼大不了的學術文章，但對於古生物學來說其實具有重大意義。由於羽毛是種相當複雜的構造，所以在兩群不同的動物身上都分別演化出來的機率並不高。因此翼龍具有羽毛的話，就意味著最原始的羽毛一開始是出現在恐龍與翼龍的共同祖先身上。

這代表類似羽毛的部位最初演化出來的時間至少比過往認為的還要早了八千萬年；同時也暗示了所有恐龍最初都擁有羽毛或原始羽毛，只不過某些族群之後又把羽毛丟掉了，像是蜥腳類——這和現今廣為接受的理論背道而馳。

Yang和同僚的證據來自於大約三十具翼龍化石身上辨識出狀似毛髮、直徑不到十分之一厘米的細絲。 但是他們只有從其中的三具樣本裡，發現這些細絲看起來具有原始羽毛典型的分岔構造。

Unwin和Martill提出這些細絲並非原始羽毛而是堅硬的纖維，其為翼龍的翼膜內部結構的組成之一，而狀似分岔的結構可能只是這些纖維分解並受到破壞的結果。

Unwin博士說：「翼龍具有羽毛的說法可以追溯至19世紀，但是當時的化石證據相當薄弱，而現在依然如此。特立獨行的理論就需要與眾不同的證據來支持——雖然我們看到了前者，但後者卻沒出現。」

Martill教授指出不管哪種說法是對的，古生物學家之後必定還是要仔細地重新思考有關這些古代飛行爬蟲類的生態細節。他說：「如果牠們真的有羽毛，那羽毛會讓牠們的外觀變得如何？會跟鳥類一樣展現出迷人的多變色彩嗎?又如果牠們沒有羽毛，那麼牠們在夜間如何保持溫暖？地理分布會受到什麼樣的限制，是和現在大部分的爬蟲類一樣，會避開寒冷的北方氣候嗎？牠們又是如何調節體溫？這些線索相當難以解讀，因此我們還有很多的研究需要進行，才能找出這些神奇的動物究竟是如何生活。」

這篇本周發表在《自然—生態學與演化》上的論文標題為「翼龍身上沒有羽毛」。

 

Naked prehistoric monsters! Evidence that prehistoric flying reptiles probably had feathers refuted

The debate about when dinosaurs developed feathers has taken a new turn with a paper refuting earlier claims that feathers were also found on dinosaurs’ relatives, the flying reptiles called pterosaurs.

Pterosaur expert Dr David Unwin from the University of Leicester’s Centre for Palaeobiology Research, and Professor David Martill, of the University of Portsmouth have examined the evidence that these creatures had feathers and believe they were in fact bald.

They have responded to a suggestion by a group of his colleagues led by Zixiao Yang that some pterosaur fossils show evidence of feather-like branching filaments, ‘protofeathers’, on the animal’s skin.

Dr Yang, from Nanjing University, and colleagues presented their argument in a 2018 paper in the journal Nature Ecology and Evolution. Now Unwin and Martill, have offered an alternative, non-feather explanation for the fossil evidence in the same journal.

While this may seem like academic minutiae, it actually has huge palaeontological implications. Feathered pterosaurs would mean that the very earliest feathers first appeared on an ancestor shared by both pterosaurs and dinosaurs, since it is unlikely that something so complex developed separately in two different groups of animals.

This would mean that the very first feather-like elements evolved at least 80 million years earlier than currently thought. It would also suggest that all dinosaurs started out with feathers, or protofeathers but some groups, such as sauropods, subsequently lost them again – the complete opposite of currently accepted theory.

The evidence rests on tiny, hair-like filaments, less than one tenth of a millimetre in diameter, which have been identified in about 30 pterosaur fossils. Among these, Yang and colleagues were only able to find just three specimens on which these filaments seem to exhibit a ‘branching structure’ typical of protofeathers.

Unwin and Martill propose that these are not protofeathers at all but tough fibres which form part of the internal structure of the pterosaur’s wing membrane, and that the ‘branching’ effect may simply be the result of these fibres decaying and unravelling.

Dr Unwin said: “The idea of feathered pterosaurs goes back to the nineteenth century but the fossil evidence was then, and still is, very weak. Exceptional claims require exceptional evidence – we have the former, but not the latter.”

Professor Martill noted that either way, palaeontologists will have to carefully reappraise ideas about the ecology of these ancient flying reptiles. He said, “If they really did have feathers, how did that make them look, and did they exhibit the same fantastic variety of colours exhibited by birds. And if they didn’t have feathers, then how did they keep warm at night, what limits did this have on their geographic range, did they stay away from colder northern climes as most reptiles do today. And how did they thermoregulate? The clues are so cryptic, that we are still a long way from working out just how these amazing animals worked.

The paper ‘No protofeathers on pterosaurs’ is published this week in Nature Ecology and Evolution.

原始文章David M. Unwin, David M. Martill. No protofeathers on pterosaurs. Nature Ecology & Evolution, 2020; DOI: 10.1038/s41559-020-01308-9

引用自：University of Portsmouth. “Naked prehistoric monsters! Evidence that prehistoric flying reptiles probably had feathers refuted.”