新的發現將羽毛的起源回推了7000萬年
一組包含布里斯托大學的國際古生物學家團隊，發現翼龍這種會飛行的爬蟲類其實擁有四種羽毛。這些跟恐龍共有的特徵將羽毛的起源回推了7000萬年左右。
研究中道虎溝翼龍的重建圖。在牠的頭頸、身體和翅膀上具有四種不同類型的羽毛，大致上呈薑茶色。圖片來源：Yuan
Zhang所繪製的重建圖
翼龍這種飛行爬蟲類在2億3000萬年前至6600萬年前跟恐龍一同生活在地球上。人們很早就知道翼龍身上覆有某種毛髮，一般稱為「密集絲狀物」(pycnofibres)。過去的假設認為它跟恐龍和鳥類的羽毛本質上是不同的。
在今日發表於期刊《自然生態學與演化》(Nature Ecology & Evolution)的新研究中，由南京、布里斯托、科克、北京、都柏林、香港等大學組成的團隊指出翼龍身上至少有四種羽毛：一般纖維(毛髮)、束狀纖維、中段成簇的纖維、絨羽。
目前知道另外兩大類恐龍身上也能見到這四種羽毛：一種是草食性的鳥臀目和包括鳥類祖先的獸腳亞目。
主持這項研究，南京大學的姜寶玉表示：「我們前往內蒙古的道虎溝層進行野外考察。」
「我們知道此處曾經挖出將密集絲狀物保存下來的極佳翼龍樣本。所以我確定只要仔細進行研究，我們就能得到更多發現。」
同為南京大學的Zixiao
Yang的博士研究一部份為道虎溝層的出露地點以及從中出產的翼龍，他說：「可以研究這些極為迷人的化石是千載難逢的機會。」
「我運用高倍顯微鏡來檢視樣本的各個角落。而這四種羽毛我們都有發現許多例子。」
科克大學的Maria
McNamara補充：「過去有些反對意見認為密集絲狀物實際上只有一種簡單的類型，但我們的研究顯示不同類型的羽毛是確實存在的。」
「我們把重點放在羽毛沒有互相重疊而比較乾淨的區塊。此處我們可以清楚地看出羽毛的構造，甚至可以看到黑色素小體(melanosome)的微小細節。這些蓬鬆的羽毛可能因這些色素而呈現出薑茶色。」
布里斯托大學地球科學院的教授Mike
Benton表示：「我們進行了一些演化方面的分析，結果清楚顯示翼龍的密集絲狀物確實是羽毛，就像在現代鳥類和許多不同類型的恐龍身上可以見到的一樣。」
「就算經過嚴格的檢視，我們還是找不出半個解剖證據顯示這四種密集絲狀物在任何方面跟鳥類和恐龍的羽毛有不同的地方。由於它們完全一樣，因此它們的演化起源也必定相同――大概是在距今2億5000萬年前，比鳥類出現的時間還早了許多。」
鳥類有兩種更加進步、用來飛行及保持外表平順的羽毛，這些廓羽(contour
feather)具有一根空心的羽根、隨後有羽枝從兩側分岔出來。
廓羽只能在鳥類和接近鳥類始祖的獸腳亞目恐龍身上發現。但現代鳥類還具有的另外幾種羽毛，像是單根纖維(monofilament)和絨羽，則可以在許多不同種類的恐龍和翼龍身上看到。
覆有裝甲的恐龍以及巨大的蜥腳亞目恐龍並沒有羽毛，不過也有可能是羽毛的生長被抑制住了。也就是這些動物刻意不讓羽毛長出來(至少在成體身上)，就像鯨魚、大象和河馬沒有毛髮一樣。豬就是一種典型例子：小豬的身上覆有一層像小狗的毛髮，但牠們長大之後就不會再長出來。
Benton教授補充：「這項發現不只對羽毛的起源有相當大的啟發，對陸上生命的重大演化事件也是如此。」
「大約距今2億5000萬年前羽毛出現的時候，生命正從二疊紀末大滅絕造成的浩劫中回復過來。」
「另一條獨立證據顯示包括哺乳類和恐龍的祖先在內，當時某些陸生脊椎動物從貼地爬行變成直立行走、開始擁有不同程度的恆溫性，生活步調一般來說也變得比較快速。」
「那時的哺乳類祖先具有毛髮，就像翼龍、恐龍和其近親長出羽毛來幫助保溫一樣。」
「越來越多研究加入在化石中搜捕羽毛的行列，而找出如此早期的羽毛形式的作用是相當迫切的。這可以改寫我們對三疊紀地球上的重大生物演化事件的瞭解，以及基因如何調控從皮膚上長出來的羽毛、鱗片和毛髮。」
New discovery pushes origin
of feathers back by 70 million years
An international team of
palaeontologists, which includes the University of Bristol, has discovered that
the flying reptiles, pterosaurs, actually had four kinds of feathers, and these
are shared with dinosaurs – pushing back the origin of feathers by some 70
million years.
Pterosaurs are the flying reptiles that lived side by
side with dinosaurs, 230 to 66 million years ago. It has long been known that
pterosaurs had some sort of furry covering often called ‘pycnofibres’, and it
was presumed that it was fundamentally different to feathers of dinosaurs and
birds.
In a new work published today in the journal Nature Ecology & Evolution, a team
from Nanjing, Bristol, Cork, Beijing, Dublin, and Hong Kong show that
pterosaurs had at least four types of feathers:
- simple filaments (‘hairs’)
- bundles of filaments
- filaments with a tuft halfway down
- down feathers.
These four types are now also known from two major
groups of dinosaurs – the ornithischians, which were plant-eaters, and the
theropods, which include the ancestors of birds.
Baoyu Jiang of Nanjing University, who led the
research, said: “We went to Inner Mongolia to do fieldwork in the Daohugou
Formation.
“We already knew that the sites had produced
excellent specimens of pterosaurs with their pycnofibres preserved and I was
sure we could learn more by careful study.”
Zixiao Yang, also of Nanjing University, has studied
the Daohugou localities and the pterosaurs as part of his PhD work. He said:
“This was a fantastic opportunity to work on some amazing fossils.
“I was able to explore every corner of the specimens
using high-powered microscopes, and we found many examples of all four
feathers.”
Maria McNamara of University College Cork, added:
“Some critics have suggested that actually there is only one simple type of
pycnofibre, but our studies show the different feather types are real.
“We focused on clear areas where the feathers did not
overlap and where we could see their structure clearly. They even show fine
details of melanosomes, which may have given the fluffy feathers a ginger
colour.”
Professor Mike Benton from the University of
Bristol’s School of Earth Sciences, said: “We ran some evolutionary analyses
and they showed clearly that the pterosaur pycnofibres are feathers, just like
those seen in modern birds and across various dinosaur groups.
“Despite careful searching, we couldn’t find any
anatomical evidence that the four pycnofibre types are in any way different
from the feathers of birds and dinosaurs. Therefore, because they are the same,
they must share an evolutionary origin, and that was about 250 million years
ago, long before the origin of birds.”
Birds have two types of advanced feathers used in
flight and for body smoothing, the contour feathers with a hollow quill and
barbs down both sides.
These are found only in birds and the theropod
dinosaurs close to bird origins. But the other feather types of modern birds
include monofilaments and down feathers, and these are seen much more widely
across dinosaurs and pterosaurs.
The armoured dinosaurs and the giant sauropods
probably did not have feathers, but they were likely suppressed, meaning they
were prevented from growing, at least in the adults, just as hair is suppressed
in whales, elephants, and hippos. Pigs are a classic example, where the piglets
are covered with hair like little puppies, and then, as they grow, the hair
growth is suppressed.
Professor Benton added: “This discovery has amazing
implications for our understanding of the origin of feathers, but also for a
major time of revolution of life on land.
“When feathers arose, about 250 million years ago,
life was recovering from the devasting end-Permian mass extinction.
“Independent evidence shows that land vertebrates,
including the ancestors of mammals and dinosaurs, had switched gait from
sprawling to upright, had acquired different degrees of warm-bloodedness, and
were generally living life at a faster pace.
“The mammal ancestors by then had hair, so likely the
pterosaurs, dinosaurs and relatives had also acquired feathers to help insulate
them.
“The hunt for feathers in fossils is heating up and
finding their functions in such early forms is imperative. It can rewrite our
understanding of a major revolution in life on Earth during the Triassic, and
also our understanding of the genomic regulation of feathers, scales, and hairs
in the skin.”
原始論文：Zixiao Yang,
Baoyu Jiang, Maria E. McNamara, Stuart L. Kearns, Michael Pittman, Thomas G.
Kaye, Patrick J. Orr, Xing Xu, Michael J. Benton. Pterosaur integumentary
structures with complex feather-like branching. Nature Ecology
& Evolution, 2018; 3 (1): 24 DOI: 10.1038/s41559-018-0728-7
引用自：University of Bristol. "New discovery
pushes origin of feathers back by 70 million years."
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