遠在天邊，近在眼前――科學家找到了第一具鳥喙
By
Jim Shelton
研究人員建構出具有牙齒的鳥類中，一種代表性物種的頭骨立體模型。此種鳥類象徵了恐龍轉變成現今鳥類的關鍵環節。
今日所能見到的鳥類是由恐龍演化而來，在此歷程中「魚鳥」(Ichthyornis dispar)具有關鍵地位。牠們生存在一億年前的北美洲，看起來像是帶有牙齒的海鳥，也因此吸引了某些知名自然學者的注意，像是耶魯大學的O.C. Marsh(他是首位命名並描述此物種的人)和達爾文。
雖然我們擁有魚鳥的局部樣品，但是自1870年代首次發現一些頭骨碎片之後，對於牠們的頭骨化石就再也沒有新的重要發現。近日，一個由耶魯大學領導的團隊，發表的新樣品中具有立體的顱骨遺骸，包括了一個完整的頭顱和兩個最初保存在耶魯大學，但之前一直被忽略的顱骨部件。它們揭示了演化史中最驚人的變化之一的全新細節。
「這隻處於過渡階段的神奇鳥類原來一直就在我們眼皮底下。」耶魯大學的古生物學家Bhart-Anjan Bhullar表示。他是這篇發表於期刊《自然》(Nature)的研究的計畫主持人。「牠的大腦看起來相當現代。令人印象深刻的是，牠同時擁有跟恐龍一樣的下顎肌肉結構。」
Bhullar說魚鳥最令人感興趣的地方，也許是牠透露出鳥喙在自然界出現時一開始的樣貌為何。
Bhullar表示：「最初的鳥喙是在顎骨末端由角質覆蓋，像是鑷子尖端的部分。」他是地質學和地球物理學的助理教授與助理研究員。「顎骨的剩餘部分都有牙齒。鳥喙最初的功用是用來精準咬住物體。在牠們的前肢變成翅膀之後，可以用來當作義肢使用。」
研究團隊運用電腦斷層掃描來進行分析，樣品則來自耶魯大學皮博迪自然史博物館、堪薩斯州海斯的斯騰伯格自然歷史博物館、阿拉巴馬州立自然史博物館、堪薩斯大學生物多樣性研究所，以及黑山地質研究院。
此篇新研究的共同主要作者為英國巴斯大學米爾納演化中心的Daniel Field和耶魯大學的Michael Hanson。共同作者則包括堪薩斯大學的David Burnham、福特喜斯州立大學的Laura Wilson 和Kristopher Super、阿拉巴馬州立自然史博物館的Dana Ehret，以及麥克韋恩科學中心的Jun Ebersole。
「生物現有的外型是由什麼事物經由演化轉變而成，我們所能擁有的直接證據就只有化石紀錄而已。」Field表示，「魚鳥是目前已知在爬蟲時代中，和現代鳥類關係最為親近的物種。這些十分特別的新樣品讓我們驚訝地發現魚鳥的頭顱中，與恐龍類似的構造保存的相當久。」
研究人員表示他們的發現提供了新觀點顯示鳥類的頭顱最後是如何變成現今的模樣。魚鳥的鳥喙正在逐漸成形的同時，牠的大腦已經跟現今鳥類頗為相似，但是頭骨的顳區卻還是跟恐龍驚人地相像――意味在鳥類的演化過程中，腦部首先產生了變化，但頭部的其他區域卻還是比較原始，跟恐龍較為相似。
「魚鳥看起來應該跟現今的海鳥十分相似，特別是海鷗和燕鷗。」Hanson表示，「雖然牠們的嘴巴並未覆蓋某種類似嘴唇的口腔外部組織，但除非牠們把嘴巴打開，否則應該是看不到牠們的牙齒。」
最近幾年Bhullar的實驗室就脊椎動物的頭骨在許多方面進行了大量研究，通常集中在有關鳥喙的起源。「每一個新發現都讓我們之前得到的結論更加有力。我們之前利用分子證據發現鳥喙和上顎的發育是由同一基因調控，而魚鳥的顱骨證明了此說法。」Bhullar表示，「鳥類是當今種類最為豐富的陸生脊椎動物，牠們的故事也是演化史上最為重要的篇章之一。也許，我們終究還是處於恐龍時代。」
Scientists find the first bird beak, right under their
noses
Researchers have pieced together the
three-dimensional skull of an iconic, toothed bird that represents a pivotal
moment in the transition from dinosaurs to modern-day birds.
Ichthyornis
dispar holds
a key position in the evolutionary trail that leads from dinosaurian species to
today’s avians. It lived nearly 100 million years ago in North America, looked
something like a toothy seabird, and drew the attention of such famous
naturalists as Yale’s O.C. Marsh (who first named and described it) and Charles
Darwin.
Yet
despite the existence of partial specimens of Ichthyornis dispar, there has been no
significant new skull material beyond the fragmentary remains first found in
the 1870s. Now, a Yale-led team reports on new specimens with three-dimensional
cranial remains — including one example of a complete skull and two previously
overlooked cranial elements that were part of the original specimen at Yale —
that reveal new details about one of the most striking transformations in
evolutionary history.
“Right
under our noses this whole time was an amazing, transitional bird,” said Yale paleontologist Bhart-Anjan Bhullar, principal investigator of a study published
in the journal Nature. “It has a modern-looking brain along with a remarkably
dinosaurian jaw muscle configuration.”
Perhaps
most interesting of all, Bhullar said, is that Ichthyornis dispar shows us what the
bird beak looked like as it first appeared in nature.
“The
first beak was a horn-covered pincer tip at the end of the jaw,” said Bhullar,
who is an assistant professor and assistant curator in geology and geophysics.
“The remainder of the jaw was filled with teeth. At its origin, the beak was a
precision grasping mechanism that served as a surrogate hand as the hands
transformed into wings.”
The
research team conducted its analysis using CT-scan technology, combined with
specimens from the Yale Peabody Museum of Natural History; the Sternberg Museum
of Natural History in Hays, Kan.; the Alabama Museum of Natural History; the
University of Kansas Biodiversity Institute; and the Black Hills Institute of
Geological Research.
Co-lead
authors of the new study are Daniel Field of the Milner Centre for Evolution at
the University of Bath and Michael Hanson of Yale. Co-authors are David Burnham
of the University of Kansas, Laura Wilson and Kristopher Super of Fort Hays
State University, Dana Ehret of the Alabama Museum of Natural History, and Jun
Ebersole of the McWane Science Center.
“The
fossil record provides our only direct evidence of the evolutionary
transformations that have given rise to modern forms,” said Field. “This
extraordinary new specimen reveals the surprisingly late retention of
dinosaur-like features in the skull of Ichthyornis —
one of the closest-known relatives of modern birds from the Age of Reptiles.”
The
researchers said their findings offer new insight into how modern birds’ skulls
eventually formed. Along with its transitional beak, Ichthyornis dispar had
a brain similar to modern birds but a temporal region of the skull that was
strikingly like that of a dinosaur — indicating that during the evolution of
birds, the brain transformed first while the remainder of the skull remained
more primitive and dinosaur-like.
“Ichthyornis would have looked very similar to
today’s seabirds, probably very much like a gull or tern,” said Hanson. “The
teeth probably would not have been visible unless the mouth was open but
covered with some sort of lip-like, extra-oral tissue.”
In
recent years Bhullar’s lab has produced a large body of research on various
aspects of vertebrate skulls, often zeroing in on the origins of the avian
beak. “Each new discovery has reinforced our previous conclusions. The skull
of Ichthyornis even
substantiates our molecular finding that the beak and palate are patterned by
the same genes,” Bhullar said. “The story of the evolution of birds, the most
species-rich group of vertebrates on land, is one of the most important in all
of history. It is, after all, still the age of dinosaurs.”
Daniel
J. Field, Michael Hanson, David Burnham, Laura E. Wilson, Kristopher Super,
Dana Ehret, Jun A. Ebersole, Bhart-Anjan S. Bhullar. Complete Ichthyornis skull illuminates mosaic assembly of the avian
head. Nature, 2018; 557
(7703): 96 DOI: 10.1038/s41586-018-0053-y
Yale University. "Scientists find the first bird
beak, right under their noses."
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