由俄亥俄大學傳統骨科醫學院的教授Patrick
O’Connor博士領導的國際團隊，今日在期刊《自然》(Nature)發表的研究指出在白堊紀的馬達加斯加，可以看到一隻烏鴉大小的鳥類穿梭天際的同時，揮動著像鐮刀一般的大型鳥喙。從牠身上得出的重要觀點，讓我們得以瞭解中生代時，現代鳥類先驅的臉部以及鳥喙形狀的演變過程。
這張圖描繪了在白堊紀晚期的馬達加斯加，一種早期的鳥類Falcatakely和非鳥類恐類與其他生物共同享有這片環境的景象。圖片來源：Mark Witton
在我們建構關於生物演化知識的過程當中，鳥類一直扮演著相當重要的腳色。早在19世紀中葉，查爾斯．達爾文就對加拉巴哥鷽鳥五花八門的鳥喙進行了深刻的觀察，進而影響他寫出演化是以天擇為動力的論文。而這副鳥類化石的發現，則讓我們可以從全新的觀點來看待鳥類和其近親的頭骨及喙部如何演化。它顯示演化可以作用在不同的發育途徑，使得關係相當疏遠的動物形成類似的頭部型態。
這種新的鳥類是由美國國家科學基金會贊助的團隊，在馬達加斯加發現的第二種白堊紀鳥類。團隊將其命名為Falcatakely，發想自拉丁文和馬達加斯加語中的小型以及鐮刀狀鳥喙，後者這項特徵是中生代鳥類全新的面部型態。對於該物種的瞭解來自於一副在6800萬年前被埋在土石流當中，保存良好、近乎完整的顱骨。由於鳥類的骨頭又輕又小，因此在化石紀錄當中很少看到鳥類的骨骸，而頭顱化石更是少之又少。
由於組成這副頭顱的骨頭相當細緻且排列方式複雜，因此這具脆弱的樣品仍有一部份鑲嵌在岩石當中。雖然這副顱骨非常小，長度估計只有8.5公分左右，但是絕佳的保存狀態讓它顯示出很多重要的細節。舉例來說，組成臉部兩側的骨頭上一系列複雜的凹槽，代表這隻鳥類活著的時候臉上有很大一部份覆蓋著角蛋白，也就是鳥喙。
「隨著這副面孔逐漸從岩石中浮現出來，我們開始理解到就算它不是獨一無二，也是相當特別的化石，」研究主要作者，俄亥俄大學解剖與神經科學的教授Patrick
O’Connor表示。「之前從未發現到臉部高度和長度都這麼長的中生代鳥類，因此Falcatakely提供了一個絕佳的機會，讓我們重新思考現代鳥類所屬譜系的頭部與喙部的演化過程。」
Falcatakely屬於一種已經滅絕的族群「反鳥類」(Enantiornithines)，已知的成員皆生活於白堊紀，主要的化石發現地點為亞洲。「反鳥類是第一批大幅多樣化的原始鳥類，牠們和其他非鳥類的近親，像是伶盜龍和霸王龍生活在同樣的生態系，」研究共同作者，石溪大學的解剖科學副教授Turner表示。「不同於始祖鳥等最早的鳥類還有長長的尾巴，而且頭顱當中還有較為原始的特徵，Falcatakely這樣的反鳥類看起來更像是現代的鳥類。」
Falcatakely活著時的重建圖或許不會讓人對這種鳥類印象深刻。但是在角蛋白組成的鳥喙底下才是演化令人感興趣的地方。由於這些骨頭大部分都極為脆弱，使得O’Connor和同事無法從岩石裡把它們一根一根取下來研究。因此研究團隊轉而使用高解析度的微電腦斷層掃描，並且建立大量的數位模型，透過虛擬的方式來剖析岩石裡每一根骨頭。同時他們也用3D列印實際做出數位模型的放大版，如此才能把頭顱重建出來並和其他物種互相比較。
「像這樣的計畫跨越了比較解剖學、古生物學和工程/材料科學等多個領域，因此和俄亥俄大學新創中心合作便成了過程中的關鍵。」共同作者，HCOM/BMS的實驗室負責人Joseph
Groenke強調，他在這項研究負責製作實體和數位模型。「成功把原型裡的每一根骨頭複製出來，是我們瞭解這具樣本並將其重建的基礎。」隨著研究有所進展，它們很快就看出雖然Falcatakely表面上看起來類似於某些現代的鳥類，但組成牠臉部的骨頭，排列方式跟所有恐龍、鳥類和非鳥類都有很大的差異。
所有現存的鳥類在形成鳥喙時，骨頭的組裝方式都非常統一，幾乎都是由一塊變得特別大的骨頭――前上頷骨所構成。相較之下，恐龍時代的大多數鳥類，包括代表性的始祖鳥在內，口鼻部都還沒有那麼特化，是由較小的前上頷骨和較大的上頷骨組成。不過讓研究人員驚訝的是，他們發現Falcatakely的骨頭雖然比較類似於這種原始的排列方式，但是臉部的形狀整體來說卻讓他們聯想到現在某些上喙又厚又長的鳥類，這跟之前所知的中生代鳥類完全不同。
「乍看之下Falcatakely的鳥喙和外型整體來說也許像是許多現代的鳥類，但是臉部下方的骨頭結構，卻是我們所知的鳥類的頭部解剖構造，在演化過程中應有的樣貌，」O’Connor說明。「顯然組織臉部骨頭的發育過程就算有所不同，最終還是能到達類似的整體目標，在這個案例中，便是形成了類似的頭部與鳥喙形狀。」
藝術家重建了這種白堊紀晚期的反鳥類Falcatakely forsterae。
為了探討這種趨向一致的解剖構造如何演化出來，O’Connor聯絡了從俄亥俄大學畢業，現為倫敦大學學院的人體解剖學講師Ryan
Felice博士。他的專長為鳥類和其他恐龍頭顱的解剖學。Felice解釋：「我們發現在Falcatakely出現過後的數千萬年，某些現代的鳥類，像是巨嘴鳥和犀鳥也演化出了相當類似的鐮刀狀鳥喙。驚人的是，雖然這些譜系的關係非常疏遠，但是這種基本的解剖構造卻可以趨向一致。」Felice在2015年與O’Connor共同探討鳥類尾部的演化而取得了生物科學的博士學位。接著便繼續在倫敦自然史博物館完成博士後研究，然後在2018年於倫敦大學學院獲得終身職。
Falcatakely的出土地點是在現今馬達加斯加的北部，白堊紀晚期(7000到6800萬年前)的岩石中。研究人員認為那裡當時的環境是半乾燥氣候且季節變化明顯。「Falcatakely的發現凸顯出地球許久以前的歷史大都仍籠罩在謎團當中，」O’Connor接著說，「特別是地球上那些至今仍少有人前去探索的地方。」
馬達加斯加總是能突破我們想像中生命的可能性。確實，馬達加斯加獨特的生物相從古至今激發了眾多自然史學者與許多領域科學家的興趣，他們著眼的地方通常是隔絕在大型島嶼上的生物會如何演化。
O’Connor表示：「我們對現今的馬達加斯加在白堊紀時的動植物和生態系了解得更多，就越能看出馬達加斯加獨樹一格的生物可以追溯到許久以前，而非只反映了近期才形成的島嶼生態系。」
「O’Connor博士、Felice博士和其同僚發表的研究具有重大貢獻，在演化和生物多樣性的知識當中為我們開啟了全新的視野。」俄亥俄大學研究和創作活動處的副主任Joseph
Shields博士表示。「這項成果凸顯出俄亥俄大學的職員和學生在全球各地進行的研究中都有一席之地，讓我們對於自然史裡的基本問題有了更深的理解。」
參與這項研究的研究人員來自俄亥俄大學、石溪大學、倫敦大學學院、瑪卡萊斯特學院、丹佛自然科學博物館、馬達加斯加的塔那那利佛大學。經費來自於美國國家科學基金會、國家地理學會、俄亥俄大學傳統骨科醫學院和俄亥俄大學。
Early bird with tall, sickle-shaped
beak reveals hidden diversity during the Age of Dinosaurs
A Cretaceous-age, crow-sized bird from
Madagascar would have sliced its way through the air wielding a large,
blade-like beak and offers important insight on the evolution of face and beak
shape in the Mesozoic forerunners of modern birds. An international team of
researchers led by Ohio University Heritage College of Osteopathic Medicine
professor Dr. Patrick O’Connor announced the discovery today in the journal Nature.
Birds have played a pivotal role in shaping our
understanding of biological evolution. As long ago as the mid-19th Century,
Charles Darwin’s keen observations on the diversity of beak shape in Galapagos
finches influenced his treatise on evolution through natural selection. This
fossil bird discovery adds a new twist on the evolution of skulls and beaks in
birds and their close relatives, showing that evolution can work through different
developmental pathways to achieve similar head shapes in very distantly related
animals.
The new bird is named Falcatakely, a combination of Latin and Malagasy words inspired by
the small size and the sickle-shaped beak, the latter representing a completely
novel face shape in Mesozoic birds. The species is known from a single
well-preserved, nearly complete skull, one that was buried in a muddy debris
flow around 68 million years ago. Bird skeletons are rare in the fossil record
because of their lightweight bones and small size. Bird skulls are an even
rarer find. Falcatakely is the second
Cretaceous bird species discovered in Madagascar by the National Science
Foundation-funded team.
The delicate specimen remains partially embedded in
rock due to the complex array of lightly built bones that make up the skull.
Although quite small, with an estimated skull length of only 8.5 cm (~ 3
inches), the exquisite preservation reveals many important details. As one
example, a complex series of grooves on the bones making up the side of the
face indicate that the animal hosted an expansive keratinous covering, or beak,
in life.
“As the face began to emerge from the rock, we knew
that it was something very special, if not entirely unique,” notes Patrick
O’Connor, professor of anatomy and neuroscience at Ohio University and lead
author on the study. “Mesozoic birds with such high, long faces are completely
unknown, with Falcatakely providing a
great opportunity to reconsider ideas around head and beak evolution in the
lineage leading to modern birds.”
Falcatakely
belongs to an extinct group of birds called Enantiornithines, a group known
exclusively from the Cretaceous Period and predominantly from fossils
discovered in Asia. “Enantiornithines represent the first great diversification
of early birds, occupying ecosystems alongside their non-avian relatives such
as Velociraptor and Tyrannosaurus,” says Turner, an associate professor of
anatomical sciences at Stony Brook University and study co-author. “Unlike the
first birds, such as Archaeopteryx, with long tails and primitive features in
the skull, enantiornithines like Falcatakely
would have looked relatively modern.”
A life reconstruction of Falcatakely might leave one with the impression that this is a
relatively unremarkable bird. But it is underneath the keratinous beak that the
evolutionary intrigue lies. O’Connor and his colleagues couldn’t remove the
individual bones of Falcatakely from the rock for study because they were much
too fragile. Instead, the research team employed high-resolution micro-computed
tomography (µCT) and extensive digital modeling to virtually dissect individual
bones from the rock, with enlarged 3D printing of the digital models being
essential for reconstructing the skull and for comparisons with other species.
“A project like this bridges disciplines ranging from
comparative anatomy, paleontology, and engineering/materials science. Our
partnership with the Ohio University Innovation Center was a key part of this
process,” noted Joseph Groenke, laboratory coordinator in HCOM/BMS, a study
co-author responsible for the physical and digital preparation. “Being able to
see each of the bones as a prototype replica formed the basis for understanding
the specimen and also in reconstructing it.” As the research progressed, it was
quickly apparent that bones making up the face in Falcatakely are organized quite unlike those of any dinosaur, avian
or nonavian, despite having a face superficially similar to a number of modern
bird groups alive today.
All living birds build the skeleton of their beaks in
a very specific way. It is mostly formed by a single enlarged bone called the
premaxilla. In contrast, most birds from the age of dinosaurs, like the iconic
Archaeopteryx, have relatively unspecialized snouts comprised of a small
premaxilla and a large maxilla. Surprisingly, the researchers found this
similar primitive arrangement of bones in Falcatakely
but with an overall face shape reminiscent of certain modern birds with a high,
long upper bill and completely unlike anything known in the Mesozoic.
“Falcatakely
might generally resemble any number of modern birds with the skin and beak in
place, however, it is the underlying skeletal structure of the face that turns
what we know about bird evolutionary anatomy on its head,” noted O’Connor.
“There are clearly different developmental ways of organizing the facial
skeleton that lead to generally similar end goals, or in this case, similar
head and beak shape.”
To explore how this type of convergent anatomy
evolved, O’Connor connected with former OHIO PhD student Dr. Ryan Felice, an
expert on skull anatomy in birds and other dinosaurs. “We found that some
modern birds like toucans and hornbills evolved very similar sickle-shaped
beaks tens of millions of years after Falcatakely.
What is so amazing is that these lineages converged on this same basic anatomy
despite being very distantly related,” noted Felice, lecturer in human anatomy
at University College London (UCL). Dr. Felice completed his PhD in biological
sciences in 2015, working with O’Connor to examine the tail evolution in birds.
Felice has since gone on to complete a postdoctoral research position at the
Natural History Museum in London, after which he began a tenure track position
at UCL in 2018.
Falcatakely
was recovered from latest Cretaceous-age (70-68 million years ago) rocks in
what is now northern Madagascar, in what has been interpreted as a semi-arid,
highly seasonal environment. “The discovery of Falcatakely underscores that much of the deep history of the Earth
is still shrouded in mystery,” added O’Connor, “particularly from those parts
of the planet that have been relatively less explored.”
Madagascar has always pushed the boundaries of
biological potential. Indeed, the unique biota of Madagascar has intrigued
natural historians and scientists across many disciplines, often framed in the
context of evolution in isolation on the large island continent.
“The more we learn about Cretaceous-age animals,
plants, and ecosystems in what is now Madagascar, the more we see its unique
biotic signature extends far back into the past and is not merely reflective of
the island ecosystem in recent times,” O’Connor said.
“The discovery reported by Drs. O’Connor and Felice
and their collaborators is a remarkable contribution that opens new vistas into
our understanding of evolution and species diversity,” said Dr. Joseph Shields,
Ohio University’s vice president for research and creative activity. “This work
highlights the role of Ohio University faculty and students in conducting
research on sites around the globe, to advance our understanding of fundamental
questions in natural history.”
The study involved researchers from Ohio University,
Stony Brook University, University College London, Macalester College, Denver
Museum of Nature & Science, and the Université d’Antananarivo, Madagascar,
and was funded by the National Science Foundation, the National Geographic
Society, and the Ohio University Heritage College of Osteopathic Medicine/Ohio
University.
原始論文：Patrick
M. O’Connor, Alan H. Turner, Joseph R. Groenke, Ryan N. Felice, Raymond R.
Rogers, David W. Krause, Lydia J. Rahantarisoa. Late Cretaceous bird from Madagascar reveals unique development of
beaks. Nature, 2020 DOI:
10.1038/s41586-020-2945-x
引用自：Ohio
University. “Early bird with tall, sickle-shaped beak reveals hidden diversity
during the Age of Dinosaurs”
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