2.28億年的烏龜化石雖然還沒有龜殼，但擁有第一副沒有牙齒的喙部

原文網址：http://www.fieldmuseum.org/about/press/228-million-year-old-fossil-turtle-didnt-have-shell-yet-had-first-toothless-turtle-beak

2.28億年的烏龜化石雖然還沒有龜殼，但擁有第一副沒有牙齒的喙部

烏龜之所以為烏龜有賴幾個關鍵特徵：其中一個是龜殼，另一個則是沒有牙齒的喙部。科學家最近發現一具年代為2.28億年的烏龜化石，有助於闡明現代的烏龜如何發展出這些特徵。這具化石擁有嘴喙，身體則呈現飛盤狀，不過加寬的肋骨還沒有長成現今烏龜身上的龜殼。

Eorhynchochelys想像中的復原圖。圖片來源：Adrienne Stroup，菲爾德博物館

「這個生物體長超過6呎(約1.8公尺)，擁有奇特的盤狀身軀及長長的尾巴，顎部前端形成特殊的喙。」芝加哥菲爾德博物館的古生物學家Olivier Rieppel表示。他是這篇新發表於《自然》(Nature)的論文作者之一。「牠可能生活於淺水地帶，挖掘泥巴來尋找食物。」

發現這具化石的為研究主要作者，中國科學院古脊椎動物與古人類研究所的李淳。這個新物種被命名為Eorhynchochelys sinensis，雖然拗口但意思相當直白。Eorhynchochelys (“Ay-oh-rink-oh-keel-is”) 意指第一個有喙的龜類；而sinensis則是「從中國來的」，代表了牠的發現地點。

Eorhynchochelys 並非科學家唯一發現的原始龜類――還有另一種原始龜類，不過牠沒有喙而是擁有不完整的龜殼。目前為止，科學家還不清楚應該把牠們放進爬蟲類演化樹的哪個位置。「數十年來，龜類的起源一直是古生物學中的未解之謎。」Rieppel表示，「有了Eorhynchochelys，龜類的演化過程變得清晰許多。」

Eorhynchochelys 比其他原始龜類早發展出喙卻沒有龜殼，證實了龜類的演化過程是「鑲嵌式演化」(mosaic evolution)。此概念是指一種動物身上的特徵各自以不同的速率演化出來，而且這些特徵在不同的祖先物種身上也會以不同的組合出現。現在的龜類同時擁有喙和龜殼，但龜類演化成這副模樣的過程並非是一條直線。而是某些龜類的近親先演化出龜殼的一部份，另一些則演化出喙；最後，基因突變才創造出擁有全部這些特徵的單一物種。

「發現這麼大一具化石令我們感到十分興奮。它給了我們另一片解開龜類演化之謎的拼圖。」研究作者之一，蘇格蘭立博物館的Nick Fraser表示。「這顯示早期龜類的演化過程並不是沿著一條路逐步累積各種特性，而是牽涉到一連串更加複雜的事件，但我們才只有初步瞭解。」

Eorhynchochelys 顱骨的細部構造解決了另一個有關龜類演化的謎題。科學家長年以來無法確定龜類的祖先是和現今的蜥蜴與蛇同屬爬蟲類之下的雙孔亞綱(diapsid)――也就是牠們的早期演化過程中頭顱兩側有兩個孔洞――或者是屬於頭顱兩側沒有孔洞的無孔亞綱(anapsid )。Eorhynchochelys的頭骨顯示的跡象指出牠屬於雙孔亞綱。Rieppel表示：「我們從Eorhynchochelys顱骨中的雙孔亞綱特徵得知龜類跟早期的無孔亞綱爬蟲類關係不大，而是跟演化上較為進階的雙孔亞綱爬蟲類更加接近。這項證據十分堅固，足以定調有關爭議。」

研究作者表示他們的發現回答了有關龜類形成龜殼的過程和時間，以及牠們在雙孔亞綱中的地位，這會改變科學家對龜類譜系的想法。「我自己也很訝異，」Rieppel表示。「Eorhynchochelys 讓龜類的家族樹更加清楚合理。在看到這具化石之前，我不認為某些跟它關係相近的物種屬於龜類。但現在，我會這麼認為。」

研究經費來自於中國科學院的古脊椎動物與古人類研究所，以及生物演化與環境卓越創新中心；另外還有蘇格蘭國立博物館、美國菲爾德博物館和加拿大自然博物館。

228-million-year-old fossil turtle didn’t have a shell yet, but had the first toothless turtle beak

There are a couple of key features that make a turtle a turtle: its shell, for one, but also its toothless beak. A newly-discovered fossil turtle that lived 228 million years ago is shedding light on how modern turtles developed these traits. It had a beak, but while its body was Frisbee-shaped, its wide ribs hadn’t grown to form a shell like we see in turtles today.

“This creature was over six feet long, it had a strange disc-like body and a long tail, and the anterior part of its jaws developed into this strange beak,” says Olivier Rieppel, a paleontologist at Chicago’s Field Museum and one of the authors of a new paper in Nature. “It probably lived in shallow water and dug in the mud for food.”

The new species has been christened Eorhynchochelys sinensis—a mouthful, but with a straightforward meaning. Eorhynchochelys (“Ay-oh-rink-oh-keel-is”) means “dawn beak turtle”—essentially, first turtle with a beak—while sinensis, meaning “from China,” refers to the place where it was found by the study’s lead author, Li Chun of China’s Institute of Vertebrate Paleontology and Paleoanthropology.

Eorhynchochelys isn’t the only kind of early turtle that scientists have discovered—there is another early turtle with a partial shell but no beak. Until now, it’s been unclear how they all fit into the reptile family tree. “The origin of turtles has been an unsolved problem in paleontology for many decades,” says Rieppel. “Now with Eorhynchochelys, how turtles evolved has become a lot clearer.”

The fact that Eorhynchochelys developed a beak before other early turtles but didn’t have a shell is evidence of mosaic evolution—the idea that traits can evolve independently from each other and at a different rate, and that not every ancestral species has the same combination of these traits. Modern turtles have both shells and beaks, but the path evolution took to get there wasn’t a straight line. Instead, some turtle relatives got partial shells while others got beaks, and eventually, the genetic mutations that create these traits occurred in the same animal.

“This impressively large fossil is a very exciting discovery giving us another piece in the puzzle of turtle evolution,” says Nick Fraser, an author of the study from National Museums Scotland. “It shows that early turtle evolution was not a straightforward, step-by-step accumulation of unique traits but was a much more complex series of events that we are only just beginning to unravel.”

Fine details in the skull of Eorhynchochelys solved another turtle evolution mystery. For years, scientists weren’t sure if turtle ancestors were part of the same reptile group as modern lizards and snakes—diapsids, which early in their evolution had two holes on the sides of their skulls—or if they were anapsids that lack these openings. Eorhynchochelys's skull shows signs that it was a diapsid. “With Eorhynchochelys’s diapsid skull, we know that turtles are not related to the early anapsid reptiles, but are instead related to evolutionarily more advanced diapsid reptiles. This is cemented, the debate is over,” says Rieppel.

The study’s authors say that their findings, both about how and when turtles developed shells and their status as diapsids, will change how scientists think about this branch of animals. “I was surprised myself,” says Rieppel. “Eorhynchochelys makes the turtle family tree make sense. Until I saw this fossil, I didn’t buy some of its relatives as turtles. Now, I do.”

This study was contributed to by the Institute of Vertebrate Paleontology and Paleoanthropology, the CAS Center for Excellence in Life and Paleoenvironment, National Museums Scotland, the Field Museum, and the Canadian Museum of Nature.

原始論文：Chun Li, Nicholas C. Fraser, Olivier Rieppel, Xiao-Chun Wu. A Triassic stem turtle with an edentulous beak. Nature, 2018; 560 (7719): 476 DOI: 10.1038/s41586-018-0419-1

引用自：Field Museum. "Fossil turtle didn't have a shell yet, but had the first toothless turtle beak."