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【中國科學報】長臂混元龍:一次意想不到的藍天之旅

文章來源:中國科學報 丁佳   發布時間:2019-05-10  【字號:     】  

  在關于對藍天的渴望方面,人類和恐龍并沒有什么區別。

  為了能飛到天上去看看,人類進行了大量的嘗試,最終發明了飛機;而恐龍在其演化歷程中,也曾進行過大量意想不到的適應飛行的嘗試——比人類更甚的是,它們連自己的樣子都改變了。

  5月9日,《自然》雜志的封面文章發表了中國科學院古脊椎動物與古人類研究所科研人員的一項研究成果,他們發現了一種生活在侏羅紀的長著膜質翅膀的恐龍,它為恐龍的飛行演化補上了缺失的一環。

  撲朔迷離的“四不像”

  在脊椎動物里,翼龍、鳥類和蝙蝠都能飛,但它們的演化過程是獨立的,各自的飛行結構也是形態迥異的。

  相較翼龍和蝙蝠不完整的化石記錄,隨著不斷發現的帶羽毛恐龍和早期鳥類化石,得益于我國中晚侏羅世的燕遼生物群和早白堊世的熱河生物群,有關鳥類飛行起源這一重要科學問題取得了重要進展。

  “尤其是恐龍家族里最為怪異的類群——善攀鳥龍類的發現,為我們揭示出了一條令人匪夷所思的征服藍天之旅。”中科院古脊椎所研究員徐星說。

  善攀鳥龍類的形態十分奇特,它們的頭骨高聳、四肢纖細,很像是恐龍和鳥類的“混合體”,一度被認為是跟鳥類具有最近親緣關系的獸腳類恐龍。

  迄今為止,這類生活在中—晚侏羅世的恐龍只發現了三個屬種——寧城樹棲龍、胡氏耀龍和奇翼龍。這類恐龍的標本也是要么不完整,要么屬于幼年個體,大量形態特征難以觀察,令其在演化樹上的位置撲朔迷離。

  尤其是2015年徐星等命名的奇翼龍,更為這一類群增添了神秘色彩。奇翼龍的前肢長著翼膜,還具有一根棒狀長骨,這樣的長骨在其他恐龍(包括鳥類)中都沒有類似的結構。徐星推測,奇翼龍“可能是以滑翔為主,輔助以撲翼飛行”。

  然而,奇翼龍的標本也只發現了一件,保存同樣并不完整,因此對于棒狀長骨和翼膜的結構,學界依然存有爭議。

  “神助攻”現身

  2017年,中科院院士周忠和帶領團隊在遼寧晚侏羅世地層考察時,獲得了一件新化石。經過長達一年的室內修理、實驗和對比研究,研究團隊認為它是一種新的善攀鳥龍類,科研人員將其命名為長臂渾元龍。

  “這只長臂渾元龍是目前已知最完整的善攀鳥龍類化石,它提供了大量形態和生態學信息。”周忠和說。

  這只生活在距今約1.63億年的渾元龍體長約32厘米,體重約306克,在很多形態方面明顯不同于其他善攀鳥龍類,卻具有原始鳥類那樣的尾綜骨。這種縮短的尾骨能進一步將身體重心前移,有利于在飛行或滑翔時保持穩定。

  更重要的是,研究人員在渾元龍上發現了和奇翼龍相似的棒狀長骨和翼膜,這一新發現為棒狀長骨和翼膜在善攀鳥龍類中的出現提供了確鑿無疑的證據。

  “此外,渾元龍體內保存有胃石和疑似尚未完全消化的骨質胃容物,這是在善攀鳥龍類中首次發現的與食性相關的證據,我們推測它是雜食性的。”中科院古脊椎所研究員王敏說。

  渾元龍的前肢異常加長,甚至超過了中生代多數鳥類。研究人員在對比恐龍前肢時,發現善攀鳥龍類的前肢比例構成非常奇特,那么,這樣的差異是否與翼膜的出現有關?

  驚鴻一瞥

  為了證實這一猜測,王敏對包括鳥類在內的中生代虛骨龍類進行了系統發育關系主成分分析研究。

  他們發現,善攀鳥龍類前肢的加長主要源自肱骨和尺骨;在鳥類、馳龍類或者傷齒龍類中,則是掌骨的加長,而這些類群的前肢是具有羽毛翅膀的。

  因此研究人員推斷,善攀鳥龍類通過加長的肱骨和尺骨、第三手指,與棒狀長骨來附著膜質的翅膀,而鳥類、馳龍類和傷齒龍類則需要較長的掌骨來附著飛羽。至此,“膜質翅膀+短掌骨”和“羽毛翅膀+長掌骨”兩種截然不同的身體結構,為人們展現出了兩種不同的飛行模式。

  然而,長臂渾元龍等善攀鳥龍可能只是動物飛行演化中的一次短暫嘗試。“因為已知的善攀鳥龍類均生活在晚侏羅世,在此之后的白堊紀恐龍中,這種類似的膜質翅膀就再也沒有出現過了。”王敏說。

  但在歷史長河的對岸,由飛羽構成的翅膀卻一直延續到了白堊紀,歷經漫長的演化形成了鳥類的羽翼,最終成功征服了藍天,并逐漸演化出多樣性最為豐富的現生四足動物。

  (原載于《中國科學報》 2019-05-10 第1版 要聞)



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