科學家對鳥類演化有了新發現

原文網址：www.sciencedaily.com/releases/2017/03/170324192645.htm

科學家對鳥類演化有了新發現

在刊登於《國家科學學報》(National Science Review)的新論文中，由古脊椎動物與古人類研究所、山東天宇自然博物館和南京地質學與古生物研究所(皆位於中國)的科學家團隊描述了迄今發現的鳥類化石中保存特別良好的一具。

從蘊藏豐富的白堊紀早期熱河生物群(年代約為1.31億至1.2億年前)中發現的這具樣本被認為是始孔子鳥屬(Eoconfuciusornis)的一員。孔子鳥目(Confuciusornithiformes)這群早期鳥類的特徵為首度出現鳥喙，而始孔子鳥屬是其中最古老也最原始的成員。雖然其年代較近的親戚—孔子鳥屬(Confuciusornis)以發現數千具的樣本而聞名，這僅僅是迄今發現的第二具始孔子鳥屬的樣本。始孔子鳥屬僅在河北1億3070萬年前的花吉营組沉積物中發現，其保存了已知的鳥類化石中第二古老者。從此地層中發現的鳥類十分稀少。

這具新的始孔子鳥樣本目前保存在中國東部的山東天宇自然博物館，為一具雌性樣本。牠的卵巢中有不同大小的卵黃正在發育，跟現生的鳥類十分相似。這代表孔子鳥目已演化出在產卵前會有快速的卵黃蓄積階段(跟鳥類同屬主龍類(archosaurs)的鱷目，會耗時數月在每個胚胎中緩慢的蓄積卵黃，而不具有快速的卵黃形成階段)，象徵牠們具有複雜的能量曲線，跟在鳥類身上觀察到的很接近。

此意謂始孔子鳥跟牠的後裔，像是現存的鳥類，具有能力處理在生長階段早期和繁殖時所需的極高代謝需求(反之鱷魚的能量需求則相當一致，缺乏複雜性) 。相較之下，包括反鳥亞綱(Enantiornithes)這種較先進的族群在內，其他白堊紀鳥類似乎擁有較緩慢的代謝速率且需要的能量較少，跟鱷目和非鳥類恐龍較為相近(牠們發育中的卵黃很少出現大小不一致的現象，代表其不具有跟繁殖相關的能量需求高峰，也意謂著牠們受限於較簡單的生理機能，使其能量曲線單純許多)。

皮膚的痕跡顯示牠們的翅膀由稱作翅膜(patagia)的皮膚支撐。現生的鳥類有大片翅膜來幫助牠們飛翔。這具化石有助於顯示鳥類翅膀的演化過程。前翅膜(propatagium，連接肩膀和腰部的皮膚)和後翅膜(postpatagium，從掌部後方延伸至尺骨的皮膚)，比始孔子鳥缺乏的小翼翅膜(alular patagium，連結拇指和掌部其餘地方的皮膚)還早演化出來。更加特別的是前翅膜保有其內部構造，顯示出跟現生鳥類同樣由膠原組織構成的網絡。這種內部網絡讓這片皮膚擁有特殊外型，使其得以產生氣動昇力而幫助鳥類飛行。

近乎完整的羽毛殘骸保存了原本的羽毛型態，其顯示出翅膀上面具有斑點，也是最早的文獻紀錄顯示鳥類羽毛的性別差異現象(sexual difference)。這具新樣品指出始孔子鳥的雌性比雄性體型要小且缺乏尾羽，跟許多具有兩性異形(sexually dimorphic)的現生鳥類和年代較近，雌雄個體的羽毛彼此之間有所差異的孔子鳥類似。在顯微鏡底下觀察羽毛樣品可以看出不同顏色特性之間顯現的差異，科學可以藉此重建出羽毛色彩。雌性始孔子鳥的翅膀具有黑色斑點，身體則是灰色，喉部具有紅斑。

研究人員還沒有從熱河生物群發現的其他鳥類化石中發現具有如此多種類軟組織的化石(包括羽毛、皮膚、膠質和卵巢濾泡)。這些遺骸讓研究人員得以創造出迄今最為精確的原始鳥類(或者恐龍)重建圖。其提供的資訊也能讓我們更加了解原始孔子鳥屬的飛行能力，以及鳥類更先進的飛行特徵是如何演化出來。

「這具新化石真的相當驚人。」共同作者Jingmai O'Connor博士表示。「我們可以從這具樣品中蒐集大量資訊，而能真的讓這個遠古物種重獲新生。我們可以理解牠如何成長、飛行、繁殖，還有他的真實樣貌。如這般從熱河生物群出土的化石持續革新了我們對於早期鳥類的認識。」

Scientists make new discovery about bird evolution

In a new paper published in National Science Review, a team of scientists from the Institute of Vertebrate Paleontology and Paleoanthropology, the Shandong Tianyu Museum of Nature, and the Nanjing Institute of Geology and Paleontology (all in China) described the most exceptionally preserved fossil bird discovered to date.

The new specimen from the rich Early Cretaceous Jehol Biota (approximately 131 to 120 million years old) is referred to as Eoconfuciusornis, the oldest and most primitive member of the Confuciusornithiformes, a group of early birds characterized by the first occurrence of an avian beak. Its younger relative Confuciusornis is known from thousands of specimens but this is only the second specimen of Eoconfuciusornis found. This species comes only from the 130.7 Ma Huajiying Formation deposits in Hebei, which preserves the second oldest known fossil birds. Birds from this layer are very rare.

This new specimen of Eoconfuciusornis, housed in the Shandong Tianyu Museum of Nature, in Eastern China, is a female. The ovary reveals developing yolks that vary in size, similar to living birds. This suggests that confuciusornithiforms evolved a period of rapid yolk deposition prior to egg-laying (crocodilians, which are archosaurs like birds, deposit yolks slowly in all eggs for months with no period of rapid yolk formation), which is indicative of complex energetic profiles similar to those observed in birds.

This means Eoconfuciusornis and its kin, like living birds, was able to cope with extremely high metabolic demands during early growth and reproduction (whereas energetic demands in crocodiles are even, lacking complexity). In contrast, other Cretaceous birds including the more advanced group the Enantiornithes appear to have lower metabolic rates and have required less energy similar to crocodilians and non-avian dinosaurs (their developing yolks show little size disparity indicating no strong peak in energy associated with reproduction, and much simpler energetic profiles, limited by simpler physiologies).

Traces of skin indicate that the wing was supplemented by flaps of skin called patagia. Living birds have numerous wing patagia that help the bird to fly. This fossil helps show how bird wings evolved. The propatagium (the flap of skin that connects the shoulder and wrist) and postpatagium (the flap of skin that extends off the back of the hand and ulna) evolved before the alular patagium (the flap of skin connecting the first digit to the rest of the hand), which is absent in Eoconfuciusornis. Even more unique is the preservation of the internal structure of the propatagium which reveal a collagenous network identical to that in living birds. This internal network gives the skin flap its shape, allowing it to generate aerodynamic lift and aid the bird in flight.

The nearly complete plumage preserves remnants of the original plumage pattern, revealing the presence of spots on the wings and the earliest documentation of sexual differences in plumage within birds. This new specimen suggests that female Eoconfuciusornis were smaller than males and lacked tail feathers, similar to many sexually dimorphic living birds and the younger Confuciusornis in which the plumage of the males and females are different from each other. Samples of the feathers viewed under a microscope reveal differences in color characteristics, allowing scientists to reconstruct the plumage. Female Eoconfuciusornis had black spotted wings and gray body with a red throat patch.

Researchers have not found fossils from any other bird from the Jehol period that reveal so many types of soft tissue (feathers, skin, collagen, ovarian follicles). These remains allow researchers to create the most accurate reconstruction of a primitive early bird (or dinosaur) to date. This information provides better understanding of flight function in the primitive confuciusornithiforms and of the evolution of advanced flight features within birds.

"This new fossil is incredible," said co-author Dr. Jingmai O'Connor. "With the amount of information we can glean from this specimen we can really bring this ancient species to life. We can understand how it grew, flew, reproduced, and what it looked like. Fossils like this one from the Jehol Biota continue to revolutionize our understanding of early birds."

原始論文：Xiaoting Zheng, Jingmai K. O’Connor, Xiaoli Wang, Yanhong Pan, Yan Wang, Min Wang, Zhonghe Zhou. Exceptional preservation of soft tissue in a new specimen of Eoconfuciusornis and its biological implications. National Science Review, 2017; DOI: 10.1093/nsr/nwx004

引用自：Oxford University Press USA. "Scientists make new discovery about bird evolution." ScienceDaily. ScienceDaily, 24 March 2017.