A fossil unearthed in Belgium dubbed the “Wonderchicken” is providing a rare glimpse into the early evolution of modern birds at the twilight of the age of dinosaurs, right before an asteroid impact altered the course of life on Earth.
Researchers on Wednesday described a partridge-sized bird named Asteriornis maastrichtensis that inhabited a seashore environment during the Cretaceous Period 66.7 million years ago. It represents the oldest-known anatomically modern bird, sharing skull traits with today’s landfowl like chickens, turkeys, quail and pheasants as well as waterfowl like ducks, geese and swans.
Birds evolved from small feathered dinosaurs roughly 150 million years ago. Many birds retained primitive features like teeth until the asteroid struck about 66 million years ago, triggering a mass extinction that erased three-quarters of Earth’s plant and animal species.
Photo: Reuters
照片：路透
A current estimate for the appearance of the common ancestor of all 11,000 living species of birds is somewhere between 100 million and 70 million years ago, based on DNA mutation rates. Until now the oldest fossil of an anatomically modern bird was one called Vegavis that lived 66.5 million years ago in Antarctica, though there is a debate about its placement on the bird family tree.
At first glance, the fossil looked unimpressively like a few broken limb bones poking out of a small rock. “When we CT scanned the rocks to get a better look at the limb bones we were shocked to discover an incredible, nearly complete skull peering out at us from the computer screen,” said University of Cambridge paleontologist Daniel Field, lead author of the research published in the journal Nature. “This skull is one of the best preserved fossil bird skulls of any age, so the fact that it is from such an important point in Earth history is amazing,” Field added.
Field coined the “Wonderchicken” nickname, owing to its chicken-like beak and its scientific importance. Its scientific name “Asteriornis” honors Asteria, an ancient Greek goddess of falling stars, a nod to the imminent asteroid. In Greek mythology, Asteria was pursued by Zeus, who transformed into an eagle. In order to escape the amorous advances of Zeus, Asteria transformed herself into a quail and flung herself into the Aegean Sea, where she metamorphosed into an island like a star falling from heaven.
Asteriornis provides clues as to which factors may have helped some birds survive the asteroid impact while others perished, said paleontologist and study co-author Daniel Ksepka of the Bruce Museum in Greenwich, Connecticut, US. Its relatively small size — a 30cm-long body — may have helped it survive as the global environmental calamity caused by the asteroid strike made it hard for larger animals to find enough to eat, Ksepka said.
Its adaptation to a shoreline environment, Ksepka added, meant it would have been spared from the wildfires believed to have destroyed most of the world’s forests after the asteroid impact, which may have incinerated tree-dwelling birds.
(Reuters, with staff writer)
日前在比利時出土的一份化石，被科學家冠上「驚奇雞」的封號，因為它罕見地透露出恐龍時代尾聲──也就是小行星撞擊事件改變地球上的生命歷程前不久──現代鳥類的早期演化樣貌。
研究人員於週三描述這種體型接近鷓鴣、名為「Asteriornis maastrichtensis」的鳥類。它們棲息於濱海環境，生活在六千六百七十萬年前的白堊紀時期。在解剖學上，這種鳥類是目前所知最古老的現代鳥類，和今日的陸禽，像是雞、火雞、鵪鶉和雉，以及包括鴨子、鵝、天鵝在內的水禽擁有共同的頭骨特徵。
鳥類大約在一億五千萬年前從小型披羽恐龍演化而來。當小行星在六千六百萬年前左右撞擊地球、引發一場生物大滅絕、消滅地球上四分之三的動植物物種之前，許多鳥類仍然保有牙齒等原始特徵。
根據DNA突變率推算的結果顯示，現存一萬一千多種鳥類的共同祖先大約出現在一億年前到七千萬年前之間。在這份化石之前，解剖學上現代鳥類最古老的化石是「維加鳥」，該物種生活在六千六百五十萬年前的南極洲。不過，目前科學界對於維加鳥在鳥類親緣關係樹上的位置仍有爭論。
乍看之下，這份化石並不起眼，像是幾根斷掉的四肢骨骼從一顆小石頭向外伸出。劍橋大學古生物學家丹尼爾‧費爾德表示：「當我們用電腦斷層掃瞄石頭，想要把這些骨頭看得更清楚時，我們震驚地發現一個令人難以置信、幾近完整的頭骨，從電腦螢幕上直直盯著我們。」這份研究日前發表於期刊《自然》，費爾德為主要作者。他補充說：「這塊頭骨是史上保存狀態最好的一塊鳥頭骨化石，而它源自地球歷史上如此重要的一個時間點，這又更讓人驚嘆。」
費爾德創造新詞「驚奇雞」作為這種鳥的暱稱，是因為它的喙與雞相似，並肯定其科學重要性。學名中的「Asteriornis」是為了向古希臘女神阿斯特里亞致敬，也是隱喻當時即將撞擊地球的小行星。在古希臘神話中，宙斯化為老鷹追求阿斯特里亞。為了逃離宙斯多情的步步進逼，她把自己變成鵪鶉，直直衝進愛琴海裡，在那邊變成一座小島，就像是從天上墜落的星辰。
這篇研究的共同作者、目前任職於美國康乃狄克州格林威治市布魯斯博物館的古生物學家丹尼爾‧賽普卡指出，Asteriornis提供許多研究線索，讓科學家得以探討哪些因素可能幫助某些鳥類從當時的小行星撞擊中存活下來，而其他物種卻就此滅亡。他說，可能是因為「驚奇雞」相對小的體型──身長三十公分──幫助它們存活，因為小行星撞擊造成的全球性環境災難，讓大型動物很難找到足夠的食物。
賽普卡補充表示，這種鳥類適應海岸線的生活環境，意味著它們可能倖免於野火之災。小行星撞擊地球時，很可能將樹棲鳥類燒成灰燼，隨後各地燒起的野火恐怕也摧毀了世界絕大部分的森林。
（譯註：鷓鴣讀作「zhe4 gu1」[ㄓㄜˋㄍㄨ]，在台灣同屬的鳥類為台灣山鷓鴣，俗稱深山竹雞，為台灣特有種）
（台北時報章厚明編譯）
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