考古學(xué)家解答先有“雞”還是先有蛋的謎團(tuán)

牧童 · 發(fā)表于 2008-11-25 11:34:22

　　到底是先有的雞還是先有的蛋，這一世紀(jì)謎團(tuán)一直讓人百思不得其解。最近，科學(xué)家通過對0.77億年前一個小型肉食恐龍巢穴的研究后終于找到答案，實為先有的蛋才有的雞。

　　先有的雞還是先有的蛋？這個存在了幾個世紀(jì)的難題，一直都是人們爭論的焦點，從生物學(xué)到哲學(xué)，都沒有得到令人信服的答案。不過，來自加拿大的科學(xué)家已經(jīng)得到了答案，這個問題的解答從發(fā)現(xiàn)一個罕見的小型肉食恐龍的巢穴開始。這個巢穴存在于7千7百年前，當(dāng)時海平面上升，恐龍媽媽不得不拋棄巢穴里面的恐龍蛋，自己逃生而去。留下的恐龍蛋成為了現(xiàn)在科學(xué)家研究的珍貴化石。

　　位于加拿大阿爾伯塔省的皇家泰瑞爾博物館恐龍館館長弗朗索瓦說：“這個巢穴有著恐龍和鳥類的共有特征，通過對這個巢穴的深入研究，可以幫助我們解決一個古老的難題：到底是先有蛋還是先有雞。”來自加拿大卡爾加里大學(xué)專門研究恐龍繁殖的古生物學(xué)家達(dá)拉·澤勒尼茨基(Darla Zelenitsky)表示：“直到現(xiàn)在先有蛋還是先有雞的問題還沒有能夠得到解答。但是隨著研究的深入，謎底逐漸清晰：恐龍首先建造了類似鳥窩的巢穴，產(chǎn)下了類似鳥蛋的蛋，然后恐龍再進(jìn)化成鳥類（雞也屬于鳥類的一種），這很明確，蛋先于雞之間就存在了。雞是由這些產(chǎn)下了類似雞蛋的肉食恐龍進(jìn)化而成。”

　　基于上面的研究表明，原來的問題應(yīng)該被改寫成：是先有恐龍還是先有蛋。

　　據(jù)報道，這個巢穴是在上世紀(jì)90年代發(fā)現(xiàn)的，早期考古成果保存在加拿大阿爾伯塔省的卡爾加里化石有限公司。最開始這個巢穴被認(rèn)為屬于一種類似于鴨子的草食恐龍。后來的深入研究才發(fā)現(xiàn)，這應(yīng)該是一種小型肉食恐龍的化石，2007年，該化石被阿爾伯塔省的皇家蒂勒爾古生物博物館收購，現(xiàn)存于館中。

　　澤勒尼茨基說：“在北美地區(qū)，這種小型肉食恐龍的巢穴是十分罕見的，基于洞穴的造型和里面的恐龍蛋化石我們可以推斷，這種小型肉食恐龍和鳥類有著密切的親屬關(guān)系?！倍P(guān)于這種恐龍的產(chǎn)卵行為，在最新一期的《古生物學(xué)》（Paleobiology）雜志上，詳細(xì)的分析了這個珍稀的巢穴，并公布許多關(guān)于恐龍產(chǎn)蛋和建立巢穴的重要信息。這可以為古生物學(xué)家對于類似這種的小型肉食恐龍的研究提供幫助，同時也為恐龍進(jìn)化成鳥類的研究提供重要的證據(jù)。

　　例如，通過對恐龍蛋在巢穴中的位置以及蛋的尺寸可以判斷：一窩蛋至少包括12個，它們成環(huán)狀逐個排列在沙土之上。每顆蛋大約有5英寸（約12厘米）長，這一點很像鳥蛋。分析還表明，恐龍一次會產(chǎn)下兩個蛋，這一點和鳥類很相似，而和一次只產(chǎn)一個蛋的鱷魚不同。這點從側(cè)面證明了是恐龍進(jìn)化成了鳥類。

　　古生物學(xué)家們不滿足對于只是解答了先有蛋還是先有雞這個問題，他們的下一個目標(biāo)是尋找擁有這種恐龍幼仔化石的巢穴。這樣可以通過對恐龍幼仔骨骼的進(jìn)一步研究，來得出更多的結(jié)論。雖然在北美地區(qū)找到下個保存這樣完整的巢穴很困難，但是考古學(xué)家還是想挑戰(zhàn)一下。接下來的研究將由艾伯塔創(chuàng)新研究基金和基拉姆獎學(xué)金基金共同資助。

Which Came First? Eggs Before Chickens, Scientists Now SayBy Jeanna Bryner, Senior Writer
posted: 14 November 2008 10:39 am ET
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The nest and eggs either belonged to a bird-like dinosaur that looked similar to an oviraptor (dino on left) or a relative of Velociraptor (dino on right). Credit: Julius T. Csotonyi.
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The nest and eggs either belonged to a bird-like dinosaur that looked similar to an oviraptor (dino on left) or a relative of Velociraptor (dino on right). Credit: Julius T. Csotonyi.
The theropod nest included a mound of sand with some egg shells and egg impressions on the side of the mound. Credit: Darla Zelenitsky/University of Calgary. A rare fossilized dinosaur nest helps answer the conundrum of which came first, the chicken or the egg, two paleontologists say.
The small carnivorous dinosaur sat over her nest of eggs some 77 million years ago, along a sandy river beach. When water levels rose, Mom seems to have fled, leaving the unhatched offspring.
Researchers have now studied the fossil nest and at least five partial eggs. The nest is a mound of sand that extends about 1.6 feet (half a meter) across and weighs as much as a small person, or about 110 pounds (50 kg).
"Some characteristics of the nest are shared with birds, and our analysis can tell us how far back in time these features, such as brooding, nest building, and eggs with a pointed end, evolved — partial answers to the old question of which came first, the chicken or the egg," said researcher Francois Therrien, curator of dinosaur paleoecology at the Royal Tyrrell Museum in Alberta, Canada.
The answer?
Well, it’s still unclear whether chicken eggs or chickens came first (the intended question in the original riddle), said Darla Zelenitsky, a paleontologist of the University of Calgary in Alberta who was the first scientist to closely analyze the dinosaur nest.
But interpreted literally, the answer to the riddle is clear. Dinosaurs were forming bird-like nests and laying bird-like eggs long before birds (including chickens) evolved from dinosaurs.
"The egg came before the chicken," Zelenitsky said. "Chickens evolved well after the meat-eating dinosaurs that laid these eggs."
So the original riddle might now be rephrased: Which came first, the dinosaur or the egg? Meanwhile, the new nest provides some of the strongest evidence in North America in favor of the bird-like egg over the chicken.
Rare dino nests
The fossil nest was collected in the 1990s and kept at Canada Fossils Limited in Calgary, Alberta. That's where Zelenitsky first spotted the remains, which were labeled at first as belonging to a duck-billed dinosaur, an herbivore. (In 2007, the fossil was acquired by the Royal Tyrrell Museum of Paleontology in Alberta.)
Zelenitsky realized that the nest and eggs actually belonged to a small theropod, a meat-eating dinosaur. In particular, the egg-layer was likely a maniraptoran, the group of theropods that paleontologists think birds derived from some 150 million years ago during the Jurassic Period.
"Nests of small theropods are rare in North America and only those of the dinosaur Troodon have been identified previously," said Zelenitsky. "Based on characteristics of the eggs and nest, we know that the nest belonged to either a caenagnathid [a family of maniraptorans] or a small raptor, both small meat-eating dinosaurs closely related to birds."
She added, "Either way, it is the first nest known for these small dinosaurs."
The only other egg clutch identified to date from a maniraptoran in North America belonged to Troodon formosus.
Egg-laying behaviors
The analysis of the nest, detailed in the latest issue of the journal Palaeontology, provides paleontologists with information about egg-laying in this particular dinosaur and others, along with the evolution of various egg-laying behaviors, Therrien said.
"Our research tells us a lot about the dinosaur that laid the eggs and how it built its nest," he said.
For instance, the position and spacing of the eggs suggest the original clutch contained at least 12 eggs arranged in a ring around the mound's flat top, where the theropod would have sat and brooded its clutch. The eggs were about 5 inches (12 cm) long and, like bird eggs, they were pointed at one end.
The analysis also suggests the dinosaur laid its eggs two at a time on the sloping sides of the mound. That's unlike, say, crocodiles, which lay all their eggs at once, and more like birds, which lay one egg at a time. (The ancestors of crocodiles gave rise to dinosaurs and later on, birds.)
As if figuring out the chicken-egg puzzle weren't enough, the researchers also have another objective: "To find the same kind of nest with babies inside," Zelenitsky told LiveScience. "There are dinosaur eggs from North America with baby bones preserved inside of them. It's entirely possible, but again these types of nests (from small meat-eating dinosaurs) are fairly rare."
The research was funded by Richard and Donna Strong, the Alberta Ingenuity Fellowship Fund and the Killam Fellowship Fund.
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