考古学家解答先有“鸡”还是先有蛋的谜团

牧童 · 发表于 2008-11-25 11:34:22

　　到底是先有的鸡还是先有的蛋，这一世纪谜团一直让人百思不得其解。最近，科学家通过对0.77亿年前一个小型肉食恐龙巢穴的研究后终于找到答案，实为先有的蛋才有的鸡。

　　先有的鸡还是先有的蛋？这个存在了几个世纪的难题，一直都是人们争论的焦点，从生物学到哲学，都没有得到令人信服的答案。不过，来自加拿大的科学家已经得到了答案，这个问题的解答从发现一个罕见的小型肉食恐龙的巢穴开始。这个巢穴存在于7千7百年前，当时海平面上升，恐龙妈妈不得不抛弃巢穴里面的恐龙蛋，自己逃生而去。留下的恐龙蛋成为了现在科学家研究的珍贵化石。

　　位于加拿大阿尔伯塔省的皇家泰瑞尔博物馆恐龙馆馆长弗朗索瓦说：“这个巢穴有着恐龙和鸟类的共有特征，通过对这个巢穴的深入研究，可以帮助我们解决一个古老的难题：到底是先有蛋还是先有鸡。”来自加拿大卡尔加里大学专门研究恐龙繁殖的古生物学家达拉·泽勒尼茨基(Darla Zelenitsky)表示：“直到现在先有蛋还是先有鸡的问题还没有能够得到解答。但是随着研究的深入，谜底逐渐清晰：恐龙首先建造了类似鸟窝的巢穴，产下了类似鸟蛋的蛋，然后恐龙再进化成鸟类（鸡也属于鸟类的一种），这很明确，蛋先于鸡之间就存在了。鸡是由这些产下了类似鸡蛋的肉食恐龙进化而成。”

　　基于上面的研究表明，原来的问题应该被改写成：是先有恐龙还是先有蛋。

　　据报道，这个巢穴是在上世纪90年代发现的，早期考古成果保存在加拿大阿尔伯塔省的卡尔加里化石有限公司。最开始这个巢穴被认为属于一种类似于鸭子的草食恐龙。后来的深入研究才发现，这应该是一种小型肉食恐龙的化石，2007年，该化石被阿尔伯塔省的皇家蒂勒尔古生物博物馆收购，现存于馆中。

　　泽勒尼茨基说：“在北美地区，这种小型肉食恐龙的巢穴是十分罕见的，基于洞穴的造型和里面的恐龙蛋化石我们可以推断，这种小型肉食恐龙和鸟类有着密切的亲属关系。”而关于这种恐龙的产卵行为，在最新一期的《古生物学》（Paleobiology）杂志上，详细的分析了这个珍稀的巢穴，并公布许多关于恐龙产蛋和建立巢穴的重要信息。这可以为古生物学家对于类似这种的小型肉食恐龙的研究提供帮助，同时也为恐龙进化成鸟类的研究提供重要的证据。

　　例如，通过对恐龙蛋在巢穴中的位置以及蛋的尺寸可以判断：一窝蛋至少包括12个，它们成环状逐个排列在沙土之上。每颗蛋大约有5英寸（约12厘米）长，这一点很像鸟蛋。分析还表明，恐龙一次会产下两个蛋，这一点和鸟类很相似，而和一次只产一个蛋的鳄鱼不同。这点从侧面证明了是恐龙进化成了鸟类。

　　古生物学家们不满足对于只是解答了先有蛋还是先有鸡这个问题，他们的下一个目标是寻找拥有这种恐龙幼仔化石的巢穴。这样可以通过对恐龙幼仔骨骼的进一步研究，来得出更多的结论。虽然在北美地区找到下个保存这样完整的巢穴很困难，但是考古学家还是想挑战一下。接下来的研究将由艾伯塔创新研究基金和基拉姆奖学金基金共同资助。

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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