為何恐龍行經赤道卻不定居下來？

原文網址：www.sciencedaily.com/releases/2015/06/150615152436.htm

Why did the dinosaur cross the equator, then not stay there?

Wild climate swings -- persisting tens of millions of years -- were too much for the dinos

為何恐龍行經赤道卻不定居下來？

持續了數千萬年之久的劇烈氣候動盪，對恐龍來說太難以承受

For more than 30 million years after dinosaurs first appeared, they remained inexplicably rare near the equator, where only a few small-bodied meat-eating dinosaurs eked out a living. The age-long absence of big plant-eaters at low latitudes is one of the great, unanswered questions about the rise of the dinosaurs.

在首批恐龍出現後的3000萬年間，赤道附近的恐龍種類難以解釋的稀少，僅有幾種體型小的肉食性恐龍在那裡勉強求生。大型草食性恐龍在如此長的時間中未出現於低緯度地區，為恐龍崛起的種種謎團中，最大的未解之謎之一。

And now the mystery has a solution, according to an international team of scientists who pieced together a remarkably detailed picture of the climate and ecology more than 200 million years ago at Ghost Ranch in northern New Mexico, a site rich with fossils from the Late Triassic Period.

新墨西哥州北部的幽靈農場(Ghost Ranch)為一座晚三疊紀的化石寶庫。現在國際科學家團隊藉由拼湊出當地超過2億年前的生態與氣候的詳盡樣貌，解答了這個謎題。

The new findings show that the tropical climate swung wildly with extremes of drought and intense heat. Wildfires swept the landscape during arid regimes and continually reshaped the vegetation available for plant-eating animals.

這項新發現顯示當時的熱帶氣候變動劇烈且受到極端的乾旱與高熱影響。在乾旱時期野火肆虐大地，迫使可供草食動物食用的植被不斷重新生長。

"Our data suggest it was not a fun place," says study co-author Randall Irmis, curator of paleontology at the Natural History Museum of Utah and assistant professor at the University of Utah. "It was a time of climate extremes that went back and forth unpredictably and large, warm-blooded dinosaurian herbivores weren't able to exist nearer to the equator -- there was not enough dependable plant food."

「我們的數據顯示當時那裡可不是個宜人的地方，」共同作者Randall Irmis說，其為猶他自然歷史博物館(the Natural History Museum of Utah)的古生物研究員與猶他大學(the University of Utah)的助理教授。「當時的極端氣候變化劇烈且捉摸不定。溫血的草食恐龍不可能生存在赤道附近，因為在那裡可靠的植物糧食來源並不足。」

The study, led by geochemist Jessica Whiteside, lecturer at the University of Southampton, is the first to provide a detailed look at the climate and ecology during the emergence of the dinosaurs. The results are important, also, for understanding human-caused climate change. Atmospheric carbon dioxide levels during the Late Triassic were four to six times current levels. "If we continue along our present course, similar conditions in a high-CO2 world may develop, and suppress low-latitude ecosystems," Irmis says.

本研究由南安普敦大學(he University of Southampton)的講師，地球化學家Jessica Whiteside所領導，為首篇詳細描述恐龍出現時的氣候與生態的研究。這些結果對於了解人類造成的氣候變遷也十分重要。晚三疊紀時大氣中的二氧化碳濃度是現今的四至六倍。「若我們還是一意孤行，環境可能會逐漸變成類似那時的高二氧化碳世界，而打擊低緯度的生態系。」Irmis說。

The other authors are Sofie Lindström, Ian Glasspool, Morgan Schaller, Maria Dunlavey, Sterling Nesbitt, Nathan Smith and Alan Turner. They report the findings today in the Proceedings of the National Academy of Sciences.

其它作者分別為Sofie Lindström、Ian Glasspool、Morgan Schaller、Maria Dunlavey、Sterling Nesbitt、Nathan Smith與Alan Turner。他們的研究成果發表在今日(2015.06.15)的《美國國家科學院院刊》(the Proceedings of the National Academy of Sciences)。

Reconstructing the deep past

重建遠古時光

The earliest known dinosaur fossils, found in Argentina, date from around 230 million years ago. Within 15 million years, multitudes of species with different diets and body sizes had evolved and were abundant beyond the tropical latitudes. In the tropics, the only dinosaurs present were small carnivores. This pattern persisted for 30 million years after the first dinosaurs appeared.

最早的恐龍化石發現於阿根廷，年代為2億3000萬年左右。之後的1500萬年間，恐龍演化出各式各樣的體型與飲食特性，並在熱帶地區以外的地方繁榮生長。而在熱帶地區唯一出現的恐龍為小型的肉食恐龍。在首批恐龍出現後的3000萬年間，牠們的分佈一直維持著這種模式。

In the new study, the authors focused on Chinle Formation rocks, which were deposited by rivers and streams between 205 and 215 million years ago at Ghost Ranch (better known to many outside of paleontology as the place where artist Georgia O'Keeffe lived and painted for much of her career). The multi-colored rocks of the Chinle Formation are a common sight on the Colorado Plateau at places such as the Painted Desert at Petrified Forest National Park in Arizona. During the Late Triassic, North America and other land masses of the world were bound together in the supercontinent Pangea. The Ghost Ranch site stood close to the equator at roughly the same latitude as present-day southern India.

在本篇新研究中，作者們將焦點放在幽靈農場2億500萬至2億1500萬年前，由河流沉積而成的欽迪組(Chinle Formation)岩石(對不熟悉古生物學的許多人來說，幽靈農場較知名之處為藝術家喬治亞·歐姬芙(Georgia O'Keeffe)職業生涯中，長期定居且完成許多畫作的地方)。色彩斑斕的欽迪組岩石為科羅拉多高原的常見景觀，亞利桑那州石化森林國家公園(Petrified Forest National Park)中的繪畫沙漠(the Painted Desert)即為一例。晚三疊紀時，北美與世界上的許多其他陸塊結合成盤古超大陸(the supercontinent Pangea)。當時的幽靈農場所在的緯度接近於赤道，約莫等同現今的南印度。

The researchers reconstructed the deep past by analyzing several kinds of data: fossils, charcoal left by ancient wildfires, and stable isotopes from organic matter and carbonate nodules that formed in ancient soils. "Each dataset complements the others, and they all point towards similar conditions," Whiteside says. "I think this is one of the major strengths of our study."

研究人員藉由分析數種資料來重建遠古時光，這些資料包括：化石、古代野火燒剩的木炭、有機物及古土壤中的碳酸鹽結核(carbonate nodule)的穩定同位素。「這些資料彼此之間能夠互補且它們傳達出的環境條件十分類似，」Whiteside說，「我認為這是我們這篇研究的有力之處之一。」

Fossilized bones, pollen grains and fern spores revealed the types of animals and plants living at different times, marked by layers of sediment. Dinosaurs remained rare among the fossils, accounting for less than 15 percent of vertebrate animal remains. They were outnumbered in diversity, abundance and body size by the reptiles known as Pseudosuchian archosaurs, the lineage that gave rise to crocodiles and alligators.

沉積岩中不同層裡面的化石骨骸、花粉顆粒與蕨類孢子代表了生活在不同時間的動植物種類。恐龍的遺骸在化石中相對來說較為稀少，僅占脊椎動物骨骸總數的15%以下。而後裔為鱷魚及短吻鱷的爬蟲類--主龍類(archosauria)的偽鱷亞目(pseudosuchia)不管在多樣性、數目與體型上都超出恐龍許多。

The sparse dinosaurs consisted mostly of small, carnivorous theropods. Big, long-necked dinosaurs, or sauropodomorphs -- already the dominant plant-eaters at higher latitudes -- did not exist at the study site or any other low-latitude site in Triassic Pangaea, as far as the fossil record shows.

這些為數不多的恐龍以小型的肉食性獸腳亞目(theropoda)為主。就現有的化石紀錄顯示，三疊紀時大型的長頸恐龍，或稱蜥腳形亞目(sauropodomorpha)，雖然已在盤古大陸較高緯度的地區成為主要的的草食動物，卻不存在於研究區域或者其他的低緯度地帶。

Abrupt changes in climate left a record in the shifting abundance of different types of pollen and fern spores between sediment layers. Fossilized organic matter from decaying plants provided another window on climate shifts. Changes in the ratio of stable isotopes of carbon in the organic matter bookmarked times when plant productivity declined during extended droughts.

突來的氣候變遷會造成沉積岩層中不同種類的花粉與孢子的數量改變而留下紀錄。植物腐爛後遺留的化石有機物也可以反映出氣候變遷。這些有機物中碳同位素比例的變化能夠標記出由於長期乾旱導致生產力下降的時期。

Drought and fire

乾旱與野火

Wildfire burn temperatures varied drastically, the researchers found, consistent with a fluctuating environment in which the amount of combustible plant matter rose and fell over time. The researchers estimated the intensity of wildfires using bits of charcoal recovered in the sediment layers. The amount of light reflected from the fossil charcoal under a light microscope relates directly to the burn temperature of the wood. The overall picture, the authors say, is that of a climate punctuated by extreme shifts in precipitation in which plant die-offs fueled hotter fires, which in turn killed more plants, damaged soils and increased erosion.

研究人員發現野火燃燒時的溫度的變化十分劇烈，這跟當時變動極大的環境中，可燃植物的數量會隨時間而有所增減相符。他們利用從沉積岩層中取出的微量木炭來估計野火的強度。在顯微鏡下木炭化石反射的光線量，會與木材燃燒時的溫度直接相關。作者說，當時的氣候整體上會穿插降雨極少的時期，在此時期火勢受乾枯的大量植物助長而變得更高溫，導致更多的植物死亡，並破壞水土保持而使侵蝕加劇。

Atmospheric carbon dioxide levels, calculated from stable isotope analyses of soil carbonate and preserved organic matter, rose from about 1,200 parts per million at the base of the section, to about 2,400 parts per million near the top. At these high CO2 concentrations, climate models predict more frequent and more extreme weather fluctuations consistent with the fossil and charcoal evidence.

分析土壤中的碳酸鹽及保存下來的有機物中的穩定同位素，他們估計研究區間的大氣二氧化碳濃度從開始時的1200 ppm，至結束時已攀升至2400 ppm。在這種二氧化碳濃度極高的環境下，氣候模型預測天氣變化會更為頻繁且愈加極端，與化石及木炭證據呈現出的情況相符。

Continuing shifts between extremes of dry and wet likely prevented the establishment of dinosaur-dominated communities found in the fossil record at higher-latitudes across South America, Europe and southern Africa, where aridity and temperatures were less extreme and humidity was consistently higher. Resource-limited conditions could not support a diverse community of fast-growing, warm-blooded, large dinosaurs, which require a productive and stable environment to thrive.

不斷發生的極端乾濕變化，可能造成當地無法建立如同南美、歐洲與非洲南部的化石紀錄中，以恐龍為主的動物群集。在那些地區的乾旱與氣溫不那麼極端，濕度也高的穩定。資源有限的環境難以支持由許多種類的成長快速、溫血的大型恐龍所組成的群集，因為牠們的生存依賴豐饒且穩定的環境。

"The conditions would have been something similar to the arid western United States today, although there would have been trees and smaller plants near streams and rivers and forests during humid times," says Whiteside. "The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive."

「當時的環境跟現在美國西部的乾燥地區有些類似。雖然在河流沿岸可能會有樹木及小型植物生長，濕潤時期也可能會有森林發育，」Whiteside說。「然而動盪且嚴峻的氣候，以及大片的野火，造成僅有某些小型的雙足肉食恐龍，例如腔骨龍(Coelophysis)才能夠生活於此。」

引用自University of Utah. "Why did the dinosaur cross the equator, then not stay there? Wild climate swings -- persisting tens of millions of years -- were too much for the dinos." ScienceDaily. ScienceDaily, 15 June 2015.