如何在大滅絕事件中存活下來？匆匆活著，早早死去

原文網址：www.sciencedaily.com/releases/2016/04/160405093103.htm

How to survive a mass extinction: Live fast, die young

如何在大滅絕事件中存活下來？匆匆活著，早早死去

Life history of ancient mammal relatives provides insight on survival tactics

古代哺乳類近親的生活史讓我們得以了解牠們的生存策略

Two hundred and fifty-two million years ago, a series of Siberian volcanoes erupted and sent the Earth into the greatest mass extinction of all time. As a result of this mass extinction, known as the Permo-Triassic Mass Extinction, billions of tons of carbon were propelled into the atmosphere, radically altering the Earth's climate. Yet, some animals thrived in the aftermath and scientists now know why.

2億5200萬年前，西伯利亞一連串的火山噴發事件引發了地球史上規模最劇的大滅絕事件。又稱作二疊紀-三疊紀滅絕事件(Permo-Triassic Mass Extinction)的那次大滅絕將數十億噸的碳噴發至大氣當中，因而劇烈改變了地球氣候。即使如此，仍有一些動物在這次浩劫過後倖存下來，而科學家現今知道牠們是如何辦到的。

In a new study published in Scientific Reports, a team of international paleontologists, including postdoctoral scholar Adam Huttenlocker of the Natural History Museum of Utah at the University of Utah, demonstrate that ancient mammal relatives known as therapsids were suited to the drastic climate change by having shorter life expectancies and would have had a better chance of success by breeding at younger ages than their predecessors.

在這篇發表於《科學報導》(Scientific Reports)期刊中的新研究，由國際古生物學家組成的團隊描述了一種哺乳類的古代表親，稱作「獸孔目」(therapsid)的動物藉由活得比預期壽命(life expectancy)要短來適應如此激烈的氣候變遷。跟牠們的祖先相比，牠們在較年輕時就開始繁殖後代來讓整體有較高的存活率。作者群之一包括了猶他州立大學猶他州立博物館的博士後研究學者 Adam Huttenlocker。

The research team studied growth patterns in therapsids from the South African Karoo Basin, a paleontologically significant area which preserves a wide range of fossils from the Permian to the Early Jurassic, or 300-180 million years ago.

研究團隊探討了來自南非卡魯盆地(Karoo Basin)的獸孔目化石的生長模式。這個盆地保存了年代為3億年至1億8000萬年前，也就是二疊紀至侏儸紀早期各式各樣的化石，因此在古生物學上具有重要地位。

By examining their bone microstructure before and after the extinction boundary, Huttenlocker and his colleagues were able to study how growth patterns in therapsids were affected by the extinction. By studying body size distributions in particularly abundant species from the Permian and Triassic, the team was able to interpret shifts in size class structure and in rates of survivorship.

Huttenlocker 和他的同僚藉由檢視大滅絕前後獸孔目化石骨骼中的微構造，來探討大滅絕如何影響牠們的生長模式。研究團隊研究了從二疊紀至三疊紀數量皆相當豐富的物種的個體體型分布情況，進而了解牠們體型分佈的結構和存活率如何變化，

In this study, special attention was paid to the genus Lystrosaurus because of its success in surviving the Permo-Triassic extinction; it dominated ecosystems across the globe for millions of years during the post-extinction recovery period, and makes up some 70-90% of the vertebrate fossils found in Early Triassic rocks in the Karoo.

此研究特別將注意力聚焦在水獸龍屬(Lystrosaurus)身上，這是因為牠們很成功地在二疊紀-三疊紀滅絕事件中倖存下來，並在大滅絕過後的回復期中統治了全球生態系數百萬年的時間。卡魯盆地早三疊統岩石裡的脊椎動物化石中，牠們就佔了總數的70至90%。

"Therapsid fossils like Lystrosaurus are important because they teach us about the resilience of our own extinct relatives in the face of extinction, and provide clues to which traits confered success on lineages during this tubulent time. Lystrosaurus was particularly prolific, making it possible to build a large dataset and to sacrifice some specimens for histology to study the growth patterns recorded in its bones," said Huttenlocker, one of the paper's authors.

「水獸龍之類的獸足類化石之所以如此重要，在於牠們告訴了我們在遭遇大滅絕事件時，我們這些已消失的近親有多麼頑強；還有在這段混亂不堪的時期，具有何種特徵的後裔子孫才能夠成功存活下來。水獸龍數目眾多的特性讓我們得以建立起內容豐富的資料庫，並且使我們可以犧牲某些樣品來進行組織學研究，以得到記錄在骨骼當中的生長模式。」作者之一 Huttenlocker說。

"Before the Permo-Triassic extinction, the famous therapsid Lystrosaurus had a life span of about 13 or 14 years based on the record of growth preserved in their bones," said Field Museum paleontologist Ken Angielczyk, another one of the paper's authors. "Yet, nearly all of the Lystrosaurus specimens we find from after the extinction are only 2¬-3 years old. This implies that they must have been breeding when they were still [relatively young] themselves."

「根據保存在骨骼當中的成長紀錄，這些著名的獸孔目水獸龍在二疊紀-三疊紀滅絕事件前的壽命大約為13至14歲。」此論文的另外一位作者，菲爾德自然史博物館的古生物學家 Ken Angielczyk說。「但是，在大滅絕之後我們發現的水獸龍樣品幾乎都僅僅只有2到3歲。這意味著牠們勢必在年幼時(相對於牠們自身所屬的物種)就已經繁衍了下一代。」

This adjustment in life history also meant a physical change for Lystrosaurus. Before the mass extinction, this creature would have been a couple meters long and weighed hundreds of pounds--about the size of a pygmy hippo. Post-extinction, its size dropped to that of a large dog, in large part due to its altered lifespan. Yet, these adaptations seemed to pay off for Lystrosaurus. Ecological simulations show that by breeding younger, Lystrosaurus could have increased its chance of survival by 40% in the unpredictable environments that existed in the aftermath of the extinction.

水獸龍在生命史長度上做出的調適意味著牠們的生理結構同時也產生了變化。在大滅絕之前，這些生物的身長大約有2公尺並且重達數百磅，跟侏儒河馬的體型相偌。但在大滅絕之後，牠們的體型迅速縮小成跟大型犬差不多大，這多半跟牠們的壽命改變有關。然而，對水獸龍而言這種適應似乎相當值得。生態模型顯示在大滅絕過後，水獸龍在這種變化莫測的環境下，可以藉由在年輕時就繁殖下一代，而提高40%的存活率。

This change in breeding behavior is not isolated to ancient animals either. In the past century, the Atlantic cod has undergone a similar effect due to human interference. Industrial fishing has removed most large individuals from the population, shifting the average size of cod significantly downward. Likewise, the remaining individuals are forced to breed as early in their lives as possible. Similar shifts have also been demonstrated in African monitor lizards exploited by humans.

這種繁殖策略的改變並不僅只局限於古代動物。在上個世紀，大西洋鱈也因為人類的干擾而經歷了這種變化。捕魚工業化幾乎撈除了族群中所有的大型個體，造成鱈魚的平均體型迅速縮小。同樣地，剩餘的個體被迫要盡量在還相當年輕時就繁殖後代。相同的變化也在被人類捕捉過後的非洲巨蜥身上描述到。

"Although it's hard to see the effects in our daily lives, there is substantial evidence that we are in the middle of a sixth mass extinction right now. It has been predicted that half of mammal species could become extinct by the end of the next century if present patterns continue; that's more than 1,000 times greater than previous estimates of natural extinctions, a trend not seen since the End-Permian or End-Cretaceous extinctions," said Huttenlocker.

「雖然在我們的日常生活中很難看到這類效應，但這的確是我們現在正處於第六次大滅絕的堅實證據。科學家預測若繼續維持現今的滅絕模式，在下個世紀末將會有一半的哺乳類動物就此消失。在這種模式下物種滅亡的速度是先前預估自然滅絕亡速率的1000倍以上，這是自二疊紀末或白堊紀末的大滅絕之後前所未見的。」 Huttenlocker說。

"With the world currently facing its sixth mass extinction, paleontological research helps us understand the world around us today," said Angielczyk. "By studying how animals like Lystrosaurus adapted in the face of disaster, we can better predict how looming environmental changes may affect modern species."

「在世界正經歷第六次大滅絕的此時，古生物研究可以讓我們更加了解周遭世界發生的一切。」 Angielczyk說。「藉著研究水獸龍這類的動物如何在面臨災禍時適應環境，我們可以更準確地預測悄然逼近的環境變遷會如何影響現存的生物。」

引用自：University of Utah. "How to survive a mass extinction: Live fast, die young: Life history of ancient mammal relatives provides insight on survival tactics." ScienceDaily. ScienceDaily, 5 April 2016.