南極底層水淡化的速率高於預期

原文網址：www.sciencedaily.com/releases/2017/01/170125145741.htm

南極底層水淡化的速率高於預期

研究人員表示此改變可能會打亂海洋環流並加速海平面上升

南極底層水(Antarctic Bottom Water)是全球海洋環流系統的一部份，其沿著冰冷的深海海床，將氧氣、碳及其他營養成分供應至全球海洋。過去十年以來，科學家一直觀察著南極底層水的變化。由伍茲霍爾海洋研究所(WHOI)最新進行的研究指出，這些變化正往出人意料的方向進行，而可能對海洋和氣候造成重大影響。

刊登在1月25日《科學前緣》(Science Advances)期刊的論文中，由WHOI的海洋學家Viviane Menezes和Alison Macdonald領導的研究團隊發表南極底層水(AABW)在2017年至2016年期間以驚人的速率變淡許多――此改變可能會改變海洋環流而最終造成海平面上升。

本研究的主要作者，WHOI的博士後研究員Menezes表示：「如果環流發生了變化，海中的一切也會隨之改變。」海洋環流驅動了全球冷暖水體彼此之間的移動過程，因此對地球的溫度和氣候來說，它在熱量的儲存及調節環節上具有重要地位。「我們仍尚未知曉其中的所有情節。雖然我們取得了一些新線索，但還無法破解整個謎題。」

這道謎題本身存在已久：過往研究指出AABW在這數十年間經歷了重大變化。自1990年代起，一項國際計畫就已經反覆對全球特定幾個海盆進行週期性採樣，以追蹤不同地區的海流及環境因子如何隨時間變化。這些採樣位置，或稱「站位」中有一串是從南極延伸至印度洋東部，研究人員即沿著這串站位來追蹤AABW的特性。AABW為流經深海，低於0℃的極冷水層(由於含鹽量，即「鹽分」較高的緣故，因此仍處於液態)。當它環繞南大洋並往北流進三大洋盆時會跟暖水混和。

AABW形成於南極的冰棚，此處颳起的強風會冷卻稱作冰間湖(polynyas)的開放水域，造成部分海水開始凍結。由於水中的鹽分不會結凍，因此冰層附近未結冰的水體就會越來越鹹。而鹽分會增加海水的密度，使得這些海水沉往海底。

「這些海水被視為大尺度全球海洋環流的基石。」本研究的共同作者，WHOI的高級研究專員Macdonald表示。「南極底層水的性質有些來自於大氣，像是溶解其中的碳和氧氣。這些性質會被南極底層水送往海洋深處。之後隨著南極底層水流往世界各地，它會跟周圍的水團混和並且開始交換彼此之間的特性。這就像是深深吸入一口氣之後，花上數十年甚至數百年的時間極為緩慢地吐出一般。」

因此，這道凜冽的海流對全球海洋的環流、溫度、含氧量及養分的調控上都有重要地位。它也可以用來衡量氣候變遷的影響程度，同時也是氣候變遷的影響因子之一。

過往利用重覆測量得到的數據所進行的研究發現，1994年至2007年AABW變得較為溫暖且較淡(也就是鹽分降低)。當Macdonald和Menezes重新探訪這條測線，他們測量了自此之後AABW歷經了什麼樣的變化。

在2016年南半球的夏季，他們參與了研究船R/V Revelle從南極往北航行至澳洲的航次，期間他們每3海浬就得在頻繁發生的暴風雨當中採集數據。在船上實驗室，他們跟研究共同作者，斯克里普斯海洋研究所的Courtney Schatzman合力以鹽溫深感測器(CTD sensors)分析樣品的鹽度、溫度與其他性質。而原始數據目前由Schatzman持有。

研究團隊發現雖然步調減緩了一些，但先前觀測到的暖化趨勢仍持續著。然而，讓他們最感驚訝之處在於樣品缺乏鹽分的程度：本區的AABW在過去十年淡化的幅度是1994至2007年的四倍。

「當我看到鹽度變化時的念頭是：『哦，哇!』」Menezes說。「有時你收集到數據後得花上2到3年才能發現其中端倪，但這次我們在數個小時內就明瞭我們找到了什麼，而且還知道它有多麼出人意料。」

若全球皆發生如此巨幅的改變，可能會嚴重打亂海洋環流並且影響海平面甚鉅。

「當水變得較淡且較暖，它的密度也會跟著下降，造成水體膨脹且占據更多空間――這會促使海平面上升。」Macdonald表示。「如果這些海水不再下沉，會對全球海洋環流的模式造成十分深遠的影響。」

關於這項改變的成因仍然環繞著許多謎團。Menezes和Macdonald推測淡化可能起源於近年發生的一齣大幅改變地貌的事件。2010年，跟羅德島差不多大(比花蓮略小)的冰山撞上南極默茨(Mertz)冰川的前緣，而開鑿出一座超過1000平方英哩的冰河斷塊，並造成阿黛利地和喬治五世地海岸的冰原地貌發生劇變，此處正是本研究觀察的AABW的推測形成位置。之後該斷塊融化而大幅沖淡了此處的海水，可能進一步也讓AABW變得更淡。未來研究可以利用化學分析來追溯AABW的來源是否為那次撞擊及崩解事件發生的位置，以證實理論是否為真。

Antarctic bottom waters freshening at unexpected rate

Shift could disturb ocean circulation and hasten sea level rise, researchers say

In the cold depths along the sea floor, Antarctic Bottom Waters are part of a global circulatory system, supplying oxygen-, carbon- and nutrient-rich waters to the world's oceans. Over the last decade, scientists have been monitoring changes in these waters. But a new study from the Woods Hole Oceanographic Institution (WHOI) suggests these changes are themselves shifting in unexpected ways, with potentially significant consequences for the ocean and climate.

In a paper published January 25 in Science Advances, a team led by WHOI oceanographers Viviane Menezes and Alison Macdonald report that Antarctic Bottom Water (AABW) has freshened at a surprising rate between 2007 and 2016 -- a shift that could alter ocean circulation and ultimately contribute to rising sea levels.

"If you change the circulation, you change everything in the ocean," said Menezes, a WHOI postdoctoral investigator and the study's lead author. Ocean circulation drives the movement of warm and cold waters around the world, so it is essential to storing and regulating heat and plays a key role in Earth's temperature and climate. "But we don't have the whole story yet. We have some new pieces, but we don't have the entire puzzle."

The puzzle itself isn't new: past studies suggest that AABW has been undergoing significant changes for decades. Since the 1990s, an international program of repeat surveys has periodically sampled certain ocean basins around the world to track the circulation and conditions at these spots over time. Along one string of sites, or "stations," that stretches from Antarctica to the southern Indian Ocean, researchers have tracked the conditions of AABW -- a layer of profoundly cold water less than 0°C (it stays liquid because of its salt content, or salinity) that moves through the abyssal ocean, mixing with warmer waters as it circulates around the globe in the Southern Ocean and northward into all three of the major ocean basins.

The AABW forms along the Antarctic ice shelves, where strong winds cool open areas of water, called polynyas, until some of the water freezes. The salt in the water doesn't freeze, however, so the unfrozen seawater around the ice becomes saltier. The salt makes the water denser, causing it to sink to the ocean bottom.

"These waters are thought to be the underpinning of the large-scale global ocean circulation," said Macdonald, a WHOI senior research specialist and the study's co-author. "Antarctic Bottom Water gets its characteristics from the atmosphere -- for example, dissolved carbon and oxygen -- and sends them deep into the ocean. Then, as the water moves around the globe, it mixes with the water around it and they start to share each other's properties. It's like taking a deep breath and letting it go really slowly, over decades or even centuries."

As a result, the frigid flow plays a critical role in regulating circulation, temperature, and availability of oxygen and nutrients throughout the world's oceans, and serves as both a barometer for climate change and a factor that can contribute to that change.

A past study using the repeat survey data found that AABW had warmed and freshened (grown less saline) between 1994 and 2007. When Macdonald and Menezes revisited the line of stations, they measured how AABW has changed in the years since.

During the austral summer of 2016, they joined the crew of the research ship R/V Revelle and cruised north from Antarctica to Australia, braving frequent storms to collect samples every 30 nautical miles. In a shipboard lab, they analyzed the samples using data from conductivity-temperature-depth (CTD) sensors, which measure the water's salinity, temperature and other properties, with support from study co-author Courtney Schatzman of the Scripps Institution of Oceanography, who processed the raw data.

The team found that the previously detected warming trend has continued, though at a somewhat slower pace. The biggest surprise, however, was its lack of saltiness: AABW in this region has grown fresher four times faster in the past decade than it did between 1994 and 2007.

"I thought, 'Oh wow!' when I saw the change in salinity," said Menezes. "You collect the data and sometimes you spend 2 to 3 years to find something, but this time we knew what we had within hours, and we knew it was very unexpected."

Such a shift, were it global, could significantly disrupt ocean circulation and sea levels.

"The fresher and warmer the water is, the less dense it will be, and the more it will expand and take up more space -- and that leads to rising sea levels," Macdonald said. "If these waters no longer sink, it could have far reaching affects for global ocean circulation patterns."

Questions remain around the cause of the shift. Menezes and Macdonald hypothesize that the freshening could be due to a recent landscape-changing event. In 2010, an iceberg about the size of Rhode Island collided with Antarctica's Mertz Glacier Tongue, carving out a more-than-1,000-square-mile piece and reshaping the icescape of the George V/Adelie Land Coast, where the AABW observed in this study is thought to form. The subsequent melting dramatically freshened the waters there, which may have in turn freshened the AABW as well. Future studies could use chemical analysis to trace the waters back to the site of the collision and calving and confirm the hypothesis.

原始論文：Viviane V. Menezes, Alison M. Macdonald and Courtney Schatzman. Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean. Science Advances, January 2017 DOI: 10.1126/sciadv.1601426

引用自：Woods Hole Oceanographic Institution. "Antarctic bottom waters freshening at unexpected rate: Shift could disturb ocean circulation and hasten sea level rise, researchers say." ScienceDaily. ScienceDaily, 25 January 2017. 

古生物學家找到了神祕的古代錐形海洋生物的分類地位

原始網址：www.sciencedaily.com/releases/2017/01/170111132815.htm

古生物學家找到了神祕的古代錐形海洋生物的分類地位

演化樹上的一根分枝在今天變得更加擁擠了一些。由多倫多大學20歲的學生Joseph Moysiuk領導的科學家團隊，終於確認了一種已經滅絕的奇特錐形動物族群的真實身分。

被稱為軟舌螺(hyolith)的這種海洋生物大約是在5.3億年前的寒武紀期間演化出來，牠們也是已知會形成礦物質外骨骼的首批動物之一。

雖然長久以來牠們被認為跟蝸牛、烏賊和其他軟體動物所屬同一家族，今日在科學期刊《自然》上刊登的一篇論文卻證實軟舌螺與腕足動物的關係更為密切。儘管腕足動物門存活至今的現生物種相當稀少，但牠們卻留下了相當豐碩的化石紀錄。

腕足動物擁有被上下外殼包覆的柔軟本體，這跟雙殼軟體動物的外殼呈現左右排列有所不同。腕足動物在攝食時會從前端打開牠們的外殼，其餘時刻則緊緊閉闔以保護他們的攝食構造以及身體的其他部分。

雖然化石紀錄中軟舌螺的骨骸為數眾多，但含有牠們軟組織構造中重要特徵的化石至今為止仍然付之闕如。

「我們最重要也最驚人的發現是找到了軟舌螺的攝食構造。這是一排從口部伸出的柔軟觸手，位在錐狀的下殼與蓋子般的上殼之間。」Moysiuk表示。「在現生動物當中只有一群動物擁有被雙殼包覆的類似攝食構造，那就是腕足動物。此發現顯示跟軟舌螺關係最密切的現存動物是腕足動物而非軟體動物。」

Moysiuk說：「這表示軟舌螺跟現今的腕足動物一樣以懸浮在水中的有機物質維生，牠們會利用牠們的觸手將食物掃進嘴裡。」

於多倫多大學主修地球科學和生態及演化生物學的Moysiuk完成的這項專題為該校研究機會培育計畫的一部份。此計畫為多倫多大學文理學院專門為大學生設立。

過往試著分類軟舌螺時都對牠們獨特的外型和構造相當棘手。所有軟舌螺都有著細長且兩側對稱的錐狀外殼，在開口處則有一小塊蓋狀甲殼(口蓋，operculum)可以將其封住。有些種類還有一雙堅硬的弧狀尖刺(海倫體，helens)從錐狀外殼和口蓋之間延伸出來。在其他動物族群當中完全沒有跟海倫體同樣的構造。

仔細檢查從伯吉斯頁岩中找到的不同軟舌螺樣品的海倫體延伸方向，研究人員認為這些尖刺可能是這些動物將牠們的身體從沉積物中撐高時所用的支架，牠們以此舉起攝食器官來增加捕食效率。

Moysiuk以及共同作者英國杜倫大學的Martin Smith，和多倫多大學與皇家安大略博物館的Jean-Bernard Caron，主要是依據從英屬哥倫比亞著名的伯吉斯頁岩中最新發現的化石來完成他們的論述。

「伯吉斯頁岩之所以獨一無二是因為它保存了在正常情況下通常難以留存下來的軟組織。」Moysiuk的指導教授Caron表示。Caron是皇家安大略博物館無脊椎古生物學的高級研究員，同時也是多倫多大學地球科學、生態學和演化生物學系的副教授。

「雖然有些論文的作者提出牠們跟軟體動物的親緣較為接近，但這些假說的依據仍然不夠充足。軟舌螺成為演化樹上無家可歸的一道分枝，成為古生物學家難以啟齒的存在。在第一次對軟舌螺做出記述約莫175年之後，我們終於從最新的野外研究成果中找到了破解牠們故事的關鍵。」

近期由Caron領導對伯吉斯頁岩進行的野外研究成果中，找到了許多成為此研究基礎的樣品。這些關鍵樣品發現於庫特尼國家公園中，史丹利冰河和大理石峽谷附近最新發現到的沉積物。該地點位於幽鶴國家公園最初的伯吉斯頁岩場址東南方40公里處。

要研究於5.42億年前開始的寒武紀，動物如何誕生及早期的演化過程時，伯吉斯頁岩是最重要的化石庫之一。在寒武紀大爆發誕生的生物群中，軟舌螺僅是眾多動物族群裡最具代表性的其中一類。牠們成為全球海洋生態系中一個成員眾多的家族長達2.8億年，卻在首批恐龍演化出現以前的2..52億年前步入滅絕。

「寒武紀大爆發是化石紀錄中最主要的動物族群迅速演化而成的時期，解開有關軟舌螺分類地位上的爭議有助於讓我們對這段時期有更多的理解。」Smith表示。他在牛津大學時開始這項研究，現在則為杜倫大學的古生物學講師。「對於要闡釋那些我們仍所知甚少的生物的演化史來說，我們的研究再次重申了伯吉斯頁岩類型的沉積物中保存的軟組織有多麼重要。」

從數個地點發現的這些樣品皆來自伯吉斯頁岩，其為加拿大洛磯山脈世界遺產的一部分。在研究於5.42億年前左右開始發生的寒武紀大爆發期間，動物的起源和早期演化過程時，伯吉斯頁岩是最重要的化石庫之一。

加拿大公園管理局保護這處享譽全球的場所，並且贊助同儕審核的科學研究以持續增進我們對這座豐富的古生物寶庫的認識。加拿大公園管理局出版的指南每年跟眾多遊客熱心分享這些早期動物演化的精采故事，而這項發現又為此更添一筆。

本研究的主要資金來源為皇家安大略博物館，以及加拿大自然科學暨工程研究委員會授予Caron的經費。

Paleontologists classify mysterious ancient cone-shaped sea creatures

One branch on the tree of life is a bit more crowded today. A team of scientists led by 20-year-old University of Toronto (U of T) undergraduate student Joseph Moysiuk has finally determined what a bizarre group of extinct cone-shaped animals actually are.

Known as hyoliths, these marine creatures evolved over 530 million years ago during the Cambrian period and are among the first animals known to have produced mineralized external skeletons.

Long believed to belong to the same family as snails, squid and other molluscs, a study published today in the scientific journal Nature shows that hyoliths are instead more closely related to brachiopods -- a group of animals which has a rich fossil record, although few living species remain today.

Brachiopods have a soft body enclosed between upper and lower shells (valves), unlike the left and right arrangement of valves in bivalve molluscs. Brachiopods open their valves at the front when feeding, but otherwise keep them closed to protect their feeding apparatus and other body parts.

Although the skeletal remains of hyoliths are abundant in the fossil record, key diagnostic aspects of their soft-anatomy remained critically absent until now.

"Our most important and surprising discovery is the hyolith feeding structure, which is a row of flexible tentacles extending away from the mouth, contained within the cavity between the lower conical shell and upper cap-like shell," said Moysiuk. "Only one group of living animals -- the brachiopods -- has a comparable feeding structure enclosed by a pair of valves. This finding demonstrates that brachiopods, and not molluscs, are the closest surviving relatives of hyoliths.

"It suggests that these hyoliths fed on organic material suspended in water as living brachiopods do today, sweeping food into their mouths with their tentacles," Moysiuk said.

Moysiuk, who studies Earth sciences and ecology & evolutionary biology, completed this project as part of the Research Opportunity Program at U of T, a special undergraduate research program in the Faculty of Arts & Science.

The distinctive appearance and structure of the hyolith skeleton has obstructed previous attempts to classify these animals. All hyoliths had an elongated, bilaterally symmetrical cone-shaped shell and a smaller cap-like shell which covered the opening of the conical shell (known as an operculum). Some species also bore a pair of rigid, curved spines (known as helens) that protruded from between the conical shell and operculum -- structures with no equivalents in any other group of animals.

Examination of the orientation of the helens in multiple hyolith specimens from the Burgess Shale suggests that these spines may have been used like stilts to lift the body of the animal above the sediment, elevating the feeding apparatus to enhance feeding.

Moysiuk and coauthors Martin Smith at Durham University in the United Kingdom, and Jean-Bernard Caron at the Royal Ontario Museum (ROM) and U of T were able to complete the descriptions based mainly on newly discovered fossils from the renowned Cambrian Burgess Shale in British Columbia.

"Burgess Shale fossils are exceptional because they show preservation of soft tissues which are not usually preserved in normal conditions," said Caron, Moysiuk's research supervisor, who is the senior curator of invertebrate palaeontology at the ROM and an associate professor in U of T's Departments of Earth Sciences and Ecology & Evolutionary Biology.

"Although a molluscan affinity was proposed by some authors, this hypothesis remained based on insufficient evidence. Hyoliths became an orphaned branch on the tree of life, an embarrassment to paleontologists. Our most recent field discoveries were key in finally cracking their story, around 175 years after the first description of a hyolith."

Caron led recent fieldwork activities to the Burgess Shale which resulted in the discovery of many specimens that form the basis of this study. The key specimens came from recently discovered deposits near Stanley Glacier and Marble Canyon in Kootenay National Park, about 40 kilometres southeast of the original Burgess Shale site in Yoho National Park.

The Burgess Shale is one of the most important fossil deposits for studying the origin and early evolution of animals that took place during the Cambrian period, starting about 542 million years ago. Hyoliths are just one of the profusion of animal groups that characterize the fauna of the 'Cambrian Explosion'. They became a diverse component of marine ecosystems around the globe for more than 280 million years, only to go extinct 252 million years ago, prior to the evolution of the first dinosaurs.

"Resolving the debate over the hyoliths adds to our understanding of the Cambrian Explosion, the period of rapid evolutionary development when most major animal groups emerge in the fossil record," said Smith, who started this research at the University of Cambridge and who is now a lecturer in paleontology at Durham University. "Our study reiterates the importance of soft tissue preservation from Burgess Shale-type deposits in illuminating the evolutionary history of creatures about which we still know very little."

The Burgess Shale, from which the specimens were recovered from several locations, is part of the Canadian Rocky Mountain Parks World Heritage Site. It is one of the most important fossil deposits for understanding the origin and early evolution of animals that took place during the Cambrian Explosion starting about 542 million years ago.

Parks Canada protects this globally significant site, and supports peer-reviewed scientific research that continues to enhance our understanding of these rich paleontological deposits. This discovery adds another element to the dramatic story of early animal evolution that Parks Canada guides share enthusiastically with hundreds of park visitors every year.

Funding for the research was provided primarily by the Royal Ontario Museum and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Caron.

原始論文：Joseph Moysiuk, Martin R. Smith, Jean-Bernard Caron. Hyoliths are Palaeozoic lophophorates. Nature, 2017; DOI: 10.1038/nature20804

引用自：University of Toronto. "Paleontologists classify mysterious ancient cone-shaped sea creatures." ScienceDaily. ScienceDaily, 11 January 2017. 

原始網址：www.sciencedaily.com/releases/2017/01/170102155018.htm

孵化一顆恐龍蛋要花多久時間？3-6個月

人類嬰兒通常要經過九個月才出生，而鴕鳥雛鳥則花上42天就能破卵而出。但是孵化一隻恐龍寶寶得用上多久的時間？

由佛羅里達州立大學(FSU)的教授進行的研究，確認了恐龍依據種類的不同，孵化所需的時間從三至六個月不等。

在此篇發表於《美國國家科學院院刊》的文章中，FSU的生物學教授Gregory Erickson和研究團隊破解了這些史前生物的複雜生理性質，並解釋如何利用胚胎的牙齒生長紀錄，來解開孵化恐龍蛋需要多久的謎團。

「關於恐龍的最大謎題有些在於牠們的胚胎學性質――事實上我們對此幾乎一無所知。」Erickson說。「牠們的蛋是慢慢孵化，與牠們的爬蟲類表親――鱷魚和蜥蜴類似？或者較為迅速，跟現存的恐龍，也就是鳥類相仿？」

科學家許久以來假設恐龍孵化所需時間跟鳥類較為相似。鳥類的蛋孵化所需時間為11至85天，

而類似大小的爬蟲類的蛋，一般來說則要花上兩倍以上的時間，從數週到數月不等。

因為恐龍蛋的尺寸相當大，某些種類甚至有4公斤重，大小跟排球差不多。所以，科學家認為它們的孵化歷程必定相當迅速，而鳥類則從牠們的恐龍祖先身上繼承了這項特色。

Erickson、FSU的研究生David Kay和來自卡爾加里大學與美國自然史博物館的其他研究人員，決定要實際證明這些理論是否正確。

為了達成此目的，他們取得了一些相當稀有的化石――恐龍的胚胎化石。

「對動物的發育來說在蛋中的時光是相當重要的時期，但由於恐龍的胚胎相當稀少，因此我們對恐龍最初的成長階段也所知不多。」共同作者，卡爾加里大學的地質科學助理教授Darla Zelenitsky表示。「胚胎或許可以告訴我們恐龍在生命史最早期的成長發育過程，以及在這方面牠們是近似於鳥類還是爬蟲類。」

研究人員探討的兩種恐龍胚胎分別來自於原角龍(Protoceratops)和亞冠龍(Hypacrosaurus)。前者是一種發現於蒙古戈壁沙漠，綿羊大小的恐龍，其產下的蛋相當小巧(194公克)；後者則是發現於加拿大亞伯達省的巨型鴨嘴龍類，蛋可重達4公斤以上。

Erickson和他的團隊將胚胎的下顎通過電腦斷層掃描儀以得到牙齒生長的影像。接著他們取出數顆牙齒，在高精密度的顯微鏡之下做進一步的觀察。

研究人員於顯微鏡切片當中發現了他們正在尋找的證據。牙齒上的生長線可以準確告訴研究人員這隻恐龍在蛋裡面已經發育了多久。

「這些線條是在動物牙齒生長時形成的，」Erickson表示。「這有點像是樹木的年輪，但它們每天就會形成一條。因此我們可以逐條計算它們的數目以得到每一隻恐龍已經發育了多久。」

他們的成果顯示嬌小的原角龍胚胎為3個月左右，而碩大的亞冠龍胚胎則是6個月左右。

「恐龍胚胎是世上最珍貴的幾種化石之一。」這篇研究的共同作者，美國自然史博物館的馬考利圖書館的館長Mark Norell說。「在此，我們利用了美國博物館於戈壁進行考察時採集的珍貴化石樣品，結合新技術與概念，而發現到一些關於恐龍的真正最新見解。」

恐龍蛋需要長時間來孵育的概念也帶來了眾多啟發。

除了發現恐龍的孵化過程跟原始爬蟲類比較相似之外，研究人員也可以從此研究當中推論出恐龍生物學當中的許多面向。

較長的孵化期會讓恐龍蛋和他們的雙親暴露在掠食者、飢餓和其他環境危險因子的威脅當中。另外，從孵蛋以及遷移所需的時間長短來看，認為某些恐龍會在較溫和的加拿大低緯地區築巢，之後夏季時遷移到極區的理論，現在似乎變得不太可行。

然而，從此研究中衍生出來的最重要概念則跟恐龍的滅絕有關。若這些溫血生物需要大量資源才能長到成年體型、一歲以上才能生育且孵化過程緩慢，跟其他度過大滅絕事件的動物相比，牠們可以說是處在明顯劣勢的位置。

「為何恐龍會在白堊紀結束時滅絕，然而兩棲類、鳥類、哺乳類以及其他爬蟲類卻能度過這場浩劫並在之後繁榮生長？我們認為我們的發現有助於釐清這個難題。」Erickson表示。

本研究由美國國家科學基金會資助。

 

How long did it take to hatch a dinosaur egg? 3-6 months

A human typically gives birth after nine months. An ostrich hatchling emerges from its egg after 42 days. But how long did it take for a baby dinosaur to incubate?

Groundbreaking research led by a Florida State University professor establishes a timeline of anywhere from three to six months depending on the dinosaur.

In an article in the Proceedings of the National Academy of Sciences, FSU Professor of Biological Science Gregory Erickson and a team of researchers break down the complicated biology of these prehistoric creatures and explain how embryonic dental records solved the mystery of how long dinosaurs incubated their eggs.

"Some of the greatest riddles about dinosaurs pertain to their embryology -- virtually nothing is known," Erickson said. "Did their eggs incubate slowly like their reptilian cousins -- crocodilians and lizards? Or rapidly like living dinosaurs -- the birds?"

Scientists had long theorized that dinosaur incubation duration was similar to birds, whose eggs hatch in periods ranging from 11-85 days. Comparable-sized reptilian eggs typically take twice as long -- weeks to many months.

Because the eggs of dinosaurs were so large -- some were about 4 kilograms or the size of a volleyball -- scientists believed they must have experienced rapid incubation with birds inheriting that characteristic from their dinosaur ancestors.

Erickson, FSU graduate student David Kay and colleagues from University of Calgary and the American Museum of Natural History decided to put these theories to the test.

To do that, they accessed some rare fossils -- those of dinosaur embryos.

"Time within the egg is a crucial part of development, but this earliest growth stage is poorly known because dinosaur embryos are rare," said co-author Darla Zelenitsky, assistant professor of geoscience at University of Calgary. "Embryos can potentially tell us how dinosaurs developed and grew very early on in life and if they are more similar to birds or reptiles in these respects."

The two types of dinosaur embryos researchers examined were those from Protoceratops -- a sheep-sized dinosaur found in the Mongolian Gobi Desert whose eggs were quite small (194 grams) -- and Hypacrosaurus, an enormous duck-billed dinosaur found in Alberta, Canada with eggs weighing more than 4 kilograms.

Erickson and his team ran the embryonic jaws through a CT scanner to visualize the forming dentition. Then, they extracted several of the teeth to further examine them under sophisticated microscopes.

Researchers found what they were looking for on those microscope slides. Growth lines on the teeth showed researchers precisely how long the dinosaurs had been growing in the eggs.

"These are the lines that are laid down when any animal's teeth develops," Erickson said. "They're kind of like tree rings, but they're put down daily. We could literally count them to see how long each dinosaur had been developing."

Their results showed nearly three months for the tiny Protoceratops embryos and six months for those from the giant Hypacrosaurus.

"Dinosaur embryos are some of the best fossils in the world," said Mark Norell, Macaulay Curator for the American Museum of Natural History and a co-author on the study. "Here, we used spectacular fossils specimens collected by American Museum expeditions to the Gobi Desert, coupled them with new technology and new ideas, leading us to discover something truly novel about dinosaurs."

The implications of long dinosaur incubation are considerable.

In addition to finding that dinosaur incubation was similar to primitive reptiles, the researchers could infer many aspects of dinosaurian biology from the results.

Prolonged incubation put eggs and their parents at risk from predators, starvation and other environmental risk factors. And theories that some dinosaurs nested in the more temperate lower latitude of Canada and then traveled to the Arctic during the summer now seem unlikely given the time frame for hatching and migration.

The biggest ramification from the study, however, relates to the extinction of dinosaurs. Given that these warm-blooded creatures required considerable resources to reach adult size, took more than a year to mature and had slow incubation times, they would have been at a distinct disadvantage compared to other animals that survived the extinction event.

"We suspect our findings have implications for understanding why dinosaurs went extinct at the end of the Cretaceous period, whereas amphibians, birds, mammals and other reptiles made it through and prospered," Erickson said.

This research was supported by the National Science Foundation.

原始論文：Gregory M. Erickson, Darla K. Zelenitsky, David Ian Kay, and Mark A. Norell. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development. PNAS, 2017 DOI:10.1073/pnas.1613716114

引用自：Florida State University. "How long did it take to hatch a dinosaur egg? 3-6 months." ScienceDaily. ScienceDaily, 2 January 2017.