大堡礁三萬年來的興衰起落

原文網址：https://sydney.edu.au/news-opinion/news/2018/05/29/rise-and-fall-of-the-great-barrier-reef-over-30-000-years.html

大堡礁三萬年來的興衰起落

全世界最大的珊瑚礁系統曾經歷過五次死亡事件

由雪梨大學的副教授Jody Webster領導的國際研究顯示大堡礁具有從劇烈的環境變遷中復原的韌性，但它們對沉積物輸入量的增加和水質惡化也極為敏感。

在一項關於大堡礁的劃時代國際研究中，研究人員顯示過去三萬年來，這座世界最大的珊瑚礁系統曾經遭受五次死亡事件，成因大都是海平面變化以及隨之產生的環境變遷。

透過在海底四處遷移的方式，大堡礁能在數千年內適應海水漲落時迅速發生的環境變化。

由雪梨大學的副教授Jody Webster領導的這篇研究發表於今日的《自然―地質科學》(Nature Geoscience)，他們首度重建出過去30000年以來，大堡礁因應劇烈且迅速的環境變遷而隨之演化的歷程。

這項歷經10年的跨國研究成果顯示，大堡礁從海平面上升或海洋溫度變化之類的重大環境變遷中復原的韌性比之前認為的還強，但它對沉積物輸入量的增加和水質惡化也極為敏感。

屬於雪梨大學地球科學院和海岸地質研究團隊的副教授Webster表示，在全世界的珊瑚礁都在減少的當下，大堡礁的韌性是否足以讓它撐過這次危機仍然是個未知數。

「我們的研究顯示大堡礁有能力從上個冰河期和冰消期發生的死亡事件中復原回來。」他說，「然而，我們也發現大堡礁對於流入的沉積物增加是極為敏感的，從現今的土地利用情形來看這很令人擔憂。」

研究用的數據為化石礁核的地形、沉積學、生物和定年資訊，它們分布於凱恩斯和麥凱的16個地點。

研究涵蓋的時間從「末次冰盛期」(Last Glacial Maximum)以前就開始了。末次冰盛期大約位於距今20000年前，當時的海平面比現在的還低了118公尺。

歷史上的死亡事件

在末次冰盛期以前數千年的海面下降過程中，大約在30000年前和22000年前發生了兩次大範圍的死亡事件，原因是珊瑚礁暴露在空氣當中(也稱作露出地表)。在這段期間，大堡礁往海洋的方向移動以試著跟上海面下降的速度。

而在末次冰盛期之後的冰消期，大約17000年和13000年前發生了另外兩起珊瑚礁死亡事件，原因為海平面迅速上升。在此同時大堡礁往陸地的方向移動以試著追上海面上升的速度。

分析礁體樣本和沉積物輸入速率的數據，顯示海平面上升時發生的珊瑚礁死亡事件可能跟沉積物大幅增加有關。

最後一次的珊瑚礁死亡事件發生在大約10000年前，不久之後，在距今9000年前左右現代的大堡礁開始形成。該次事件跟已知的任何一次海平面突然急遽上升，或者是冰消期出現的「冰融水脈衝」(meltwater pulse)都沒有關聯。反之，它看起來跟全球海平面上升伴隨而來的沉積物大量增加以及水質下降有所關聯。

作者提出隨著時間經過大堡礁可以憑藉自身之力東山再起，是因為可供珊瑚和珊瑚藻建立礁體的棲地一直都在，加上珊瑚礁能以每年0.2至1.5公尺的速度橫向遷移。

未來的存活機會

然而，Webster副教授表示這種速度恐怕沒辦法讓大堡礁撐過它現在面臨的危機：包括海溫快速上升、珊瑚覆蓋面積急速降低、年復一年的珊瑚白化，以及自歐洲人遷入之後持續惡化的水質，而沉積物進入環境的量也越來越多。

他說：「從現況看來，我很擔心大堡礁的能耐是否可以讓它度過目前這些眾多的壓力，以及在不遠之後的未來預計產生的更多壓力所造成的整個變化歷程。」

Webster副教授表示前人研究得出海洋表面溫度曾經上升幾度，但這是建立在數萬年的時間尺度之上。然而，在現今的預測中，海洋表面溫度在一個世紀之內就會上升大約0.7度。

他說：「我們的研究顯示過去大堡礁除了能因應海面變化而做出調整以外，它對沉積物的輸入量特別敏感，意味著我們需要瞭解第一級產業的運作，會對現在這段期間的沉積物輸入量以及礁體上方的水質造成什麼影響。」

Rise and fall of the Great Barrier Reef over 30,000 years

World’s largest reef system has suffered five death events

An international study led by University of Sydney's Associate Professor Jody Webster has shown the reef is resilient to major environmental changes but is highly sensitive to increased sediment input and poor water quality.

A landmark international study of the Great Barrier Reef has shown that in the past 30,000 years the world’s largest reef system has suffered five death events, largely driven by changes in sea level and associated environmental change.

Over millennia, the reef has adapted to sudden changes in environment by migrating across the sea floor as the oceans rose and fell.

The study published today in Nature Geoscience, led by University of Sydney’s Associate Professor Jody Webster, is the first of its kind to reconstruct the evolution of the reef over the past 30 millennia in response to major, abrupt environmental change.

The 10-year, multinational effort has shown the reef is more resilient to major environmental changes such as sea-level rise and sea-temperature change than previously thought but also showed a high sensitivity to increased sediment input and poor water quality.

Associate Professor Webster from the University’s School of Geosciences and Geocoastal Research Group said it remains an open question as to whether its resilience will be enough for it to survive the current worldwide decline of coral reefs.

“Our study shows the reef has been able to bounce back from past death events during the last glaciation and deglaciation,” he said. “However, we found it is also highly sensitive to increased sediment input, which is of concern given current land-use practices.”

The study used data from geomorphic, sedimentological, biological and dating information from fossil reef cores at 16 sites at Cairns and Mackay.

The study covers the period from before the “Last Glacial Maximum” about 20,000 years ago when sea levels were 118 metres below current levels.

History of death events

As sea levels dropped in the millennia before that time, there were two widespread death events (at about 30,000 years and 22,000 years ago) caused by exposure of the reef to air, known as subaerial exposure. During this period, the reef moved seaward to try to keep pace with the falling sea levels.

During the deglaciation period after the Last Glacial Maximum, there were a further two reef-death events at about 17,000 and 13,000 years ago caused by rapid sea level rise. These were accompanied by the reef moving landward, trying to keep pace with rising seas.

Analysis of the core samples and data on sediment flux show these reef-death events from sea-level rise were likely associated with high increases in sediment.

The final reef-death event about 10,000 years ago, from before the emergence of the modern reef about 9000 years ago, was not associated with any known abrupt sea-level rise or “meltwater pulse” during the deglaciation. Rather it appears to be associated with a massive sediment increase and reduced water quality alongside a general rise in sea level.

The authors propose that the reef has been able to re-establish itself over time due to continuity of reef habitats with corals and coralline-algae and the reef’s ability to migrate laterally at between 0.2 and 1.5 metres a year.

Future survival

However, Associate Professor Webster said it was unlikely that this rate would be enough to survive current rates of sea surface temperature rises, sharp declines in coral coverage, year-on-year coral bleaching or decreases in water quality and increased sediment flux since European settlement.

“I have grave concerns about the ability of the reef in its current form to survive the pace of change caused by the many current stresses and those projected into the near future,” he said.

Associate Professor Webster said previous studies have established a past sea surface temperature rise of a couple of degrees over a timescale of 10,000 years. However, current forecasts of sea surface temperature change are around 0.7 degrees in a century.

“Our study shows that as well as responding to sea-level changes, the reef has been particularly sensitive to sediment fluxes in the past and that means, in the current period, we need to understand how practices from primary industry are affecting sediment input and water quality on the reef,” he said.

原始論文：Jody M. Webster, Juan Carlos Braga, Marc Humblet, Donald C. Potts, Yasufumi Iryu, Yusuke Yokoyama, Kazuhiko Fujita, Raphael Bourillot, Tezer M. Esat, Stewart Fallon, William G. Thompson, Alexander L. Thomas, Hironobu Kan, Helen V. McGregor, Gustavo Hinestrosa, Stephen P. Obrochta, Bryan C. Lougheed. Response of the Great Barrier Reef to sea-level and environmental changes over the past 30,000 years. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0127-3

引用自：University of Sydney. “Rise and fall of the Great Barrier Reef over 30,000 years.”

24億年前陸地抬升出海面，也改變了地球

原文網址：www.sciencedaily.com/releases/2018/05/180523133221.htm

24億年前陸地抬升出海面，也改變了地球

一篇由奧勒岡大學領導的研究探討頁岩中經由風化所形成的訊號，而指出改變地球氣候、地質和生物的事件發生於何時。

頁岩是地球上最常見的沉積岩，其中的化學訊號指出24億年前陸地以相當快的速度抬升出海面，可能對氣候和生物造成了巨大的影響。

此篇研究發表在5月24日發行的《自然》(Nature)期刊，研究人員表示他們從世界各地採集的頁岩樣品含有品質絕佳的證據，顯示地球最早從35億年前就開始有極為微量的雨水對陸地造成了風化作用。

主要作者，奧勒岡大學的地質學家Ilya Bindeman表示，氧17與氧18和較為常見的氧16之間的比例出現的顯著變化，讓研究人員得以解讀這些岩石化學成分的變化歷史。

藉此他們可以確定從什麼時候開始，新形成的地殼表面會受到物理和化學作用造成的風化，或者更廣義來說，在今日進行的水文作用中，水流經大片陸地途中蒸發成水氣的過程是從什麼時候開始發生。

證據來自於三種氧同位素的分析結果，尤其是稀有的穩定同位素氧17。他們分析的278具頁岩樣品來自於全球各地的露頭和鑽井，年代則涵蓋了地球歷史當中的37億年；分析進行的地點為Bindeman的穩定同位素實驗室。

Bindeman表示根據他自己先前進行的模擬以及其他研究，24億年前地球陸塊的總體積或許已經達到今日可觀測體積的三分之二了。而且這些新生陸地出現的速度相當快，同時地函動力學也發生了大規模變化。

此時的頁岩樣品中的同位素紀錄也吻合理論中陸塊碰撞而形成地球第一座超大陸――凱諾蘭大陸(Kenorland)――的時間點，並形成了高聳的山脈和高原。

「地殼要有足夠的厚度才能凸出水面，」Bindeman表示。「厚度除了跟地殼本身的體積有關，也和地函的熱力學機制和黏滯性有關。當地球溫度較高而地函較軟弱時，地函無法支持高山形成。我們的數據指出情況在24億年前出現了劇變，地函溫度降低使得地函可以支撐大片陸地挺出海面。」

他說當時從海面冒出的新生陸地，表面溫度可能比今天的還要高上數十度。

研究發現氧的三種同位素在這段時間附近出現了階段式變化。科學家表示這解決了過往對於11億年前至35億年前，陸地究竟是逐漸形成還是分段出現的爭議。Bindeman表示在24億年前，這些新形成的陸地開始藉由化學風化作用消耗大氣中的二氧化碳。

這個時間點也和太古代過渡至元古代時的其他轉變同時發生，此時地球上的生命從存活於海裡的簡單原核生物，包括古菌和細菌，開始出現真核生物，像是藻類、植物和真菌。

「在此研究中，我們著眼於35億年來風化作用的歷程。」Bindeman表示，「抬升出海面的陸地會改變地球的反照率(albedo )。地球最初從宇宙看起來是一顆有些許白雲繚繞的深藍色星球。新形成的陸地會增加地球反射陽光的能力。現在我們的陸地顏色較深是因為有大量的植被覆蓋。」

他說新生陸地遭受風化作用之後或許可以儲集二氧化碳之類的溫室氣體，因而打亂地球的輻射平衡，造成地球在24億至22億年前出現一連串冰河期。他說這段冰河期可能又釀成了之後的大氧化事件，大氣變化使得大量自由氧進入到空氣當中。岩石因為氧化作用而變成赤紅色，反觀太古代的岩石則是灰色的。

Bindeman表示在沒有太多陸地的情況下，從太陽來的光子主要是跟水互相作用並使其溫度提高。新生陸地呈現的明亮表面會把陽光反射回太空，使得輻射作用和溫室氣體之間的平衡出現新的變數，造成氣候變化。

「我們猜測大型陸塊一旦出現，就會有許多陽光被反射回太空，而啟動一發不可收拾的冰河期。」Bindeman表示，「地球因此第一次看見雪。」

頁岩是由地殼風化的產物組成。

「它們(頁岩)可以告訴你許多關於地殼受到空氣、光和降水作用的情形。」Bindeman表示，「形成頁岩的過程中會捕捉到有機物，最終會成為土壤形成過程的一部份。頁岩提供了我們關於風化作用的連續紀錄。」

Land rising above the sea 2.4 billion years ago changed planet Earth

A University of Oregon-led study of chemical signatures in shales formed through weathering points to changes that transformed climate, geology and life on the planet

 Chemical signatures in shale, the Earth's most common sedimentary rock, point to a rapid rise of land above the ocean 2.4 billion years ago that possibly triggered dramatic changes in climate and life.

In a study published in the May 24 issue of the journal Nature, researchers report that shale sampled from around the world contains archival quality evidence of almost imperceptible traces of rainwater that caused weathering of land from as old as 3.5 billion years ago.

Notable changes in the ratios of oxygen 17 and 18 with more common oxygen 16, said lead author Ilya Bindeman, a geologist at the University of Oregon, allowed researchers to read the chemical history in the rocks.

In doing so, they established when newly surfaced crust was exposed to weathering by chemical and physical processes, and, more broadly, when the modern hydrologic process of moisture distillation during transport over large continents started.

The evidence is from analyses of three oxygen isotopes, particularly the rare but stable oxygen 17, in 278 shale samples drawn from outcrops and drill holes from every continent and spanning 3.7 billion years of Earth's history. The analyses were done in Bindeman's Stable Isotope Laboratory.

Based on his own previous modeling and other studies, Bindeman said, total landmass on the planet 2.4 billion years ago may have reached about two-thirds of what is observed today. However, the emergence of the new land happened abruptly, in parallel with large-scale changes in mantle dynamics.

Isotopic changes recorded in the shale samples at that time also coincides with the hypothesized timing of land collisions that formed Earth's first supercontinent, Kenorland, and high-mountain ranges and plateaus.

"Crust needs to be thick to stick out of water," Bindeman said. "The thickness depends on its amount and also on thermal regulation and the viscosity of the mantle. When the Earth was hot and the mantle was soft, large, tall mountains could not be supported. Our data indicate that this changed exponentially 2.4 billion years ago. The cooler mantle was able to support large swaths of land above sea level."

Temperatures on the surface when the new land emerged from the sea would have likely been hotter than today by several tens of degrees, he said.

The study found a stepwise change in triple-isotopes of oxygen around that time frame. That, the scientists said, resolves previous arguments for a gradual or stepwise emergence of land between 1.1 and 3.5 billion years ago. At 2.4 billion years ago, Bindeman said, the newly emerged land began to consume carbon dioxide from the atmosphere amid chemical weathering.

The timing also coincides with the transition from the Archean Eon, when simple prokaryotic life forms, archaea and bacteria, thrived in water, to the Proterozoic Eon, when eukaryotes, such as algae, plants and fungi, emerged.

"In this study, we looked at how weathering proceeded over 3.5 billion years," Bindeman said. "Land rising from water changes the albedo of the planet. Initially, Earth would have been dark blue with some white clouds when viewed from space. Early continents added to reflection. Today we have dark continents because of lots of vegetation."

Exposure of the new land to weathering, he said, may have set off a sink of greenhouse gases such carbon dioxide, disrupting the radiative balance of the Earth that generated a series of glacial episodes between 2.4 billion and 2.2 billion years ago. That, he said, may have spawned the Great Oxygenation Event in which atmospheric changes brought significant amounts of free oxygen into the air. Rocks were oxidized and became red. Archean rocks are gray.

In the absence of much land, he said, photons from the sun interacted with water and heated it. A bright surface, provided by emerging land, would reflect sunlight back into space, creating additional torque on radiative-greenhouse balance and a change in climate.

"What we speculate is that once large continents emerged, light would be reflected back into space and initiate runaway glaciation," Bindeman said. "Earth would have seen its first snowfall."

Shales are formed by the weathering of crust.

"They tell you a lot about the exposure to air and light and precipitation," Bindeman said. "The process of forming shale captures organic products and eventually helps to generate oil. Shales provide us with a continuous record of weathering."

原始論文：I. N. Bindeman, D. O. Zakharov, J. Palandri, N. D. Greber, N. Dauphas, G. J. Retallack, A. Hofmann, J. S. Lackey, A. Bekker. Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago. Nature, 2018; 557 (7706): 545 DOI: 10.1038/s41586-018-0131-1

引用自：University of Oregon. "Land rising above the sea 2.4 billion years ago changed planet Earth: A study of chemical signatures in shales formed through weathering points to changes that transformed climate, geology and life on the planet." ScienceDaily. ScienceDaily, 23 May 2018.

猶他州出土的化石揭櫫了哺乳類近親曾經出走到全球主要陸塊

原文網址：www.sciencedaily.com/releases/2018/05/180523133203.htm

猶他州出土的化石揭櫫了哺乳類近親曾經出走到全球主要陸塊

這具小型化石證實了地球古代的超大陸之一――盤古大陸――分裂的時間比以往認為的還要晚了1500萬年左右。

這具在猶他州發現，年代將近1億3000萬年的顱骨化石在某方面來說是個驚天動地的發現。

這具小型化石證實了盤古大陸分裂的時間可能比科學家以往認為的還要晚發生，也意外證實了某一群介於哺乳類和爬蟲類之間的過渡生物，曾在數個大陸上經歷迅速的演化。

「這具幾近完整的顱骨化石是個令人難以置信的發現，據此我們辨識出一種新的原始哺乳類近親，牠們在世界各地皆有分布。」這項研究的主要作者Adam Huttenlocker表示，他是南加州大學凱克醫學院的臨床綜合解剖科學助理教授。

這項研究5月16日刊登在期刊《自然》(Nature)，它對於哺乳類在恐龍時代的演化歷程以及擴散至各大陸塊的過程提供了最新見解。作者提出盤古古陸的分裂持續時間比過往認為的還要再往後延長1500萬年；同時在白堊紀早期哺乳類仍然可以遷徙到各地，而牠們關係最近的親戚也還存活著。

「長久以來人們認為白堊紀時(1億4500萬年至6600萬年前)的早期哺乳類在解剖學上的構造皆很相似，且生態並不複雜。」Huttenlocker表示，「但我們和其他團隊的發現鞏固了以下理論：在現代哺乳類崛起許久以前，哺乳類的遠古親戚就已經發展出各種相當專一的生態棲位，像是食蟲動物、草食動物、肉食動物、水棲動物、飛行動物等。基本上，牠們佔有的各種生態區位跟我們現今看到哺乳類佔據的生態區位是相同的。」

本研究的資深作者，芝加哥大學的古生物學家Zhe-Xi Luo表示，此研究顯示了這些早期哺乳類的前身先從亞洲遷徙到歐洲，然後再進入北美洲，最後前往南半球的主要陸塊。

發現新物種的化石

Huttenlocker和猶他州地質調查具與芝加哥大學的同事將此新物種命名為Cifelliodon wahkarmoosuch。

這具化石在猶他州東部的白堊紀岩層發現，屬名的由來是紀念知名古生物學家Richard Cifelli；種名「wahkarmoosuch」在生活於發現區域的猶特族語言中的意思是「黃貓」。

科學家利用高解析度的電腦斷層掃描儀來分析這具頭骨。

「在西部內陸富含大量化石的地區中可以找到超過150種的哺乳類和類似爬蟲類的哺乳類前身，但牠們發現時大都只有幾顆牙齒或是僅有顎骨，因此Cifelliodon的頭骨確實是極其罕有的發現。」主要負責挖掘過程的研究共同作者，猶他州立大學的古生物學家James Kirkland表示。

他們估計這隻Cifelliodon的體重只有2.5磅(約1.5公斤)，跟現生的多數哺乳類相比小了許多，但牠和生活於白堊紀的其他同類相比已經是巨無霸了。一隻成年的Cifelliodon的體型大概跟一隻小型兔子或鼠兔差不多(鼠兔是一種小型哺乳類，其耳朵為圓形，四肢短小，尾巴很短)。

牠的牙齒類似於果蝠，可以咬下、撕裂並嚼碎食物。飲食當中可能包括了植物。

這個新命名的物種的腦容量並不大，負責處理嗅覺的「嗅球」相對而言卻很巨大。而顱骨上的眼窩相當小巧，因此Cifelliodon的視力或者辨色能力或許不太好。Huttenlocker表示牠可能是一種夜行型動物，依靠嗅覺來找到牠的食物。

超大陸的存在時間比以往認為的還要久

Huttenlocker和他的同事將Cifelliodon 分類在小賊獸目(Haramiyida)之下，這支已經滅絕的哺乳類祖先和真正的哺乳類之間有親緣關係。這具化石為其所屬的亞群Hahnodontidae第一次在北美發現的種類。

這具化石的發現強調出在侏儸紀和白堊紀交替之際，小賊獸目和其他某些種類的脊椎動物在全球皆有分布，代表可以讓這些動物遷徙至盤古大陸各處的廊道在白堊紀早期仍然完好無缺。

侏儸紀和白堊紀的小賊獸目化石大都發現在歐洲、格陵蘭和亞洲的三疊紀和侏儸紀岩層中(原文就是這樣)。先前已知的Hahnodontidae化石只出現在非洲北部的白堊紀岩層中。Huttenlocker主張Cifelliodon 屬於Hahnodontidae這件事提供了證據顯示當時依然存在可以讓動物遷徙的路徑，使牠們分布至現在分散於南北半球的陸塊。

「不只是小賊獸目之下的這個族群有此現象。」Huttenlocker表示，「今年以及之前的其它研究也根據在非洲和歐洲的白堊紀岩層發現類似的恐龍化石，得出盤古大陸在此時依然連結的結論。而我們的發現也反映出同一件事。」

Utah fossil reveals global exodus of mammals' near relatives to major continents

A small fossil is evidence that Earth's ancient supercontinent, Pangea, separated some 15 million years later than previously believed

A nearly 130-million-year-old fossilized skull found in Utah is an Earth-shattering discovery in one respect.

The small fossil is evidence that the super-continental split likely occurred more recently than scientists previously thought and that a group of reptile-like mammals that bridge the reptile and mammal transition experienced an unsuspected burst of evolution across several continents.

"Based on the unlikely discovery of this near-complete fossil cranium, we now recognize a new, cosmopolitan group of early mammal relatives," said Adam Huttenlocker, lead author of the study and assistant professor of clinical integrative anatomical sciences at the Keck School of Medicine of USC.

The study, published in the journal Nature on May 16, updates the understanding of how mammals evolved and dispersed across major continents during the age of dinosaurs. It suggests that the divide of the ancient landmass Pangea continued for about 15 million years later than previously thought and that mammal migration and that of their close relatives continued during the Early Cretaceous (145 to 101 million years ago).

"For a long time, we thought early mammals from the Cretaceous (145 to 66 million years ago) were anatomically similar and not ecologically diverse," Huttenlocker said. "This finding by our team and others reinforce that, even before the rise of modern mammals, ancient relatives of mammals were exploring specialty niches: insectivores, herbivores, carnivores, swimmers, gliders. Basically, they were occupying a variety of niches that we see them occupy today."

The study reveals that the early mammal precursors migrated from Asia to Europe, into North America and further onto major Southern continents, said Zhe-Xi Luo, senior author of the study and a paleontologist at the University of Chicago.

Fossil find: a new species

Huttenlocker and his collaborators at the Utah Geological Survey and The University of Chicago named the new species Cifelliodon wahkarmoosuch.

Found in the Cretaceous beds in eastern Utah, the fossil is named in honor of famed paleontologist Richard Cifelli. The species name, "wahkarmoosuch" means "yellow cat" in the Ute tribe's language in respect of the area where it was found.

Scientists used high-resolution computed tomography (CT) scanners to analyze the skull.

"The skull of Cifelliodon is an extremely rare find in a vast fossil-bearing region of the Western Interior, where the more than 150 species of mammals and reptile-like mammal precursors are represented mostly by isolated teeth and jaws," said James Kirkland, study co-author in charge of the excavation and a Utah State paleontologist.

With an estimated body weight of up to 2.5 pounds, Cifelliodon would seem small compared to many living mammals, but it was a giant among its Cretaceous contemporaries. A full-grown Cifelliodon was probably about the size of a small hare or pika (small mammal with rounded ears, short limbs and a very small tail).

It had teeth similar to fruit-eating bats and could nip, shear and crush. It might have incorporated plants into its diet.

The newly named species had a relatively small brain and giant "olfactory bulbs" to process sense of smell. The skull had tiny eye sockets, so the animal probably did not have good eyesight or color vision. It possibly was nocturnal and depended on sense of smell to root out food, Huttenlocker said.

Supercontinent existed longer than previously thought

Huttenlocker and his colleagues placed Cifelliodon within a group called Haramiyida, an extinct branch of mammal ancestors related to true mammals. The fossil was the first of its particular subgroup -- Hahnodontidae -- found in North America.

The fossil discovery emphasizes that haramiyidans and some other vertebrate groups existed globally during the Jurassic-Cretaceous transition, meaning the corridors for migration via Pangean landmasses remained intact into the Early Cretaceous.

Most of the Jurassic and Cretaceous fossils of haramiyidans are from the Triassic and Jurassic of Europe, Greenland and Asia. Hahnodontidae was previously known only from the Cretaceous of Northern Africa. It is to this group that Huttenlocker argues Cifelliodon belongs, providing evidence of migration routes between the continents that are now separated in northern and southern hemispheres.

"But it's not just this group of haramiyidans," Huttenlocker said. "The connection we discovered mirrors others recognized as recently as this year based on similar Cretaceous dinosaur fossils found in Africa and Europe."

原始論文：Adam K. Huttenlocker, David M. Grossnickle, James I. Kirkland, Julia A. Schultz, Zhe-Xi Luo. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature, 2018; DOI: 10.1038/s41586-018-0126-y

引用自：University of Southern California. "Utah fossil reveals global exodus of mammals' near relatives to major continents: A small fossil is evidence that Earth's ancient supercontinent, Pangea, separated some 15 million years later than previously believed." ScienceDaily. ScienceDaily, 23 May 2018.