原文網址：https://news.ucsc.edu/2021/03/ocean-chemistry.html
鍶同位素的最新分析結果揭露了在地質時間當中，氣候和海平面的變化如何影響全球碳循環
By Tim Stephens
分析海洋沉積物中鍶同位素的最新結果讓科學家得以重建過去3500萬年以來，海洋化學成分的波動跟氣候因子的變化之間有什麼樣的關係。
這幅掃描式電子顯微鏡的影像是從深海沉積物岩芯中萃取出來的海洋重晶石，其提供了海洋化學性質隨著地質時間經過而變動的紀錄。(圖片來源：Adina Paytan)
遭到遺忘許久的冰芯提供了直接證據，顯示在過去一百萬年之間這座巨大的冰層曾經完全消融，也顯示了它在暖化的氣候之下有多麼岌岌可危
1966年，美國陸軍的科學家鑽入格陵蘭西北部將近一英里厚的冰層，並從底下取出一管十五英吋長的泥土。這些冰封的沉積物之後在冰庫中消失了數十年，直到2017年才在無意之間被人重新發現。
新研究探討三億多年前的八目鰻化石之後，結果質疑了關於脊椎動物演化起源廣為接受的理論。這項發現3月10日發表在科學期刊《自然》(Nature)。
藝術家重建了八目鰻化石Priscomyzon riniensis生命史的不同階段，這種魚類生活在3億6000萬年前左右一座海邊的潟湖當中。從最右邊開始，往順時針方向依序為：體型小眼睛大，帶有卵黃囊的剛孵化個體；未成年的個體；成年的個體，展露出帶有牙齒的吸盤。圖片來源：Kristen
Tietjen
八目鰻是種古老、無頷、類似鰻魚的魚類，牠們大約出現於五億年前，長久以來提供給科學家有關脊椎動物演化過程的許多觀點。最近加拿大自然博物館、芝加哥大學和南非奧爾巴尼博物館的科學家發表的文章中，他們分析了數十具微小的化石，藉此追蹤古代八目鰻生命中的不同階段，以及孵化之後從幼魚成長至成魚的過程。
現代八目鰻的幼魚無法視物並以濾食為生，普遍認為這是所有現生脊椎動物的共同祖先殘留下來的特徵。但這項研究的結果與之相反，新發現的化石顯示古代八目鰻的幼魚跟現代的完全不同。(八目鰻的幼魚和成魚在英文中各有專屬的單字，分別為ammocoete和lamprey。)
「基本上，我們把八目鰻從脊椎動物的祖先型態這個位置上移除了，」主要作者，加拿大自然博物館的古生物學家Tetsuto
Miyashita博士解釋。「因此，我們現在需要一個新的來取代。」
Miyashita解釋了八目鰻奇特的生命週期：「現代八目鰻的幼魚一孵化便會立刻將自己埋在河床當中，並以過濾食物為生。最後牠們會蛻變為成魚，改以吸血為生。由於幼魚跟成魚的樣子差異很大，科學家最初還以為牠們是不同的物種。即便之後發現牠們只是八目鰻生命週期中的早期階段，科學家還是從這些狀似原始的幼魚身上看見了我們祖先許久之前的影子。」
新發現的八目鰻化石卻改變了上述情節。這些化石屬於四個已經滅絕的不同物種，發現地點為南非與美國(伊利諾斯州和蒙大拿州)，年代為3億1000萬年至3億6000萬年前。在長度只有15公厘(跟指甲差不多大)體型最小的個體當中，研究人員發現牠們身上還帶有卵黃囊，代表牠們死前才剛孵化。更進一步的研究顯示這些未成年的個體已經有著大大的眼睛並且配備帶有牙齒的吸盤，這些特徵在現代八目鰻是成年之後才會發展出來的。
「最棒的是，我們發現的樣品數量足以使我們把八目鰻早期這幾個獨立的譜系，從出生到成年的軌跡給重建出來，」芝加哥大學的生物學教授Michael
Coates博士表示。他和奧爾巴尼博物館的Rob
Gess博士與堪薩斯大學的Kristen
Tietjin皆為研究共同作者。「牠們每一個都顯示出相同的模式：幼魚就像縮小版的成魚。」
研究人員表示結果跟演化研究中已有150年歷史的論述相悖，過往認為現代八目鰻的幼魚以及特殊的生命週期可以讓我們窺見脊椎動物遠祖的情況。但他們證實古代的八目鰻不同於現代的，生命中不需經歷盲目、以濾食維生的階段，因此推翻了一般教科書中常見的祖先模型。
在探討這些化石紀錄之後，研究人員現在認為脊椎動物演化樹的根部，更有可能是甲皮魨(ostracoderms)這類已經滅絕的盾皮魚，而現代的八目鰻幼魚則是更加近代的演化革新。
如果是這樣，為什麼現代八目鰻幼魚的樣貌會如此原始？團隊提出以濾食為生的幼魚可能是讓八目鰻得以在河川湖泊繁衍的演化革新。因為在新研究中發表的八目鰻化石皆是採自海洋沉積物，但是幼魚為盲目的濾食動物的現代八目鰻，大部分都是生活在淡水。
對於以吸血為生的古代八目鰻來說，河川湖泊是個難以遷入的艱困環境，因為其中的獵物有限且數量不穩。「八目鰻解決這項難題的方法是讓幼魚鑽進沙中，吸取所有能找到的食物碎屑，直到牠們成熟至可以開始尋找獵物來吸血，」Miyashita說。「但是這種生活方式不需要變得很複雜才能達成。因此，就算八目鰻重新發明出以濾食為生的幼魚階段，幼魚本身看起來還是很原始、簡單。一直以來我們都誤把簡單解讀成原始。」
研究人員表示這是足以重新改寫教科書的發現。「我們過去認為八目鰻是游動的時光膠囊，但牠們不只如此，」Coates說。「當我們想要了解非常久以前脊椎動物多樣化的歷史，八目鰻還是必須研讀的重要對象；不過，我們也需要體認到牠們本身同樣地會演化而有特化的情形發生。」
Long-accepted
theory of vertebrate origin upended by fossilized lamprey larvae
A new study of fossilized lampreys dating
from more than 300 million years ago is challenging a long-held theory about
the evolutionary origin of vertebrates (all animals with a backbone). The
findings are published March 10 in the science journal Nature.
Lampreys are ancient, jawless, eel-like fishes that
arose around half a billion years ago and they have long provided insights into
vertebrate evolution. Now, scientists with the Canadian Museum of Nature, the
University of Chicago and the Albany Museum in South Africa are reporting their
analysis of dozens of tiny fossils that track the life stages and growth of
ancient lampreys, from hatchlings to juveniles to adults.
Their results counter the established view that the
blind, filter-feeding larvae of modern lampreys (called ammocoetes) are a holdover
from the ancestors of all living vertebrates. The new fossil discoveries show
that ancient lamprey hatchlings were completely unlike their modern larvae
counterparts.
“We’ve basically removed lampreys from the position
of the ancestral condition of vertebrates,” explains lead author Tetsuto
Miyashita, Ph.D, a palaeontologist with the Canadian Museum of Nature. “So now
we need an alternative.”
Miyashita explains that lampreys have a curious life
cycle. “Once hatched, the larvae of modern lampreys bury themselves in a
riverbed and filter feed before eventually metamorphosing into blood-sucking
adults. The larvae are so different from adults that scientists originally
thought they were different species. Even after finding out they are just an
early phase in the lamprey life cycle, scientists saw the image of our distant
ancestors in these seemingly primitive larvae.”
The newly discovered lamprey fossils are now changing
this story. The fossils, belonging to four extinct species, were discovered in
South Africa and the United States (Illinois and Montana) and range in age from
310 to 360 million years old. The researchers found that the smallest
individuals, barely 15mm in length (fingernail sized), still carried a yolk
sac, signalling that these had only just hatched before dying. Further
examination revealed that these youngsters already had large eyes and were
armed with a toothed sucker, characteristics that in modern lamprey species
only develop in the adults.
“Remarkably, we’ve got enough specimens to
reconstruct a trajectory from hatchling to adult in several independent
lineages of early lampreys,” said Michael Coates, Ph.D, a biology professor at
UChicago and co-author of the study along with Rob Gess, Ph.D at the Albany
Museum and Kristen Tietjin at the University of Kansas. “They each show the
same pattern: the larval form was like a miniature adult.”
The researchers say that these results run counter to
the 150-year-old evolutionary narrative that modern lamprey larvae, with their
curious life cycle, offer a glimpse of deep ancestral vertebrate conditions. By
demonstrating that ancient lampreys never went through the same blind,
filter-feeding stage seen in modern species, the researchers have falsified the
conventional ancestral model shared in textbooks.
After their examination of the fossil record, the
researchers now believe that extinct armored fishes known as ostracoderms might
instead represent better candidates for the root of the vertebrate family tree,
whereas modern lamprey larvae are a more recent evolutionary innovation.
If so, then why do modern lamprey larvae appear so
primordial? The team suggests that the evolution of filter-feeding larvae may
have been an innovation that allowed lampreys to populate rivers and lakes.
Fossil lampreys reported in the new study all came from marine sediments, but
modern lampreys, with their filter-feeding, blind larvae, mostly live in
freshwater.
For ancient blood-sucking lampreys, rivers and lakes
would be tough environments in which to migrate because of the limited and
unpredictable supply of prey. “Lampreys solved this problem by burrowing in
sand and slurping up whatever food particles were available until they could
mature enough to start looking for prey with blood,” said Miyashita. “But you
don’t have to be too complicated for this kind of lifestyle. So even though
lampreys invented this filter feeding larval phase anew, the larvae themselves
looked simple and primordial. Until now we were misinterpreting this simplicity
for primitiveness.”
The researchers say that this is the sort of
discovery that can rewrite textbooks. “Lampreys are not quite the swimming time
capsules that we once thought they were,” said Coates. “They remain important
and essential for understanding the deep history of vertebrate diversity, but
we also need to recognize that they, too, have evolved and specialized in their
own right.”
原始論文：Tetsuto
Miyashita, Robert W. Gess, Kristen Tietjen, Michael I. Coates. Non-ammocoete
larvae of Palaeozoic stem lampreys. Nature, 2021; DOI: 10.1038/s41586-021-03305-9
引用自：Canadian Museum of Nature. "Long-accepted
theory of vertebrate origin upended by fossilized lamprey larvae."
原文網址：http://hpstar.ac.cn/contents/27/11981.html
Pierfranco Demontis在1988年說：「冰在高溫高壓之下會變成快離子導體(fast-ion conductor)。」但他的預測不久之前才脫離假說的層次。2018年，經過30年的研究之後才經由實驗證明超離子水冰的存在。而超離子性或許最後還能解釋巨型行星內部的強力磁場如何產生。
原文網址：http://news.unm.edu/news/did-teenage-tyrants-outcompete-other-dinosaurs
研究人員探討肉食性恐龍的後代如何減少物種多樣性
新墨西哥大學和內布拉斯加大學林肯分校的古生態學家，證明霸王龍這類巨型肉食性恐龍的後代可能會淘汰其他與之競爭的較小型物種，因此徹底重新改造了牠們所處的群集。
恐龍體型分布出現的空缺v.s.現代的肉食性哺乳類。圖片來源：UNM Newsroom
