古代的貝類顯示七千萬年前的一天比現在短了半小時
現代蛤蜊的遠親、啤酒杯形狀的貝殼紀錄了白堊紀晚期的炎熱日子
發表在美國地球物理聯盟的期刊《古海洋學與古氣候學》(Paleoceanography and Paleoclimatology)的新研究中，科學家探討了白堊紀晚期軟體動物的殼體化石之後，發現恐龍時代末期地球轉動的速度比現在還快。相較於現在地球一年自轉365次，當時地球一年自轉了372次，也代表一天只有23個小時半。
一種雙殼貝，厚殼蛤的化石。採集自阿拉伯聯合大公國的哈傑爾山。圖片來源：Wikipedia,
Wilson44691 – Own work, Public Domain
這種古代軟體動物屬於一種極為多樣但已經滅絕的族群——厚殼蛤(rudist)，由於牠們的生長速度很快，因此每天都能形成生長輪。新研究運用雷射取得殼體的微小切片來計算生長輪的數目，得到的結果比人類用顯微鏡來計數還要精確許多。
研究人員透過生長輪推斷七千萬年前的一年有多少天，進而更加準確地算出一天的長度。這項新的測量結果還可以讓科學家模擬月球形成的過程，以及45億年來地球與月球的重力之舞讓兩者的距離改變了多少。
新研究也找到證據支持了這些厚殼蛤體內居住著可以進行光合作用的共生物。它們可能協助厚殼蛤建構出規模堪比現今珊瑚的礁體。
新研究擁有的高解析度方法加上快速生長的古代軟體動物，使得研究人員在分析牠們的日常生活以及周遭水體的環境條件時可以獲得前所未有的細節，甚至能看到一天當中的某個時段。
「我們在一天以內就有數個資料點，這在地質歷史上是幾乎不可能辦到的事情。基本上我們可以看見七千萬年前的某一天發生了什麼事，真的是相當驚人。」研究主要作者，布魯塞爾自由大學的分析地球化學家Niels
de Winter表示。
重建許久以前的氣侯時通常是以數萬年為尺度來描述長期的變化歷程。但類似本文的研究可以讓我們從生物的時間尺度來瞥見氣候的變化，還有機會讓我們縮小氣候與天氣模型之間的差異。
分析殼體化學成分的結果指出白堊紀晚期的海洋溫度比之前估計的還高：夏天可以到達攝氏40度，冬天則超過攝氏30度。de
Winter表示夏季的高溫可能已經逼近軟體動物的生理極限。
開放大學的古生物學退休講師以及厚殼蛤的專家Peter
Skelton並未參與這項新研究，他說：「由於這組資料具有相當高的還原程度，使得研究人員據此做出兩種令人特別感興趣的推論，這能讓我們更加瞭解白堊紀的天文年代學以及厚殼蛤的古生物學。」
古代的造礁生物
新研究的分析對象為在熱帶淺海的海床活了九年多的一具貝殼——該地在七千萬年之後的今日已經變成了阿曼山上的乾燥陸地。
Torreites
sanchezi這種古代軟體動物像是瘦高的啤酒杯，頂端有著熊掌麵包形狀的蓋子。牠們就像蛤蜊般有兩片透過絞齒扣在一起，但是不對稱的外殼；並跟現在的牡蠣一樣會密集地聚在一起而形成礁岩。在牠們繁榮生長的時候，全世界的海水比現在的海洋高了幾度。
在白堊紀晚期，T. sanchezi這類厚殼蛤是全世界熱帶海洋中主要的造礁生物，牠們佔據的生態棲位就和今日的珊瑚一樣。但在6600萬年前消滅非鳥類恐龍的事件發生時，厚殼蛤也跟著消失了。
「厚殼蛤是種相當特別的雙殼貝，跟現存的生物完全不同。」de
Winter表示。「尤其是在白堊紀晚期，全世界的造礁生物大部分都是這些雙殼貝。因此當時牠們在生態系中擔任的角色，其實就和現今珊瑚扮演的建築師一樣。」
他們用的新研究方法是把雷射聚焦在貝殼上的一點，鑽出直徑只有10微米的小洞，大概跟紅血球差不多大而已。這些微量樣品中的的稀有元素揭露出貝殼形成當下水體的溫度與化學資料。分析結果可以讓研究人員準確地測量日生長輪的寬度與數目，以及季節變化的模式。接著他們就能利用殼體化石的季節變化來得出年分。
新研究發現殼體的成分在一天當中的變化比不同季節的還要明顯，也就是隨著潮汐的循環而變動。每日形成的殼層在細微尺度下的解析結果顯示了殼體在白天的生長速度比晚上快。
「這顆雙殼貝的作息和日循環有很密切的關係，代表有光合生物跟牠共生。」de
Winter表示。「你可以看到貝殼當中記錄了光線的日夜節律。」
作者表示結果顯示出如果這種古代的軟體動物和現在的蛤蜊和牡蠣一樣，主要的進食方式是從海水過濾食物，那麼日光對牠們的生活方式來說應該不會那麼重要。De
Winter表示這種軟體動物可能和某種居住在牠體內，以日光維生的物種有共生關係，就和今日的巨硨磲蛤體內有共生藻居住一樣。
Skelton表示：「目前為止發表過的文獻中，聲稱厚殼蛤有光合生物共生的說法本質上都只是臆測而已，它們不過是根據某些型態上的特徵來進行聯想，有些例子還能證明是錯的。而此論文是首度有人能提出具有說服力的證據來支持這項假說。」不過他也提醒這篇新研究的結論可能僅限於Torreites，無法類推至其他的硬殼蛤。
後退的月亮
De Winter仔細計算每天產生的殼層之後發現一年當中有372層。這項結果並不令人驚訝，因為科學家早就知道過去的一天比較短。不過這是目前對於白堊紀晚期的計算結果中最精確的；而且令人更加驚訝的是，這還能應用在模擬地月系統的演化。
在地球歷史上，由於地球繞日的軌道不會改變，因此每年的長度都是固定的。但是一年當中的天數卻越來越少，這是因為每天都在逐漸變長。背後的原因為月球的重力會產生潮汐，其造成的摩擦力會讓地球自轉速度減緩，使得一天的長度持續增加。
潮汐的拉力會稍微加快月球的公轉速度，同時減緩地球的自轉速度並讓月球遠離地球。阿波羅計畫在月球表面留下了可以反射雷射的儀器，使我們可以運用雷射來準確地測量地球和月球的距離，結果證明月球確實正以每年3.82公分的速率遠離我們。
但是科學家認為月球不可能從古至今都以同樣的速度後退，因為若把此速度用線性的方式回推，大概到了14億年前月球就會跑到地球裡面。而科學家從其他證據知道月球伴隨我們的時間要久了許多——月球很有可能是在地球歷史早期的巨型撞擊事件之後由碎塊聚集而成，這發生在45億多年前。因此月球後退的速度必定會隨著時間變化。而來自遠古的訊息，比方說古代一顆蚌殼生命當中的某一年，就能幫助科學家重建月球的歷史並模擬月球的形成過程。
就月球的歷史來看7000萬年不過是一眨眼之間的事，因此de
Winter和同事希望可以把他們的新方法運用到更加古老的化石，進而得到更加久遠的過去裡曾經把某些日子記錄下來的影像。
Ancient shell shows days were
half-hour shorter 70 million years ago
Beer stein-shaped distant relative of
modern clams captured snapshots of hot days in the late Cretaceous
Earth turned faster at the end of the
time of the dinosaurs than it does today, rotating 372 times a year, compared
to the current 365, according to a new study of fossil mollusk shells from the
late Cretaceous. This means a day lasted only 23 and a half hours, according to
the new study in AGU’s journal Paleoceanography
and Paleoclimatology.
The ancient mollusk, from an extinct and wildly
diverse group known as rudist clams, grew fast, laying down daily growth rings.
The new study used lasers to sample minute slices of shell and count the growth
rings more accurately than human researchers with microscopes.
The growth rings allowed the researchers to determine
the number of days in a year and more accurately calculate the length of a day 70
million years ago. The new measurement informs models of how the Moon formed
and how close to Earth it has been over the 4.5-billion-year history of the
Earth-Moon gravitational dance.
The new study also found corroborating evidence that
the mollusks harbored photosynthetic symbionts that may have fueled
reef-building on the scale of modern-day corals.
The high resolution obtained in the new study
combined with the fast growth rate of the ancient bivalves revealed
unprecedented detail about how the animal lived and the water conditions it
grew in, down to a fraction of a day.
“We have about four to five datapoints per day, and
this is something that you almost never get in geological history. We can
basically look at a day 70 million years ago. It’s pretty amazing,” said Niels
de Winter, an analytical geochemist at Vrije Universiteit Brussel and the lead
author of the new study.
Climate reconstructions of the deep past typically
describe long term changes that occur on the scale of tens of thousands of
years. Studies like this one give a glimpse of change on the timescale of
living things and have the potential to bridge the gap between climate and
weather models.
Chemical analysis of the shell indicates ocean
temperatures were warmer in the Late Cretaceous than previously appreciated,
reaching 40 degrees Celsius (104 degrees Fahrenheit) in summer and exceeding 30
degrees Celsius (86 degrees Fahrenheit) in winter. The summer high temperatures
likely approached the physiological limits for mollusks, de Winter said.
“The high fidelity of this data-set has allowed the
authors to draw two particularly interesting inferences that help to sharpen
our understanding of both Cretaceous astrochronology and rudist palaeobiology,”
said Peter Skelton, a retired lecturer of palaeobiology at The Open University
and a rudist expert unaffiliated with the new study.
Ancient
reef-builders
The new study analyzed a single individual that lived
for over nine years in a shallow seabed in the tropics—a location which is now,
70-million-years later, dry land in the mountains of Oman.
Torreites
sanchezi mollusks look like tall pint glasses
with lids shaped like bear claw pastries. The ancient mollusks had two shells,
or valves, that met in a hinge, like asymmetrical clams, and grew in dense
reefs, like modern oysters. They thrived in water several degrees warmer
worldwide than modern oceans.
In the late Cretaceous, rudists like T. sanchezi dominated the reef-building
niche in tropical waters around the world, filling the role held by corals
today. They disappeared in the same event that killed the non-avian dinosaurs
66 million years ago.
“Rudists are quite special bivalves. There’s nothing
like it living today,” de Winter said. “In the late Cretaceous especially,
worldwide most of the reef builders are these bivalves. So they really took on
the ecosystem building role that the corals have nowadays.”
The new method focused a laser on small bits of
shell, making holes 10 micrometers in diameter, or about as wide as a red blood
cell. Trace elements in these tiny samples reveal information about the
temperature and chemistry of the water at the time the shell formed. The
analysis provided accurate measurements of the width and number of daily growth
rings as well as seasonal patterns. The researchers used seasonal variations in
the fossilized shell to identify years.
The new study found the composition of the shell
changed more over the course of a day than over seasons, or with the cycles of
ocean tides. The fine-scale resolution of the daily layers shows the shell grew
much faster during the day than at night
“This bivalve had a very strong dependence on this
daily cycle, which suggests that it had photosymbionts,” de Winter said. “You
have the day-night rhythm of the light being recorded in the shell.”
This result suggests daylight was more important to
the lifestyle of the ancient mollusk than might be expected if it fed itself
primarily by filtering food from the water, like modern day clams and oysters,
according to the authors. De Winter said the mollusks likely had a relationship
with an indwelling symbiotic species that fed on sunlight, similar to living
giant clams, which harbor symbiotic algae.
“Until now, all published arguments for
photosymbiosis in rudists have been essentially speculative, based on merely
suggestive morphological traits, and in some cases were demonstrably erroneous.
This paper is the first to provide convincing evidence in favor of the
hypothesis,” Skelton said, but cautioned that the new study’s conclusion was
specific to Torreites and could not
be generalized to other rudists.
Moon retreat
De Winter’s careful count of the number of daily
layers found 372 for each yearly interval. This was not a surprise, because
scientists know days were shorter in the past. The result is, however, the most
accurate now available for the late Cretaceous, and has a surprising
application to modeling the evolution of the Earth-Moon system.
The length of a year has been constant over Earth’s
history, because Earth’s orbit around the Sun does not change. But the number
of days within a year has been shortening over time because days have been
growing longer. The length of a day has been growing steadily longer as
friction from ocean tides, caused by the Moon’s gravity, slows Earth’s
rotation.
The pull of the tides accelerates the Moon a little
in its orbit, so as Earth’s spin slows, the Moon moves farther away. The moon
is pulling away from Earth at 3.82 centimeters (1.5 inches) per year. Precise
laser measurements of distance to the Moon from Earth have demonstrated this
increasing distance since the Apollo program left helpful reflectors on the
Moon’s surface.
But scientists conclude the Moon could not have been
receding at this rate throughout its history, because projecting its progress
linearly back in time would put the Moon inside the Earth only 1.4 billion years
ago. Scientists know from other evidence that the Moon has been with us much
longer, most likely coalescing in the wake of a massive collision early in
Earth’s history, over 4.5 billion years ago. So the Moon’s rate of retreat has
changed over time, and information from the past, like a year in the life of an
ancient clam, helps researchers reconstruct that history and model of the
formation of the moon.
Because in the history of the Moon, 70 million years
is a blink in time, de Winter and his colleagues hope to apply their new method
to older fossils and catch snapshots of days even deeper in time.
原始論文：Niels J. Winter, Steven Goderis, Stijn J.M. Van
Malderen, Matthias Sinnesael, Stef Vansteenberge, Christophe Snoeck, Joke
Belza, Frank Vanhaecke, Philippe Claeys. Subdaily‐Scale Chemical Variability in a Torreites
Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous
Day‐Night Cycle. Paleoceanography
and Paleoclimatology, 2020; 35 (2) DOI: 10.1029/2019PA003723
引用自：American Geophysical Union. "Ancient shell
shows days were half-hour shorter 70 million years ago.”
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