過去有多少隻暴龍？數十億吧

 原文網址：https://news.berkeley.edu/2021/04/15/how-many-t-rexes-were-there-billions/

By Robert Sanders

白堊紀時有多少隻暴龍漫步在北美洲？

根據加州大學柏克萊分校的研究，北美洲在250萬年當中總共出現了25億隻暴龍，只有非常小的一部分被古生物學家挖掘出來並加以研究。(圖片來源：Julius Csotonyi/Science magazine)

這項多年來縈繞在Charles Marshall與他的古生物同僚心中的問題，最近終於在他和他的學生合作之下找到了答案。

在本周發表於《科學》(Science)期刊的文章當中，團隊發現任一時間活著的成年暴龍大約有20,000隻，誤差為10倍左右，這落在他的多數同僚猜測的範圍裡面。

不過Marshall表示包括他本人在內，只有少數古生物學家完全領悟到，這意味著在暴龍行走於地球上的將近250萬年之中，共有25億隻左右的暴龍出生死亡。

在此之前，沒有人能計算出已經滅亡許久的某種動物的族群總數有多少。上個世紀最具影響力的古生物學家之一George Gaylord Simpson甚至認為這是無法辦到的。

Marshall是加州大學古生物博物館的主任、古生物學的講座教授、加州大學柏克萊分校的整合生物學與地球及行星科學教授。他也對這類計算真的可以實行而感到驚訝。

「在某種層面上，我們開始這項計畫時只是一時興起而已，」他說。「當我手中握著一個化石，總是忍不住讚嘆這種難得的機會――在數百萬年前這是一隻活生生的巨獸，此時此刻我卻可以拿著牠的骨頭，真的是千載難逢。而我腦中總是會浮現這個問題：『到底有多難發生？是千分之一，萬分之一，還是億分之一？』之後我漸漸發覺我們或許真的能估計牠們曾經活著的數目有多少，如此一來就能回答這個問題。」

在生命科學大樓裡的加州大學古生物博物館外面展示著這副暴龍的化石翻模，原版的化石目前展示於華盛頓的史密森尼學會。這副近乎完整的暴龍骨骸1990年出土於蒙大拿州東部的惡地。圖片來源： Keegan Houser / UC Berkeley

Marshall立刻指出估計結果有很大的不確定性。雖然在任一時間活著的成年暴龍數目最有可能為20,000，但是95%的信賴區間(真正的族群大小有95%的可能性位在這個範圍)卻是1,300到328,000。因此，該物種存活的期間內，曾經出現的個體總數可能是1億4,000萬到420億之間的任何一個數字。

「如同Simpson的觀察結果，要用化石紀錄進行定量化的估計是非常困難的，」他接著表示。「在我們的研究當中，重點不是放在估計本身要得出最好的結果，而是對於進行計算時所需的變數，如何透過確切的證據來限制出它們的範圍。」

他和團隊接著運用稱為蒙地卡羅法的電腦模擬方法來得出數據裡的不確定性，會如何轉變成結果裡的不確定性。

Marshall表示變數中最大的不確定性，主要圍繞在他們無法確定暴龍生態裡的某些性質，像是牠們血液的恆溫程度有多高。研究依據的數據源自加州大學聖塔芭芭拉分校的John Damuth之前發表現生動物的身體質量與族群密度之間的關係，也稱為達姆斯定律。他說雖然兩者之間的關係非常強烈，但是生理條件和生態棲位相同的動物，還是會因為在生態上的差異而讓族群密度有很大的差別。舉例來說，雖然美洲虎和土狼的大小差不多，但是土狼在棲地中的密度卻比美洲虎高出50倍。

分析過程相當重要的一部份是估計暴龍的生態棲位，他們根據的圖表為現生哺乳類的生體質量與族群密度之間的關係，稱作達姆斯定律。圖表來源：John Damuth, UC Santa Barbara

「我們的計算依據是現生動物身體質量與族群密度之間的關係，但是此關係中的不確定性卻可以跨越兩個數量級，」Marshall表示。「而讓我們感到驚訝的是，估計過程當中的不確定性主要是這類關於生態的變異度，而非我們採用的古生物資料中的不確定性。」

Marshall在計算過程中把暴龍作為掠食者的能量需求，選擇位在獅子與世界上最大的蜥蜴――科摩多龍的中間。

關於暴龍在生態系中的地位的爭議，促使Marshall和團隊忽略未成年的暴龍，牠們在化石紀錄中相當少見，事實上，牠們或許不會跟成年暴龍一同生活，追逐的獵物也不同。當暴龍逐漸成年的時候，顎骨的強度可以提升一個數量級，使得牠們可以咬碎骨頭。這意謂成年和未成年暴龍的獵物不同，甚至可以算是不同的掠食動物。

新墨西哥大學的演化古生物學家Felicia Smith最近的研究也支持了此可能性，他們推測白堊紀晚期暴龍這種巨型掠食者出現的時候，中型掠食者相當少見的原因便是牠們的生態棲位被未成年的暴龍佔據了。

化石透露的訊息

這群加州大學柏克萊分校的科學家鑽研科學文獻，並向古生物學家同僚尋求專業意見，以獲得他們進行估計時要用的數據。他們推測暴龍可能是在15歲半達到性成熟；最大年齡為20幾歲將近30；成年時的平均體重(也稱作生態身體質量)大約為5200公斤，也就是5.2公噸。此外，他們也運用了暴龍一生當中各階段的成長速度：牠們在大約性成熟時成長速度會大幅增加，最重可以到達7000公斤，也就是7公噸。

圖中是一具暴龍的顎骨。加州大學古生物博物館已故的古生物學家Harley Garbani在1977年從蒙大拿州的地獄溪地層發現這具化石。圖片來源：加州大學古生物博物館，2011

根據這些估計值，他們也算出暴龍每一個世代的長度大概為19年，族群平均密度約為每100平方公里一隻。

接著，他們估計暴龍的地理分布範圍約為230萬平方公里，整個物種的存活時間差不多是250萬年，並且算出長期的族群大小為兩萬隻暴龍。該物種存在的整體期間總共歷經了大約127,000個世代，換算成個體總數為25億隻左右。

在暴龍的歷史上曾經有這麼大量的成年個體，更別提未成年的數目應該還多出許多，那牠們的骨骸都到哪去了？這些暴龍被古生物學家發現的比例有多少？目前為止，人類發現的暴龍個體不到100具，而且大部分都是只有一根骨頭化石而已。

「現今的公立博物館保存相對良好的已成年暴龍大概是32具，」他說。「根據曾經活著的成年暴龍數目來看，這代表每八千萬隻暴龍我們才找到一隻。」

「如果將化石尋找速率的分析侷限在暴龍化石最多的地方，也就是蒙大拿州著名的地獄溪地層的一部份，我們估計在岩石沉積的期間活在該處的暴龍之中，大約每16,000隻就被我們找到一隻，」他接著表示。「這個數字讓我們蠻驚訝的，該處化石紀錄比我最初猜測的還能代表這些生物。如果該處幾乎沒有暴龍生活，那代表性可以好到每1000隻就能被我們找到一隻；但由於族群密度推測結果中的不確定性，這也可能低到每25萬隻才被我們找到一隻。」

Marshall預期他的同僚對於這些數字中的大部分甚至全部都會有所微詞，但他還是相信他估計滅絕動物的族群大小所用的計算框架是成立的，而且可以用來估計其他生物化石的族群大小。

「從某種層面上來說，這是在古生物學上的一道演練，藉此了解我們還能學到多少，以及應該透過什麼樣的方法，」他說。「令人驚訝的是我們實際上竟然如此瞭解暴龍，並且能從這些知識計算出更多東西。在過去幾十年我們對於暴龍的知識能夠增加這麼多，都是因為發現了更多化石，運用更多方法分析它們，並且把不同化石得到的資訊以更好的方式整合起來。」

他們已經公開這份研究用的計算框架的程式碼。Marshall說未來古生物學家在挖掘化石的時候，如果想估計漏掉了多少物種，也可以同樣的框架作為基礎。

「有了這些數字，我們可以開始計算在化石紀錄中，我們遺漏了多少壽命不長、侷限在特定地理範圍的物種，」他說。「或許經由這道方法我們可以開始定量過去未知的事物。」

 

How many T. rexes were there? Billions.

How many Tyrannosaurus rexes roamed North America during the Cretaceous period?

That’s a question Charles Marshall pestered his paleontologist colleagues with for years until he finally teamed up with his students to find an answer.

What the team found, to be published this week in the journal Science, is that about 20,000 adult T. rexes probably lived at any one time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed.

What few paleontologists had fully grasped, he said, including himself, is that this means that some 2.5 billion lived and died over the approximately 2 1/2 million years the dinosaur walked the earth.

Until now, no one has been able to compute population numbers for long-extinct animals, and George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that it couldn’t be done.

Marshall, director of the University of California Museum of Paleontology, the Philip Sandford Boone Chair in Paleontology and a UC Berkeley professor of integrative biology and of earth and planetary science, was also surprised that such a calculation was possible.

“The project just started off as a lark, in a way,” he said. “When I hold a fossil in my hand, I can’t help wondering at the improbability that this very beast was alive millions of years ago, and here I am holding part of its skeleton — it seems so improbable. The question just kept popping into my head, ‘Just how improbable is it? Is it one in a thousand, one in a million, one in a billion?’ And then I began to realize that maybe we can actually estimate how many were alive, and thus, that I could answer that question.”

Marshall is quick to point out that the uncertainties in the estimates are large. While the population of T. rexes was most likely 20,000 adults at any give time, the 95% confidence range — the population range within which there’s a 95% chance that the real number lies — is from 1,300 to 328,000 individuals. Thus, the total number of individuals that existed over the lifetime of the species could have been anywhere from 140 million to 42 billion.

“As Simpson observed, it is very hard to make quantitative estimates with the fossil record,” he said. “In our study, we focused in developing robust constraints on the variables we needed to make our calculations, rather than on focusing on making best estimates, per se.”

He and his team then used Monte Carlo computer simulation to determine how the uncertainties in the data translated into uncertainties in the results.

The greatest uncertainty in these numbers, Marshall said, centers around questions about the exact nature of the dinosaur’s ecology, including how warm-blooded T. rex was. The study relies on data published by John Damuth of UC Santa Barbara that relates body mass to population density for living animals, a relationship known as Damuth’s Law. While the relationship is strong, he said, ecological differences result in large variations in population densities for animals with the same and ecological niche. For example, jaguars and hyenas are about the same size, but hyenas are found in their habitat at a density 50 times greater than the density of jaguars in their habitat.

“Our calculations depend on this relationship for living animals between their body mass and their population density, but the uncertainty in the relationship spans about two orders of magnitude,” Marshall said. “Surprisingly, then, the uncertainty in our estimates is dominated by this ecological variability and not from the uncertainty in the paleontological data we used.”

As part of the calculations, Marshall chose to treat T. rex as a predator with energy requirements halfway between those of a lion and a Komodo dragon, the largest lizard on Earth.

The issue of T. rex‘s place in the ecosystem led Marshall and his team to ignore juvenile T. rexes, which are underrepresented in the fossil record and may, in fact, have lived apart from adults and pursued different prey. As T. rex crossed into maturity, its jaws became stronger by an order of magnitude, enabling it to crush bone. This suggests that juveniles and adults ate different prey and were almost like different predator species.

This possibility is supported by a recent study, led by evolutionary biologist Felicia Smith of the University of New Mexico, which hypothesized that the absence of medium-size predators alongside the massive predatory T. rex during the late Cretaceous was because juvenile T. rex filled that ecological niche.

What the fossils tell us

The UC Berkeley scientists mined the scientific literature and the expertise of colleagues for data they used to estimate that the likely age at sexual maturity of a T. rex was 15.5 years; its maximum lifespan was probably into its late 20s; and its average body mass as an adult — its so-called ecological body mass, — was about 5,200 kilograms, or 5.2 tons. They also used data on how quickly T. rexes grew over their life span: They had a growth spurt around sexual maturity and could grow to weigh about 7,000 kilograms, or 7 tons.

From these estimates, they also calculated that each generation lasted about 19 years, and that the average population density was about one dinosaur for every 100 square kilometers.

Then, estimating that the total geographic range of T. rex was about 2.3 million square kilometers, and that the species survived for roughly 2 1/2 million years, they calculated a standing population size of 20,000. Over a total of about 127,000 generations that the species lived, that translates to about 2.5 billion individuals overall.

With such a large number of post-juvenile dinosaurs over the history of the species, not to mention the juveniles that were presumably more numerous, where did all those bones go? What proportion of these individuals have been discovered by paleontologists? To date, fewer than 100 T. rex individuals have been found, many represented by a single fossilized bone.

“There are about 32 relatively well-preserved, post-juvenile T. rexes in public museums today,” he said. “Of all the post-juvenile adults that ever lived, this means we have about one in 80 million of them.”

“If we restrict our analysis of the fossil recovery rate to where T. rex fossils are most common, a portion of the famous Hell Creek Formation in Montana, we estimate we have recovered about one in 16,000 of the T. rexes that lived in that region over that time interval that the rocks were deposited,” he added. “We were surprised by this number; this fossil record has a much higher representation of the living than I first guessed. It could be as good as one in a 1,000, if hardly any lived there, or it could be as low as one in a quarter million, given the uncertainties in the estimated population densities of the beast.”

Marshall expects his colleagues will quibble with many, if not most, of the numbers, but he believes that his calculational framework for estimating extinct populations will stand and be useful for estimating populations of other fossilized creatures.

“In some ways, this has been a paleontological exercise in how much we can know, and how we go about knowing it,” he said. “It’s surprising how much we actually know about these dinosaurs and, from that, how much more we can compute. Our knowledge of T. rex has expanded so greatly in the past few decades thanks to more fossils, more ways of analyzing them and better ways of integrating information over the multiple fossils known.”

The framework, which the researchers have made available as computer code, also lays the foundation for estimating how many species paleontologists might have missed when excavating for fossils, he said.

“With these numbers, we can start to estimate how many short-lived, geographically specialized species we might be missing in the fossil record,” he said. “This may be a way of beginning to quantify what we don’t know.”

原始論文：Charles R. Marshall, Daniel V. Latorre, Connor J. Wilson, Tanner M. Frank, Katherine M. Magoulick, Joshua B. Zimmt, Ashley W. Poust. Absolute abundance and preservation rate of Tyrannosaurus rex. Science, 2021 DOI: 10.1126/science.abc8300

引用自：University of California - Berkeley. "How many T. rexes were there? Billions”