原文網址：https://biologicalsciences.uchicago.edu/news/new-fossil-shows-four-legged-fishapod-returned-water
研究人員發現的新化石跟過渡到陸生的魚類有密切關係，但其特徵卻更適合游泳以及水棲生活
By Matt Wood
疫情期間網路上流傳著一張梗圖，主角是提塔利克魚(Tiktaalik roseae)，一種四腳「魚足類」的代表，牠們是3億7500萬年前率先從水中過渡到陸地的生物。這張梗圖大多數的版本都是提塔利克魚正從水面探出頭準備爬到岸上，接著一隻手伸出來，拿著木裩或是捲起來的報紙把牠嚇回水裡。笑點在於被現代社會弄到無法喘息的人類希望可以回到過去，把提塔利克魚趕回水裡阻止牠繼續演化，這樣我們就不用面臨當今的戰爭、瘟疫和網路迷因。
Qikiqtania wakei(中間)和牠體型較大的表親提塔利克魚(下方)一樣都有四條腿，但是卻更適合游泳而非走在陸上。圖片來源：Alex Boersma
事實證明提塔利克魚的近親之一就這麼做了，選擇返回開放水域生活而不是探索陸地。Neil
Shubin博士在2004年共同發現了提塔利克魚，最近由他主持的實驗室進行的研究描述了一種跟提塔利克魚十分相似的物種，但是與牠勇於冒險的表親相比，其特徵卻更適合生活在水裡。相較能長到9英尺(約274公分)的提塔利克魚，Qikiqtania wakei並不大，身長只有30英吋(約76公分)
。新發現的化石包括了部份上下顎、一段頸椎以及鱗片。最重要的特徵是有片完整的胸鰭，其中一根特殊的肱骨缺乏了隆起，代表Qikiqtania沒有走在陸上所需的關節與肌肉；反之，牠平滑且弧形的上臂更適合在水中划行。Qikiqtania手臂骨頭的獨特之處代表牠在祖先開始利用附肢行走之後，卻回去繼續划水。
「最初因為牠的體型較小使得我們認為牠可能是未成年的提塔利克魚，某些過程可能還沒發展出來，」Shubin表示。「但是這根肱骨平滑的表面以及像是迴力鏢的外型，加上缺乏在陸地上把身體支撐起來所需的要件，都跟提塔利克魚有顯著的差異，顯示其為新的物種。」
這篇論文2022年7月20日發表在《自然》(Nature)，題目為「加拿大極區泥盆紀晚期的希望螈目新種化石以及四足類幹群的多樣性」。
疫情間的遠古計畫
Shubin是芝加哥大學有機體生物學與解剖學的傑出貢獻教授。他在發現提塔利克魚的幾天前找到了這具化石，地點為加拿大極區北部的努納武特政區，埃爾斯米爾島南方距東岸一英里處。屬名Qikiqtania來自因紐特語當中對化石場址所在地區的傳統稱謂Qikiqtaaluk或Qikiqtani。種名wakei則是紀念加州大學柏克萊分校已故的傑出演化生物學家David
Wake。
Shubin和德雷克塞爾大學自然科學院的Ted
Daeschler博士一起進行了野外調查，他們在一座採石場看到了幾顆石頭表面有特殊的白色鱗片，認為可能會有化石在裡面而採集回去。但是團隊之後的重心放在清理提塔利克魚的化石，使得這些樣品大部分都沒有受到檢查而擱置在儲藏室當中。
十五年之後，Qikiqtania的發現過程是另一則和疫情有關的故事。博士後研究員Justin
Lemberg以及Tom
Stewart博士在2020年三月用電腦斷層掃描了一個較大的岩石樣品，發覺裡面有片胸鰭。不幸的是它的所在位置太深，無法得到高解析度的影像，而在實驗室因為疫情被迫關閉之後，他們也只能停止對它進行更多研究。
「我們試著在封城之前盡可能地用電腦斷層掃描蒐集這些樣品的數據。最後幾個掃描的樣品中有個不顯眼的大石塊，只能從表面看到斑駁的鱗片，」Lemberg表示。他最近在南加州從事文化資源管理的實地考察。「當第一批粗糙的影像顯示出有片胸鰭的時候我們幾乎是不可置信。我們知道還有時間的話可以更仔細地掃描這塊石頭，但當時是2020年3月13日，芝加哥大學在接下來一個禮拜便全面停止了非必要的作業。」
透過電腦斷層掃描Qikiqtania
2020年夏季，學校設施重新開放之後，他們聯繫了地球物理學系的副教授Mark
Webster博士。他擁有的鋸子可以把石頭樣品削薄，使電腦斷層掃描的距離拉近而產生更加清晰的影像。Stewart和Lemberg仔細在石塊上標示了邊界，然後在位於卡爾弗館的實驗室外排了這次學術交流。他們得出來的影像顯示了一片幾乎完整的胸鰭與上肢，包括那根特別的肱骨。
「結果讓我們十分震驚，」Shubin表示。「這塊石頭最初完全沒有引人注目的地方，但我們在COVID封城而無法進實驗室的期間，瞭解到原本的掃描結果並不夠好，因此需要把它切得更薄。這麼做的結果是我們在疫情期間得到了令人興奮的研究成果，過程真的相當精彩。」
一窺脊椎動物的歷史
Qikiqtania的年代只比提塔利克魚早了一些。團隊分析了牠在演化樹上的位置，認為就像提塔利克魚一樣，靠近最早擁有類似手指的生物。雖然Qikiqtania特殊的胸鰭更適合游泳，但跟魚類卻一點也不像。牠弧形的槳狀肢是種獨特的適應構造，跟今日四足類具有關節與肌肉的雙腿，或是魚類呈現扇形的鰭都完全不同。
我們經常會認為動物從遠古以前的型態演化成現今某些生物的時候是沿著一條直線進行，但是Qikiqtania顯示某些動物會待在不同的道路，最終走向死胡同。或許這可以讓希望提塔利克魚留在水中的人學到一些東西。
「由於很容易就能從提塔利克魚身上看出生物從水生變成陸生的漸變模式，因此牠常常被當成一個過渡物種。但是我們知道演化並非總是如此單純，」Stewart表示。他在今年夏天將會成為賓州州立大學的職員。「以往我們不常窺見這部分的脊椎動物演化史。現在我們開始發現這些動物所擁有的多樣性，並且瞭解牠們在生態系中的腳色以及獨特的適應構造。這段歷史並非只是少數幾個物種的逐漸轉變這麼簡單。」
研究經費來自布林森基金會、德雷克塞爾大學自然科學院、芝加哥大學生物科學學門、加拿大自然資源部的極區大陸棚計畫、努納武特文化遺產部、格里斯峽灣村落捕獵者協會、美國國家科學基金會。
New fossil shows four-legged fishapod
that returned to the water
Researchers discover a new fossil that
is closely related to other animals that made the transition to land, but with
features more suited for swimming and life in the water.
A meme has been circulating online during
the pandemic featuring Tiktaalik roseae,
the iconic, four-legged “fishapod” that first made the transition from water to
land 375 million years ago. Most variations show Tiktaalik poking its head out of the water and ready to crawl
ashore, while an out of frame hand threatens it with a rolled-up newspaper or a
stick. The joke is that those of us exhausted by the modern world wish we could
go back in time, shoo it back into the water, and stop evolution in its tracks,
sparing ourselves the present day of war, pestilence, and internet memes.
As it turns out, one of Tiktaalik’s close relatives did just that, opting to return to
living in open water instead of venturing onto land. A new study from the
laboratory of Neil Shubin, PhD, who co-discovered Tiktaalik in 2004, describes a fossil species that closely
resembles Tiktaalik but has features
that made it more suited to life in the water than its adventurous cousin. Qikiqtania wakei was small—just 30
inches long—compared to Tiktaalik,
which could grow up to nine feet. The new fossil includes partial upper and
lower jaws, portions of the neck, and scales. Mostly importantly, it also
features a complete pectoral fin with a distinct humerus bone that lacks the
ridges that would indicate where muscles and joints would be on a limb geared
toward walking on land. Instead, Qikiqtania’s
upper arm was smooth and curved, more suited for a life paddling underwater. The
uniqueness of the arm bones of Qikiqtania
suggest that it returned to paddling the water after its ancestors began to use
their appendages for walking.
“At first we thought it could be a juvenile Tiktaalik, because it was smaller and
maybe some of those processes hadn’t developed yet,” Shubin said. “But the
humerus is smooth and boomerang shaped, and it doesn’t have the elements that
would support it pushing up on land. It’s remarkably different and suggests
something new.”
The paper, “A New Elpistostegalian from the Late
Devonian of the Canadian Arctic and the diversity of stem tetrapods,” was
published July 20, 2022, in Nature.
A prehistoric
pandemic project
Shubin, who is the Robert R. Bensley Distinguished
Service Professor of Organismal Biology and Anatomy at the University of
Chicago, found the fossil days before Tiktaalik
was discovered, at a site about one mile east on southern Ellesmere Island in
the territory of Nunavut in northern Arctic Canada. The name Qikiqtania (pronounced
kick-kick-TAN-ee-ya) comes from the Inuktitut word Qikiqtaaluk or Qikiqtani,
the traditional name for the region where the fossil site is located. The
species designation wakei is in memory of the late David Wake, an eminent
evolutionary biologist from the University of California at Berkeley.
Shubin and his field partner, Ted Daeschler, PhD,
from the Academy of Natural Sciences of Drexel University, collected the
specimens from a quarry after spotting a few promising looking rocks with
distinctive, white scales on the surface. But they sat in storage, mostly
unexamined, while the team focused on preparing Tiktaalik.
Fifteen years later, the discovery of Qikiqtania became another pandemic
story. Postdoctoral researchers Justin Lemberg, PhD, and Tom Stewart, PhD,
CT-scanned one of the larger rock specimens in March 2020 and realized that it
contained a pectoral fin. Unfortunately, it was too deep inside the rock to get
a high-resolution image, and they couldn’t do much more with it once the
pandemic forced labs to close.
"We were trying to collect as much CT-data of
the material as we could before the lockdown, and the very last piece we
scanned was a large, unassuming block with only a few flecks of scales visible from
the surface,” said Lemberg, who is now doing cultural resource management
fieldwork in Southern California. “We could hardly believe it when the first,
grainy images of a pectoral fin came into view. We knew we could collect a
better scan of the block if we had the time, but that was March 13th, 2020, and
the University shut down all non-essential operations the following week."
Qikiqtania CT scan
In the summer of 2020 when campus facilities
reopened, they contacted Mark Webster, PhD, Associate Professor of Geophysical
Sciences, who had access to a saw that could trim pieces off the specimen so
that a CT scanner could get closer and produce a better image. Stewart and
Lemberg carefully marked the boundaries on the block and arranged an exchange
outside their lab in Culver Hall. The resulting images revealed a nearly
complete pectoral fin and upper limb, including the distinctive humerus bone.
“That’s what blew our minds,” Shubin said. “This was
by no means a fascinating block at first, but we realized during the COVID
lockdown when we couldn’t get in the lab that the original scan wasn’t good
enough and we needed to trim the block. And when we did, look at what happened.
It gave us something exciting to work on during the pandemic. It’s a fabulous
story.”
Glimpses into vertebrate
history
Qikiqtania
is slightly older than Tiktaalik but
not by much. The team’s analysis of where it sits on the tree of life places
it, like Tiktaalik, adjacent to the
earliest creatures known to have finger-like digits. But even though Qikiqtania’s distinct pectoral fin was
more suited for swimming, it wasn’t entirely fish-like either. Its curved
paddle shape was a distinct adaptation, different from the jointed, muscled
legs or fan-shaped fins we see in tetrapods and fish today.
We tend to think animals evolved in a straight line
that connects their prehistoric forms to some living creature today, but Qikiqtania shows that some animals
stayed on a different path that ultimately didn’t work out. Maybe that’s a
lesson for those wishing Tiktaalik
had stayed in the water with it.
“Tiktaalik
is often treated as a transitional animal because it’s easy to see the stepwise
pattern of changes from life in the water to life on land. But we know that in
evolution things aren’t always so simple,” said Stewart, who will be joining
the faculty at Penn State University this summer. “We don’t often get glimpses
into this part of vertebrate history. Now we’re starting to uncover that
diversity and to get a sense of the ecology and unique adaptations of these
animals. It’s more than simple transformation with just a limited number of
species.”
The research was supported by the Brinson Foundation,
the Academy of Natural Sciences of Drexel University, the University of Chicago
Biological Sciences Division, the Polar Continental Shelf Program of Natural
Resources Canada, the Nunavut Department of Culture and Heritage, the Hamlet of
Grise Fiord and its Iviq Hunters and Trappers Association, and the National
Science Foundation.
原始論文：Thomas A.
Stewart, Justin B. Lemberg, Ailis Daly, Edward B. Daeschler, Neil H.
Shubin. A new elpistostegalian from the Late Devonian of the Canadian
Arctic. Nature, 2022; DOI: 10.1038/s41586-022-04990-w
引用自：University of Chicago.”
New fossil shows
four-legged fishapod that returned to the water.”
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