原始網址：http://www.ucalgary.ca/utoday/issue/2017-06-21/ancient-fossil-holds-new-insights-how-fish-evolved-land

古代化石擁有魚類如何演化上岸的新線索

「它就像是披著蛇皮的魚一樣」

By Collene Ferguson

過去3億4000萬年來，這具看起來像蛇一樣，稱作Lethiscus stocki的遠古化石一直守著它所持有關於演化的秘密。

由卡爾加里大學主導的國際研究團隊對這具蘇格蘭的化石做出了新的闡述，從根本層面上挑戰了科學家以往對四足類(tetrapod)，也就是具有四肢的動物早期演化歷程的理解。

他們的發現甫刊登於頂尖國際研究期刊《自然》(Nature)。

主持計畫的卡爾加里大學獸醫學院的古生物學家和教授Jason Anderson表示：「這迫使我們從頭思考演化在第一批四足類身上達到了什麼樣的作用。」

在此研究之前，一般認為原始四足類――其為人類和其他現存陸上脊椎動物的祖先――從魚類演化成具有四肢動物的過程是相當緩慢的。

他說：「過去我們認為從魚鰭轉變成四肢是個緩慢的演化歷程，過程中牠們逐漸變得不像魚類。但是Lethiscus呈現出的是一種迅速且劇烈的演化實驗。這支譜系在牠們首度演化出來之後體型就開始縮小，且幾乎是立刻就失去了牠們的鰭。牠們就像是披著蛇皮的魚一樣。」

利用3D醫學成像技術來揭露Lethicus的秘密

利用微電腦斷層掃描儀以及嶄新的計算軟體，Anderson和研究主要作者，他所指導的博士生Jason Pardo可以更仔細地觀察Lethiscus化石的內部解剖構造。在重建出電腦斷層影像之後，研究人員可以看見Lethiscus頭顱的整個構造，得到相當驚人的發現。

Pardo解釋：「牠們的解剖構造跟我們預期的並不相符。牠們身上的許多特徵不適合從兩棲類或爬蟲類解剖學的前提下解讀。」但是跟原始魚類相比時，牠們的解剖特徵就顯得有其意義。

Pardo進一步解說：「我們可以看見牠們頭顱的整體構造、牠們的腦袋位於何處以及內耳空腔的型態。所有的一切看起來都跟魚類極為相似。」他強調除了非常原始的四足類之外，在魚類常見的解剖構造無法在其他四足類身上見到。研究人員因此用匙吻鱘，一種有許多原始特徵的現代魚類，來當作Lethiscus解剖構造中某些方面的類比。

改變Lethiscus在四足類族譜中的定位

當他們納入這些解剖學新資訊來分析Lethiscus和其他動物的關係，他們將Lethiscus族譜中的位置下移至魚鰭轉變成四肢過程中的最初階段。Anderson表示：「可以和其他生存於同時代的動物一樣，得到牠的演化地位為何，是相當令人滿意的結果。」

結論也跟從地質紀錄中推出的演化順序更加符合。Anderson表示：「對於演化生物學和利用現生動物重建分子時鐘的科學家來說，Lethiscus在許多層面也具有重要影響。科學家係利用化石來校正分子時鐘。透過把Lethiscus從現代四足類的直系祖先中剔除，這會改變進行分子時鐘分析時所用的校正時間。」

 

Ancient fossil holds new insights into how fish evolved onto land

 ‘It’s like a snake on the outside, but a fish on the inside’

The fossil of an early snake-like animal — called Lethiscus stocki — has kept its evolutionary secrets for the last 340 million years.

Now, an international team of researchers, led by the University of Calgary, has revealed new insights into the ancient Scottish fossil that dramatically challenge our understanding of the early evolution of tetrapods, or four-limbed animals with backbones.

Their findings have just been published in the prestigious international research journal Nature.

“It forces a radical rethink of what evolution was capable of among the first tetrapods,” said project lead Jason Anderson, a paleontologist and professor at the University of Calgary Faculty of Veterinary Medicine (UCVM).

Before this study, ancient tetrapods — the ancestors of humans and other modern-day vertebrates — were thought to have evolved very slowly from fish to animals with limbs.

 “We used to think that the fin-to-limb transition was a slow evolution to becoming gradually less fish-like,” he said. “But Lethiscus shows immediate, and dramatic, evolutionary experimentation. The lineage shrunk in size, and lost limbs almost immediately after they first evolved. It’s like a snake on the outside but a fish on the inside.”

Lethicus’ secrets revealed with 3D medical imaging

Using micro-computer tomography (CT) scanners and advanced computing software, Anderson and study lead author Jason Pardo, a doctoral student supervised by Anderson, got a close look at the internal anatomy of the fossilized Lethiscus. After reconstructing CT scans, its entire skull was revealed, with extraordinary results.

“The anatomy didn’t fit with our expectations,” explains Pardo. “Many body structures didn’t make sense in the context of amphibian or reptile anatomy.” But the anatomy did make sense when it was compared to early fish.

“We could see the entirety of the skull. We could see where the brain was, the inner ear cavities. It was all extremely fish-like,” explains Pardo, outlining anatomy that’s common in fish but unknown in tetrapods except in the very first. The anatomy of the paddlefish, a modern fish with many primitive features, became a model for certain aspects of Lethiscus’ anatomy.

Changing position on the tetrapod ‘family tree’

When they included this new anatomical information into an analysis of its relationship to other animals, Lethiscus moved its position on the “family tree,” dropping into the earliest stages of the fin-to-limb transition. “It’s a very satisfying result, having them among other animals that lived at the same time,” says Anderson.

The results match better with the sequence of evolution implied by the geologic record. “Lethiscus also has broad impacts on evolutionary biology and people doing molecular clock reproductions of modern animals,” says Anderson. “They use fossils to calibrate the molecular clock.  By removing Lethiscus from the immediate ancestry of modern tetrapods, it changes the calibration date used in those analyses.”

原始論文：Jason D. Pardo, Matt Szostakiwskyj, Per E. Ahlberg, Jason S. Anderson. Hidden morphological diversity among early tetrapods. Nature, 2017; DOI: 10.1038/nature22966

引用自：University of Calgary. "Fossil holds new insights into how fish evolved onto land: 'It's like a snake on the outside, but a fish on the inside'."