首次在非洲發現的四足類生活於泥盆紀時的南極圈
兩種新物種的化石顯示這些四足脊椎動物也在極區演化出來，並非像以往認為的只在熱帶出現。
藝術家想像中泥盆紀當時Waterloo Farm的景象，包括了Tutusius和Umzantsi。由Maggie Newman繪製。
首次在非洲發現的泥盆紀四足類(tetrapod，具有四隻腳的脊椎動物)化石，顯示這些陸生動物的先驅在3億6000萬年前生活於當時的南極圈。
於泥盆紀時四足類從魚類演化出來，是我們祖先久遠以前的歷史中發生的關鍵事件。發表在今日《科學》(Science)上的這些年代為泥盆紀末的新化石，發現於南非東開普省格拉罕鎮附近的Waterloo Farm。它們使得我們需要大幅重新評估這起事件的全貌。「雖然之前發現泥盆紀四足類化石的地點都是位於泥盆紀時的熱帶地區，這些新樣品卻是生活在南極圈以內。」主要作者，格拉罕鎮奧爾巴尼博物館的Robert Gess博士，以及共同作者，瑞典烏普薩拉大學的Per Ahlberg教授如此解釋。這項研究由南非DST-NRF古科學卓越研究中心補助，此單位設立於金山大學和Millennium Trust組織。
第一次在非洲發現的泥盆紀四足類
這兩種命名為Tutusius和Umzantsi的新物種是非洲已知最古老的四足脊椎動物化石，年代為驚人的七億年。Tutusius umlambo(紀念榮休大主教戴斯蒙.屠圖)的體長大約為一公尺，而Umzantsia amazana則稍微小一些。雖然兩者化石都不完整――Tutusius只有一根肩帶骨頭，而Umzantsia的骨頭數目則較多，但兩者看起來都跟先前已知的泥盆紀四足類頗為相似。如果還活著的話，它們看起來會是一種介於鱷魚和魚類之間的動物：擁有像鱷魚的頭、粗短的四肢，而尾巴則像魚類一樣有鰭。
發現這些四足類的Waterloo Farm場址為一座道路邊坡，2016年南非國家道路局沿著格拉罕鎮和魚河之間的N2高速公路進行引爆作業以開挖道路時首次發現其存在。這道邊坡出露的深灰色泥岩屬於衛普特層(Witpoort
Formation)，它代表的環境在古代為半鹹水、受潮汐影響的河口，並且含有許多動植物化石。
在熱帶地區之外發現的第一具原始四足類
Tutusius和Umzantsia的真正重要之處在於他們的發現地點。
泥盆紀的四足類發現地點散落在相當廣的範圍中。然而，若畫出泥盆紀當時各個大陸的位置，就可以看出之前發現這些化石的岩石在古代的沉積環境都是熱帶――也就是赤道以南和以北的30度之間。幾乎所有化石皆來自於勞倫西亞大陸，這個超大陸之後分裂成北美、格陵蘭和歐洲。
而面積大上許多的南方超大陸――岡瓦那大陸――則包含了現今的非洲、南美洲、澳洲、南極洲和印度，迄今在這些地方幾乎沒有找到過屬於泥盆紀的四足類，除了在澳洲東部曾發現一塊單獨的顎骨化石(學名為Metaxygnathus)和足跡。由於澳洲是岡瓦納大陸延伸至赤道的最北端，因此有假說認為四足類是從赤道演化出來，而最有可能的地點便是勞倫西亞大陸。他們進一步地假設脊椎動物從水中爬到陸地的行為(陸生化，terrestrialisation)也是在熱帶發展出來。因此，想要了解這個宏觀演化上的一大進展究竟成因為何的研究，通常都集中於探討當時熱帶水體的環境普遍為何。
Waterloo Farm發現的四足類不只是位於岡瓦納大陸，而且還是在它的最南端――重建結果顯示當時此處的緯度大於南緯70度，在南極圈之內。豐富的植物化石顯示有森林生長在周圍，所以這裡並非冰天雪地，但也可以肯定此處並非熱帶環境，而且每年都會歷經數個月的完全黑暗。這項發現改變了我們對泥盆紀四足類分布範圍的認知。現在我們知道在泥盆紀末，四足類的分布範圍已經廣及全球，因此牠們的演化和陸生化實際上也可能發生在任何一地。
南非的化石紀錄之豐十分驚人，包含了哺乳類從類似爬蟲類的祖先演化而來的紀錄，以及人類演化的過程；現在，陸生動物的崛起也加入此列。在世上或許沒有另外一個國家擁有如此完整的文獻，記下了我們自身一路走來漫長且充滿戲劇性的演化歷史。
First tetrapods of Africa lived within the
Devonian Antarctic Circle
Fossils of two new species of these four-legged
vertebrates also evolved in polar regions, and not just in the tropics as
previously believed.
The first
African fossils of Devonian tetrapods (four-legged vertebrates) show these
pioneers of land living within the Antarctic circle, 360 million years ago.
The evolution of tetrapods
from fishes during the Devonian period was a key event in our distant ancestry.
Newly found fossils from the latest Devonian Waterloo Farm locality near
Grahamstown in the Eastern Cape, South Africa, published today in Science,
force a major reassessment of this event. "Whereas all previously found
Devonian tetrapods came from localities which were in tropical regions during
the Devonian, these specimens lived within the Antarctic circle," explains
lead author, Dr. Robert Gess of the Albany Museum in Grahamstown, and co-author
Professor Per Ahlberg of Uppsala University in Sweden. The research was
supported by the South African DST-NRF Centre of Excellence in Palaeosciences,
based at the University of the Witwatersrand and the Millennium Trust.
The first
African Devonian tetrapods
Two new species,
named Tutusius and Umzantsia, are Africa's earliest known four-legged
vertebrates by a remarkable 70 million years. The approximately metre-long
Tutusius umlambo (named in honour of Archbishop Emeritus Desmond Tutu) and the
somewhat smaller Umzantsia amazana are both incomplete. Tutusius is represented
by a single bone from the shoulder girdle, whereas Umzantsia is known from a
greater number of bones, but they both appear similar to previously known
Devonian tetrapods. Alive, they would have resembled a cross between a
crocodile and a fish, with a crocodile-like head, stubby legs, and a tail with
a fish-like fin.
The Waterloo
Farm locality (where the tetrapods were discovered) is a roadcut first revealed
in 2016 after controlled rock-cutting explosions by the South African National
Roads Agency (SANRAL) along the N2 highway between Grahamstown and the Fish
River. This cutting exposed dark grey mudstones of the Witpoort Formation that
represent an ancient environment of a brackish, tidal river estuary that
contain abundant fossils of animals and plants.
The first
tetrapod found outside of tropical regions
The real
importance of Tutusius and Umzantsia lies in where they were found.
Devonian tetrapod fossils are found in
widely scattered localities. However, if the continents are mapped back to
their Devonian positions, it emerges that all previous finds are from rocks
deposited in the palaeotropics—between 30 degrees north and south of the equator.
Almost all come from Laurussia, a supercontinent that later fragmented into
North America, Greenland and Europe.
The much
larger southern supercontinent, Gondwana, which incorporated present-day
Africa, South America, Australia, Antarctica, and India, has hitherto yielded
almost no Devonian tetrapods, with only an isolated jaw (named Metaxygnathus)
and footprints, being found in eastern Australia. Because Australia was the
northernmost part of Gondwana, extending into the tropics, an assumption
developed that tetrapods evolved in the tropics, most likely in Laurussia. By
extension it was assumed that movement of vertebrates from water onto land
(terrestrialisation) also occurred in the tropics. Attempts to understand the
causes of these major macroevolutionary steps therefore focussed on conditions
prevalent in tropical water bodies.
The Waterloo Farm tetrapods
not only come from Gondwana, but from its southernmost part: reconstructed to
have been more than 70 degrees south, within the Antarctic circle. Abundant
plant fossils show that forests grew nearby, so it wasn't frozen, but it was
definitely not tropical and during winter it will have experienced months of
complete darkness. This finding changes our understanding of the distribution
of Devonian tetrapods. We now know that tetrapods occurred throughout the world
by the Late Devonian and that their evolution and terrestrialisation could
realistically have occurred anywhere.
South Africa now
adds insights into the emergence of land animals to its incredible fossil
record, which also includes transition to mammals from reptile-like ancestors
and the evolution of humans. There is probably not another country on the
planet that so fully documents the long and dramatic evolutionary history of
our own lineage.
原始論文：Robert Gess, Per Erik Ahlberg. A
tetrapod fauna from within the Devonian Antarctic Circle. Science, 2018; 360 (6393): 1120 DOI: 10.1126/science.aaq1645
引用自：University of the Witwatersrand.
"First tetrapods of Africa lived within the Devonian Antarctic Circle.”
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