發現3.85億年前的森林
研究人員詳細研究紐約開羅附近卡茨基爾地區的化石土壤之後，發現了3.85億年前原始樹木在此形成了範圍廣大的根系。這些化石距離之前被認為世界最早森林的所在地點大概25英里，它們證明了森林在泥盆紀已經逐漸轉變成今日我們所知的樣貌，比一般認為的時間點還要更早。
「泥盆紀代表了第一座森林出現在地球上的時代。」研究第一作者，紐約賓漢頓大學的生物學名譽教授William
Stein表示。「從生態系的轉變，以及地球的陸地海洋、全球大氣二氧化碳濃度、和全球氣候發生的事物來看，當時這些原始森林的影響規模已經達到最基本的層面。由於它們對許多方面都造成了劇烈變化，往後的世界從根本上來說已經變得完全不一樣了。」
與Stein一起進行這項研究的包括卡迪夫大學的Christopher
Berry和Jennifer
Morris、雪菲爾大學的Jonathan
Leake。他們長久以來在紐約卡茨基爾地區進行研究，2012年他們在吉爾柏亞發現另一片化石森林留下的「痕跡」，使其多年來被視為地球最早的森林。最近這項發現位於距吉爾柏亞車程40分鐘的開羅，證據指出這座年代更為古老的森林擁有大不相同的組成。
開羅的化石場址擁有三種不同的根系，這讓Stein和團隊推論泥盆紀的森林跟現在的很像，依據每個地點的環境不同而長有不同的樹木。
Stein和團隊認為他們最先辨認出來的根系屬於一種類似棕櫚樹的植物，稱為始籽羊齒屬(Eospermatopteris)。這種樹木最初發現在吉爾柏亞的化石場址，擁有較為原始的根部。始籽羊齒屬可能就和雜草一樣能在許多環境中生長，因此才會在兩個場址中都出現。它們的根系範圍相對來說較為狹小；而壽命可能只有一到兩年，死亡之後同一地點生長的其他根系就會取代它們。研究人員還發現了另一種樹木「古蕨屬(Archaeopteris)」在此生長的證據，它們跟現在的種子植物有幾種共同特性。
「古蕨屬就像是日後森林的雛型，它們透露了未來森林最終會長成什麼樣子。」Stein表示，「從我們已知的古蕨屬實體化石證據，再加上我們現在從開羅找到的根部證據，可以得知這些植物跟其他泥盆紀的植物相比已經十分現代。雖然古蕨屬和當今樹木仍有相當大的差異，但是古蕨屬似乎已經指出了森林未來發展的道路上會出現哪些要素。」
Stein和團隊在開羅的化石土壤中還有一項驚人的發現：它們找到的第三種根系屬於一種過去認為只存在於石炭紀和更晚年代的物種――屬於石松綱(Lycopsida)的「鱗木」。
「我們在開羅找到的根部構造和石炭紀沼澤中，根部延展範圍驚人的巨大樹木似乎是一樣的，但是之前從未有人在這麼早的泥盆紀地層中發現這類樹木的實體化石證據。」Stein表示。「我們的發現或許證實了這類植物構成森林的年代比我們認為的還早，只不過是存在於不同的環境之中。但是目前我們只有痕跡化石，希望之後有更多化石證據可供確認。」
Stein和團隊接下來的目標是繼續調查卡茨基爾地區，並且和世界其他地方的化石森林比較他們的發現。
「依我來看，全世界許多地方的土壤化石都有把這類環境保存下來。我希望可以知道當時發生了什麼事――不只是卡茨基爾地區，而是每個地方。」Stein表示。「對我來說最有成就感的工作，莫過於了解演化和生態學的歷史進程。」
385-million-year-old forest discovered
While sifting through fossil soils in the
Catskill region near Cairo, New York, researchers uncovered the extensive root
system of 385-million-year old primitive trees. The fossils, located about 25 miles
from the site previously believed to have the world's oldest forests, is
evidence that the transition toward forests as we know them today began earlier
in the Devonian Period than typically believed.
"The Devonian Period represents a time in which
the first forest appeared on planet Earth," says first author William
Stein, an emeritus professor of biological science at Binghamton University,
New York. "The effects were of first order magnitude, in terms of changes
in ecosystems, what happens on the Earth's surface and oceans, in global
atmosphere, CO2 concentration in the atmosphere, and global climate. So many
dramatic changes occurred at that time as a result of those original forests
that basically, the world has never been the same since."
Stein, along with collaborators, including
Christopher Berry and Jennifer Morris of Cardiff University and Jonathan Leake
of the University of Sheffield ,have been working in the Catskill region in New
York, where in 2012 they uncovered "footprint evidence" of a
different fossil forest at Gilboa, which, for many years has been termed the
Earth's oldest forest. The discovery at Cairo, about a 40-minute drive from the
original site, now reveals an even older forest with dramatically different
composition.
The Cairo site presents three unique root systems,
leading Stein and his team to hypothesize that much like today, the forests of
the Devonian Period were composed of different trees occupying different places
depending on local conditions.
First, Stein and his team identified a rooting system
that they believe belonged to a palm tree-like plant called Eospermatopteris. This tree, which was
first identified at the Gilboa site, had relatively rudimentary roots. Like a
weed, Eospermatopteris likely
occupied many environments, explaining its presence at both sites. But its
roots had relatively limited range and probably lived only a year or two before
dying and being replaced by other roots that would occupy the same space. The
researchers also found evidence of a tree called Archaeopteris, which shares a number of characteristics with modern
seed plants.
"Archaeopteris seems to reveal the beginning of
the future of what forests will ultimately become," says Stein.
"Based on what we know from the body fossil evidence of Archaeopteris prior to this, and now
from the rooting evidence that we've added at Cairo, these plants are very
modern compared to other Devonian plants. Although still dramatically different
than modern trees, yet Archaeopteris nevertheless seems to point the way toward
the future of forests elements."
Stein and his team were also surprised to find a
third root system in the fossilized soil at Cairo belonging to a tree thought
to only exist during the Carboniferous Period and beyond: "scale
trees" belonging to the class Lycopsida.
"What we have at Cairo is a rooting structure
that appears identical to great trees of the Carboniferous coal swamps with
fascinating elongate roots. But no one has yet found body fossil evidence of
this group this early in the Devonian." Stein says. "Our findings are
perhaps suggestive that these plants were already in the forest, but perhaps in
a different environment, earlier than generally believed. Yet we only have a
footprint, and we await additional fossil evidence for confirmation."
Moving forward, Stein and his team hope to continue
investigating the Catskill region and compare their findings with fossil
forests around the world.
"It seems to me, worldwide, many of these kinds
of environments are preserved in fossil soils. And I'd like to know what
happened historically, not just in the Catskills, but everywhere," Says
Stein. "Understanding evolutionary and ecological history -- that's what I
find most satisfying."
原始論文：William E.
Stein, Christopher M. Berry, Jennifer L. Morris, Linda VanAller Hernick, Frank
Mannolini, Charles Ver Straeten, Ed Landing, John E.A. Marshall, Charles H.
Wellman, David J. Beerling, Jonathan R. Leake. Mid-Devonian
Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests. Current
Biology, 2019; DOI: 10.1016/j.cub.2019.11.067
引用自：Cell Press. "385-million-year-old forest
discovered." ScienceDaily.
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