原文網址：www.sciencedaily.com/releases/2015/06/150624140239.htm
Newly found
ring of teeth uncovers what common ancestor of molting animals looked like
新發現的環狀排列牙齒揭露了蛻皮動物的共祖樣貌
A new study of an
otherworldly creature from half a billion years ago -- a worm-like animal with
legs, spikes and a head difficult to distinguish from its tail -- has
definitively identified its head for the first time, and revealed a previously
unknown ring of teeth and a pair of simple eyes. The results, published today
in the journal Nature, have helped
scientists reconstruct what the common ancestor of everything from tiny
roundworms to huge lobsters might have looked like.
一篇對一種生活在500萬年前，猶如來自異世界的生物—看起來像蟲，擁有許多腳與尖刺且首尾莫辨的動物—的新研究首次明確辨識出其頭部的樣貌，並且發現之前從未見過的環狀排列牙齒以及一對單眼。這篇刊登於本日《自然》(Nature)的新研究，重建了小至迴蟲，大至龍蝦的這些生物的共同祖先的可能模樣。
Researchers from the
University of Cambridge, the Royal Ontario Museum and the University of Toronto
have found that the creature, known as Hallucigenia due to its strange appearance, had a
throat lined with needle-like teeth, a previously unidentified feature which
could help connect the dots between it, modern velvet worms and arthropods --
the group which contains modern insects, spiders and crustaceans.
來自劍橋大學(the University of
Cambridge)、皇家安大略博物館(the Royal Ontario Museum)與多倫多大學(the University of Toronto)的研究人員，發現這種稱為「怪誕蟲」(Hallucigenia，命名顯然來自於牠奇怪的外表)的生物，擁有佈滿針狀牙齒的喉嚨。這種之前未發現的特徵幫助研究人員建立起活在現代的天鵝絨蟲(velvet worm)及節肢動物(arthropod，包括現代的昆蟲、蜘蛛與甲殼類)跟這種古代生物之間的關聯。
Arthropods, velvet worms
(onychophorans) and water bears (tardigrades) all belong to the massive group
of animals that moult, known as ecdysozoans. Though Hallucigenia is not the common ancestor of all
ecdysozoans, it is a precursor to velvet worms. Finding this mouth arrangement
in Hallucigeniahelped scientists determine
that velvet worms originally had the same configuration -- but it was
eventually lost through evolution.
節肢動物、天鵝絨蟲(為有爪動物(onychophoran))及水熊(water bear，為緩步動物(tardigrade))皆屬於蛻皮動物(ecdysozoan)大家族的一分子，這類動物的特徵便是會蛻皮。雖然怪誕蟲並非所有蛻皮動物的共同祖先，但其為天鵝絨蟲的前驅。發現怪誕蟲的口腔形態讓科學家確認原本天鵝絨蟲的口腔也有這些特徵，但經過漫長的演化後最終失去了這些特徵。
"The early
evolutionary history of this huge group is pretty much uncharted," said Dr
Martin Smith, a postdoctoral researcher in Cambridge's Department of Earth
Sciences, and the paper's lead author. "While we know that the animals in
this group are united by the fact that they moult, we haven't been able to find
many physical characteristics that unite them."
「這一大類動物的早期演化歷史大半仍隱藏在迷霧當中，」Martin
Smith博士說，他是劍橋大學地質科學系的博士後研究員，也是本篇論文的第一作者。「即使我們知道這群動物的共同特徵是牠們都會蛻皮，我們卻未能發現更多的共同生理特徵。」
"It turns out that the
ancestors of moulting animals were much more anatomically advanced than we ever
could have imagined: ring-like, plate-bearing worms with an armoured throat and
a mouth surrounded by spines," said Dr Jean-Bernard Caron, Curator of
Invertebrate Palaeontology at the Royal Ontario Museum and Associate Professor
in the Departments of Earth Sciences and Ecology & Evolutionary Biology at
the University of Toronto. "We previously thought that neither velvet
worms nor their ancestors had teeth. But Hallucigenia tells us that actually, velvet worm
ancestors had them, and living forms just lost their teeth over time."
「顯然蛻皮動物的祖先的構造較我們之前認為的更加先進：牠們的環狀蟲體上覆有硬板，並擁有批甲的喉部及被尖刺環繞的嘴部。」Jean-Bernard Caron博士說，他是皇家安大略博物館無脊椎古生物部門的研究人員，同時也是多倫多大學地球科學暨生態與演化生物學系的副教授。「我們之前認為天鵝絨蟲與牠們的祖先都沒有牙齒。但怪誕蟲告訴我們天鵝絨蟲的祖先實際上是有牙齒的，而現生種不過是在演化過程中失去了它們。」
Hallucigenia was just one of the weird creatures that lived during the
Cambrian Explosion, a period of rapid evolutionary development starting about
half a billion years ago, when most major animal groups first emerge in the
fossil record.
怪誕蟲不過是生活在寒武紀大爆發(the Cambrian Explosion)時的眾多奇特生物的其中之一，此時期發生於約5000萬年前，當時動物的演化程度大幅躍進，大多數主要的動物類別首次出現於這段時間的化石紀錄中。
At first, Hallucigenia threw palaeontologists for a bit of a
loop. When it was identified in the 1970s, it was reconstructed both backwards
and upside down: the spines along its back were originally thought to be legs,
its legs were thought to be tentacles along its back, and its head was mistaken
for its tail.
起初，怪誕蟲讓科學家有些困惑。發現於1970年代，科學家在重建牠的樣貌時卻將上下前後都顛倒了：長在背部的成排釘刺原先被認為是腳，腿被認為是長在背上的觸鬚，頭部則被誤認成尾端。
Right side up and right way
round, Hallucigenia still looks pretty strange: it had
pairs of lengthy spines along its back, seven pairs of legs ending in claws,
and three pairs of tentacles along its neck. The animals were between 10 and 50
millimetres in length and lived on the floor of the Cambrian oceans.
即使將上下前後導正過來，怪誕蟲看起來還是相當奇異：背上有兩排長長的尖刺，七對末端帶爪的足部，頸部長有三對觸鬚。這種生物長約10至50釐米，生活在寒武紀的海床上。
More significantly, Hallucigenia's
unearthly appearance has made it difficult to link it to modern animal groups
and to find its home in the Tree of Life. In 2014, research from Cambridge
partially solved this problem by studying the structure of Hallucigenia's
claws, which helped definitively link it to modern velvet worms.
更重要的是，怪誕蟲怪異的外表讓人很難將牠跟其他的現生生物連在一起，因此難以確認牠在演化樹中的定位。在2014年，劍橋大學的科學家解決了這個難題的一部分。藉由研究怪誕蟲的爪子構造，他們認為牠與現代的天鵝絨蟲絕對有所關連。
In the new work,
researchers used electron microscopy to examine fossils from the collections of
the Royal Ontario Museum and the Smithsonian Institution, definitively sorting Hallucigenia's
front from back, and making some surprising observations.
在本篇新研究中，研究人員利用電子顯微鏡檢視皇家安大略博物館與史密森尼學會(the
Smithsonian Institution)蒐集的怪誕蟲標本，而確認了牠們的首尾方向，並且得到一些驚人的發現。
"Prior to our study
there was still some uncertainty as to which end of the animal represented the
head, and which the tail," said Smith. "A large balloon-like orb at
one end of the specimen was originally thought to be the head, but we can now
demonstrate that this actually wasn't part of the body at all, but a dark stain
representing decay fluids or gut contents that oozed out as the animal was
flattened during burial."
「在我們的研究之前，大家仍不太確定這種動物哪一端是頭，哪一端是尾巴，」Smith說。「位於其中一端的氣球外型球體原先被認為是頭部，但現在我們認為那根本就不是身體的任何一部份，而是液體分解後留下的暗色汙漬，或是蟲體被埋藏而壓扁時，從腸道擠出來的內容物。」
Identifying this end as the
tail led Caron to revisit the fossils and dig away the sediment that was
covering the head: the animals died as they were buried in a mudslide, and
their floppy head often ended up pointing down into the mud. "This let us
get the new images of the head," said Caron. "When we put the fossils
in the electron microscope, we were initially hoping that we might find eyes,
and were astonished when we also found the teeth smiling back at us!"
確認哪邊是尾部後，Caron重新檢視那些化石，並挖除覆蓋頭部的沉積物。由於這些動物被一場泥流活埋而死亡，造成牠們柔軟的頭部常常會朝下埋在泥中。「這讓我們可以看到全新的頭部影像。當我們將牠擺在電子顯微鏡下，本來我們希望可以看到牠的眼睛，而讓人驚喜的是牠竟然還對我們露齒微笑！」
The new images show an
elongated head with a pair of simple eyes, which sat above a mouth with a ring
of teeth. In addition, Hallucigenia's throat
was lined with needle-shaped teeth. The fossils originated in the Burgess Shale
of Yoho National Park in western Canada, one of the world's richest sources of
fossils from the Cambrian period.
這些新的影像顯示牠們的長型頭部帶有一對單眼，位在有環狀排列牙齒的口部上方。此外，怪誕蟲的喉部有許多細小的針狀牙齒排列其中。這些化石出土於加拿大西部幽鶴國家公園(Yoho National Park)的伯吉斯頁岩(the Burgess Shale)，為世上最豐富的寒武紀化石寶庫之一。
The ring of teeth that
surrounded Hallucigenia's mouth
probably helped to generate suction, flexing in and out, like a valve or a
plunger, in order to suck its food into its throat. The researchers speculate
that the teeth in the throat worked like a ratchet, keeping food from slipping
out of the mouth each time it took another 'suck' at its food.
環繞在怪誕蟲口部週遭的牙齒也許能夠幫助產生吸力。怪誕蟲將牙齒伸進伸出，達到類似活塞或活瓣的功能，以將食物吸入至牠的喉部。研究人員猜測牠喉部的牙齒可以鉤住食物，防止在另一次吸入食物時之前的食物溜出牠的嘴巴。
"These teeth resemble
those we see in many early moulting animals, suggesting that a tooth-lined
throat was present in a common ancestor," said Caron. "So where
previously there was little reason to think that arthropod mouths had much in
common with the mouths of animals such as penis worms, Hallucigenia tells us that arthropods and velvet
worms did ancestrally have round-the-mouth plates and down-the-throat teeth --
they just lost or simplified them later."
「這些牙齒與許多早期的蛻皮動物十分相似，顯示牠們的共同祖先擁有佈滿牙齒的喉嚨。」Caron說。「先前幾乎沒有理由認為節肢動物與陰莖蟲(penis worm)這類生物的口部會有許多共同之處，但怪誕蟲告訴我們節肢動物與天鵝絨蟲都有環繞在口部的硬板，與深至喉嚨的的牙齒，只是之後這些特徵在後者身上失去或者退化了。」
The material for this study
was collected between 1992 and 2000 and represents more than 165 additional Hallucigenia specimens -- including many rare
orientations and well-preserved specimens.
此研究的對象包含了從1992年至2000年所收集的超過165個怪誕蟲樣品，包含許多呈現少見方位與保存良好的樣品。
Parks Canada, which holds
jurisdiction over the Burgess Shale sites located in Yoho and Kootenay national
parks, is thrilled by this discovery and eager to share this exciting new piece
of the ever-unfolding Burgess Shale story with their visitors.
擁有管裡幽鶴國家公園與庫特尼國家公園(Kootenay National Park)境內的伯吉斯頁岩的加拿大公園管理局(Parks Canada)，對這些發現十分興奮並積極將這些未曾公開的伯吉斯頁岩的故事與他們的遊客分享。
The research was funded by
Clare College, Cambridge, the Natural Sciences and Engineering Research Council
of Canada, and the Royal Ontario Museum.
此研究由劍橋大學克萊爾學院(Clare College, Cambridge)、加拿大自然科學及工程研究委員會(the Natural Sciences and Engineering Research Council of Canada)及皇家安大略博物館(the Royal Ontario Museum)資助。
引用自：University of Cambridge. "Newly found ring
of teeth uncovers what common ancestor of molting animals looked like."
ScienceDaily. ScienceDaily, 24 June 2015.
沒有留言:
張貼留言