科學家發現形成於5.5億年前，迄今最古老的消化道化石
在美國內華達州沙漠發現年代為5.5億年的消化道化石，或許是瞭解地球動物早期歷史的關鍵。
這兩幅立體圖顯示了5.5億年前形成的管狀化石(左圖紅色部分)以及內部的消化道(兩圖的橘色部分)。圖片來源：密蘇里大學
地球上的生命在五億多年前只有簡單的海洋生物，它們跟現今住在海洋裡的物種毫無相似之處。接著，大約從5.4億年前開始，動物的構造出現了重大改變。
現今我們所知的動物有許多族群的祖先在這段期間開始出現，像是原始的甲殼類以及蠕蟲，但是長年以來科學家一直無法確知這兩種看似無關的動物群集之間究竟有何關聯。不過，最近由密蘇里大學的Jim
Schiffbauer領導的科學家團隊分析了一種管狀化石之後，得到的證據顯示其為5.5億年前形成的消化道。這不只是生物體內構造形成的化石中已知最早的案例之一，它也顯示了科學家認為這兩類動物之間有何關聯的可能答案。
這項研究發表在「自然」集團之下的期刊《自然通訊》(Nature Communications)。
「這些構造不只是迄今發現最為古老的腸子，也有助於解決一項長久以來的爭議――這類重要的化石族群在演化上的地位為何。」Schiffbauer表示。他是密蘇里大學文理學院的地質科學副教授，也是該大學X光微量分析核心設施的主任。「這些化石隸屬於一類相當好認的生物，稱為克勞德管蟲(cloudinids)。牠們被科學家用來辨識埃迪卡拉紀的最後一千萬年到一千五百萬年，也就是寒武紀大爆發不久之前的時期。現在我們認為牠們的解剖構造比起珊瑚更類似於蠕蟲。」
科學家普遍認為在地球的生命史當中，我們今日所知的動物族群有許多的祖先是在寒武紀大爆發這個時間點出現。
研究人員在內華達州帕朗附近的蒙哥馬利山找到形似克勞德管蟲的化石。上圖是此研究分析的化石代表。圖片來源：密蘇里大學
在這項研究中，科學家建立化石的立體數位影像時，運用了密蘇里大學X光微量分析核心設施來進行對地質科學來說相當特別的分析方法――微型電腦斷層掃描造影。這種技術可以讓科學家看見化石的內部構造。
「利用電腦斷層掃描造影，我們可以很快地得到重要的內部特徵，接著分析整個化石，而不用擔心可能對它造成損害。」共同作者Tara
Selly表示。她是地質科學系的助理研究教授以及X光微量分析核心設施的助理主任。
這項研究「Discovery
of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran
Period」發表在《自然通訊》。其他作者包括密蘇里大學的Sarah
Jacquet、斯沃斯莫爾學院的Rachel
Merz、內華達大學拉斯維加斯分校的Michael
Strange、中國西安西北大學的Yaoping
Cai、約翰霍普金斯大學的Lyle
Nelson和Emmy
Smith。
Scientists find oldest-known
fossilized digestive tract — 550 million years
A 550 million-year-old fossilized
digestive tract found in the Nevada desert could be a key find in understanding
the early history of animals on Earth.
Over a half-billion years ago, life on Earth was
comprised of simple ocean organisms unlike anything living in today’s oceans.
Then, beginning about 540 million years ago, animal structures changed
dramatically.
During this time, ancestors of many animal groups we
know today appeared, such as primitive crustaceans and worms, yet for years
scientists did not know how these two seemingly unrelated communities of
animals were connected, until now. An analysis of tubular fossils by scientists
led by Jim Schiffbauer at the University of Missouri provides evidence of a 550
million-year-old digestive tract — one of the oldest known examples of
fossilized internal anatomical structures — and reveals what scientists believe
is a possible answer to the question of how these animals are connected.
The study was published in Nature Communications, a journal of Nature.
“Not only are these structures the oldest guts yet
discovered, but they also help to resolve the long-debated evolutionary
positioning of this important fossil group,” said Schiffbauer, an associate
professor of geological sciences in the MU College of Arts and Science and
director of the X-ray Microanalysis Core facility. “These fossils fit within a
very recognizable group of organisms — the cloudinids — that scientists use to
identify the last 10 to 15 million years of the Ediacaran Period, or the period
of time just before the Cambrian Explosion. We can now say that their
anatomical structure appears much more worm-like than coral-like.”
The Cambrian Explosion is widely considered by
scientists to be the point in history of life on Earth when the ancestors of
many animal groups we know today emerged.
In the study, the scientists used MU’s X-ray
Microanalysis Core facility to take a unique analytical approach for geological
science — micro-CT imaging — that created a digital 3D image of the fossil.
This technique allowed the scientists to view what was inside the fossil
structure.
“With CT imaging, we can quickly assess key internal
features and then analyze the entire fossil without potentially damaging it,”
said co-author Tara Selly, a research assistant professor in the Department of
Geological Sciences and assistant director of the X-ray Microanalysis Core
facility.
The study, “Discovery of bilaterian-type through-guts
in cloudinomorphs from the terminal Ediacaran Period,” was published in Nature Communications. Other authors
include Sarah Jacquet from MU; Rachel Merz from Swarthmore College; Michael
Strange from the University of Nevada, Las Vegas; Yaoping Cai from Northwest
University in Xi’an, China; and Lyle Nelson and Emmy Smith from Johns Hopkins
University.\
原始論文：James D. Schiffbauer, Tara Selly, Sarah M.
Jacquet, Rachel A. Merz, Lyle L. Nelson, Michael A. Strange, Yaoping Cai, Emily
F. Smith. Discovery
of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran
Period. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-019-13882-z
引用自：University of Missouri-Columbia. "
A gutsy proposition"
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