地球最古老的動物組成了複雜的生態群集

原文網址：https://news.vanderbilt.edu/2018/09/17/earths-oldest-animals-formed-complex-ecological-communities/

地球最古老的動物組成了複雜的生態群集

Heidi Hall

謎樣的埃迪卡拉生物群(Ediacara biota)生活在5.7億至5.4億年前，是地球第一批肉眼可見的動物。一項新的分析結果使我們對牠們有了更深的理解。

Darroch最近於納米比亞進行研究時發現了這些埃迪卡拉生物群的化石。圖片來源：Simon A.F. Darroch

埃迪卡拉化石略顯怪異的外型跟現生的動物族群都不一樣。數十年來，研究人員相信這些謎樣的化石組成的生態體系較為單純。不過美國范德比大學的古生物學家 Simon A.F. Darroch和他的團隊借用當代生態學的分析方法――描述物種常見度跟相對豐度之間的關係――從而得知這些生物跟現代動物之間的相似度比過去認為得還要高。

分析顯示埃迪卡拉生物群的大部分化石組合都含有複雜的生態系所具備的特徵，因此在寒武紀大爆發的數千萬年前，埃迪卡拉生物群就已經組成了複雜的群集。這些生物一部分的身體埋在海床當中，牠們有些以攝食懸浮物維生，有些則是靠過濾海水來取得食物，還有一些則是被動地吸收養分。少數埃迪卡拉生物甚至可以自由移動。

當今許多生態系都含有複雜的動物群集，這意謂其中的物種會競爭各式各樣的資源，或者它們可以創造出更多的生態棲位給其他物種。研究團隊發現複雜群集的特徵可以一路追溯到最老的埃迪卡拉化石。也就是說，當生物演化成肉眼可見大小的時候，它們便立刻開始形成跟今天一樣的繁複生態群集。

范德比大學地球與環境科學系的助理教授Darroch表示：「我們主要的研究成果是驗證了關於埃迪卡拉生態系的兩種模型誰才是對的：簡單或者複雜。」

「支持埃迪卡拉生態系較為簡單的科學家提出這些神秘的生物一般來說都相當原始、生態單純且一致，競爭的資源類型也完全相同。」他說。「支持其生態系較為複雜的科學家則提出牠們就跟現在的動物一樣，會為了各式各樣的資源發生競爭。我們的分析結果支持了複雜生態系的模型，代表這些神秘的生物儘管看起來可能十分怪異，牠們和現今動物的共通點或許比我們過往認為的還多出許多。」

他們的論文今日發表在期刊《自然生態學和演化》(Nature Ecology & Evolution)的線上版，標題為「埃迪卡拉底棲群集的高度生態複雜性」(“High ecological complexity in benthic Ediacaran communities.”)。

團隊首先從已發表的文獻中匯集所有關於埃迪卡拉化石的資料，接著加入團隊從納米比亞南部的野外工作中蒐集到的資料。納米比亞南部的埃迪卡拉化石是世上最年輕的埃迪卡拉化石之一，它記錄的生物群集就活在寒武紀大爆發的前夕。

在他們的分析中僅有少數單純的化石群集，納米比亞南部的化石正是其中之一，代表這些生物受到許多生態壓力。這支持了隨著寒武紀大爆發的加速進行，埃迪卡拉生物群逐漸邁向滅絕的想法。Darroch說此想法雖然相當吸引人，但納米比亞南部不過是一個數據點位，因此還要更多研究才能證明。

團隊也利用化石紀錄建立出來的三維模型來深入了解埃迪卡拉生物群。這些生物在5.4億年前澈底消失，同一時間原始的節肢動物、軟體動物和海綿也開始出現。

Earth’s oldest animals formed complex ecological communities

A new analysis is shedding light on the earth’s first visible animals: the 570-540-million-year-old, enigmatic Ediacara biota.

Ediacaran fossils have a slightly bizarre appearance not shared by any modern animal groups. For decades, researchers believed these enigmatic fossils were ecologically simple. However, borrowing a method from modern ecology – fitting species to relative abundance distributions – ­Vanderbilt University paleontologist Simon A.F. Darroch and his team learned that these organisms were more like modern animals than once thought.

The analysis showed that a majority of fossil assemblages bear the hallmarks of being ecologically complex, and Ediacara biota were forming complex communities tens of millions of years before the Cambrian explosion. The creatures lived partially submerged in what was once the ocean floor, some of them suspension feeding, others filter feeding, still others passively absorbing nutrition. A few were even mobile.

Complex communities are ones that comprise species competing for numerous different resources or species that create niches for others (as in many modern-day ecosystems). The team found that the signature of complex communities extends all the way back to the oldest Ediacaran fossils. In other words, as soon as macroscopic life evolved, it began forming diverse ecological communities not unlike those in the present day.

“The main impact of our work was testing between the simple and complex models for Ediacaran ecosystems,” said Darroch, an assistant professor in the Department of Earth and Environmental Sciences.

“Supporting a simple model would suggest that these mysterious organisms were universally primitive, sharing the same basic ecology and all competing for the same resources,” he said. “Support for the complex model would instead suggest that they likely competed for a variety of different resources, just like modern animals. Our analyses support the complex model, illustrating that – even though they may look bizarre – these mysterious fossils may have far more in common with modern animals than we thought.”

Their paper, “High ecological complexity in benthic Ediacaran communities,” is available online today in Nature Ecology & Evolution.

The team first compiled all Ediacaran fossil data from the published literature then added a dataset collected during fieldwork in southern Namibia. These Namibian fossils are the some of the youngest from anywhere in the world and record communities that were living immediately prior to the onset of the Cambrian explosion.

The fossils formed one of the few simple communities in the analyses, suggesting that these organisms were ecologically stressed. That lends support to the idea that the Ediacara biota were gradually going extinct in the run-up to the Cambrian explosion. Although it’s an exciting idea, Darroch said, it’s only one data point and will need much more research to prove.

The team is also using 3D modeling based on the fossil record to better characterize Ediacara biota, which completely disappeared 540 million years ago – as early arthropods, mollusks and sponges began to appear.

原始論文：Simon A. F. Darroch, Marc Laflamme, Peter J. Wagner. High ecological complexity in benthic Ediacaran communities. Nature Ecology & Evolution, 2018; DOI: 10.1038/s41559-018-0663-7

引用自：Vanderbilt University. "Earth's oldest animals formed complex ecological communities."