Quantitative analyses of Ediacaran Ecosystems

在过去的至少35亿年里，生物一直占据着地球，但直到最近，它们的化石才大到足以用肉眼看到。第一个真正宏观的化石出现在大约5.8亿年前，在埃迪卡拉纪中期，比可识别的动物的出现和贝壳，骨骼和洞穴的传统化石记录早了不到5000万年。埃迪卡拉纪化石记录的重要性在于，它提供了一个直接的解释，说明了从最初30亿年的完全微生物世界到明显迟来的现代生物圈的建立。不幸的是，埃迪卡拉纪的记录也存在深刻的问题。我们不仅不知道这些最早的大型生命形式与什么有关，甚至不清楚它们是什么样的东西，或者它们如何谋生。本研究的目的是重建这些化石的原始群落生态-它们如何相互作用以及与环境的相互作用-着眼于理解它们在现代世界中的作用。研究埃迪卡拉巨型化石的优势之一是它们在生活中没有移动过，因此它们的化石位置提供了它们原始空间相互关系的准确描述。通过分析这些空间数据，可以提取出数量惊人的原始生态细节。就像森林中的树木一样，埃迪卡拉化石在层面上的分布反映了许多影响的相互作用，包括生物生活史，种间竞争和促进，以及物理环境。因此，这些生态过程可以使用现代森林生态学家开发的许多复杂的统计和建模技术进行逆向工程。我们已经在最近的《自然》杂志上发表了一篇论文，证明了这种方法的潜力，现在计划将其系统地应用于已知最早的埃迪卡拉巨型化石，即“阿瓦隆组合”--几乎只在查恩伍德森林、莱斯特郡和纽芬兰东南部的标志性事件中发现。研究方案分为两个主要部分。第一是数据收集。在这里，我们计划组装一个全面的数据库，高分辨率，数字化的所有已知的阿瓦隆层面的3D图像，保存大量（> 100个标本）的大型化石。由于激光扫描技术的最新进展，这将产生约20，000个化石的精确大小，位置和方向，包括44个独立的床面群落。该项目的第二个也是主要重点是分析这些数据的主要生态内容。使用一系列互补的定量技术，我们计划解决早期埃迪卡拉生态学中的三个主要问题：1）这些生物如何与环境相互作用，以及与生物繁殖相比，这些环境相互作用有多大？这是现代群落生态学长期争论的主题，埃迪卡拉纪记录提供了第一个大型生物体之间平衡的独特观点。2）埃迪卡拉纪生态系统中身体大小的分布是什么，什么生态过程对它们特定的大小结构负责？在现代水生系统中，体型大小与动物的捕食和摄食关系直接相关，但在这些早期宏观群落中没有任何捕食动物的证据。3)上覆水柱内的资源竞争是否主导了阿瓦隆生物之间的相互作用？这通常被认为是埃迪卡拉生态学中的一个重要因素，但从未被定量测试过。我们对埃迪卡拉生态学的这一问题和其他问题的分析方法有望为现代生物圈的起源提供重要的新线索。

Organisms have occupied the planet for at least the past 3.5 billion years, but only relatively recently have their fossil remains been large enough to see with the naked eye. The first properly macroscopic fossils turn up around 580 million years ago, in the middle of the Ediacaran Period, less than 50 million years before the appearance of recognizable animals and the conventional fossil record of shells, bones and burrows. The significance of the Ediacaran fossil record is that is provides a direct account of the transition from the exclusively microbial world of the first three billion years, to the conspicuously belated establishment of a recognizably modern biosphere. Unfortunately, the Ediacaran record is also profoundly problematic. Not only do we not know what these earliest large life-forms were related to, it is not even clear what kinds of things they were, or how they made a living. The aim of this research proposed here is to reconstruct the original community ecology of these fossils - how they interacted with one another and with their environment - with an eye to understanding their role in bringing about the modern world.One of the advantages of studying Ediacaran macrofossils is that none of them moved during life, so their fossilized positions provide an exact account of their original spatial inter-relationships. By analysing these spatial data, it is possible extract a surprising amount of original ecological detail. Like trees in a forest, the distribution of Ediacaran fossils on bedding surfaces reflects the interplay of numerous effects including organism life-history, inter-specific competition and facilitation, and the physical environment. As such, these ecological processes can be reverse engineered using many of the sophisticated statistical and modelling techniques developed by modern forest ecologists. We have already demonstrated the potential of this approach with a recent paper in Nature, and now plan to apply it systematically to the earliest known Ediacaran macrofossils, the "Avalonian assemblage" - known almost exclusively from iconic occurrences in Charnwood Forest, Leicestershire and SE Newfoundland. The research programme is divided into two main parts. The first is data collection. Here we plan to assemble a comprehensive database of high-resolution, digitized 3D images of all known Avalonian bedding surfaces preserving large populations (> 100 specimens) of macrofossils. Made possible by recent advances in laser-scanning technology, this will yield the precise size, position and orientation of ~20,000 fossils comprising 44 separate bedding-surface communities. The second, and primary, focus of this project is to analyse these data for their primary ecological content. Using a range of complementary quantitative techniques, we plan to address three main issues in early Ediacaran ecology:1) How did these organisms interact with their environment, and how much did these environmental interactions matter compared to organism reproduction? This is the subject of a longstanding debate in modern community ecology, and the Ediacaran record offers a unique view of the balance among the first large organisms.2) What was the distribution of body-sizes in Ediacaran ecosystems and what ecological processes were responsible for their particular size-structure? In modern aquatic systems, body-size is directly related to predation and the feeding relationships of animals, but there is no evidence of predatory animals in any of these early macroscopic communities. 3) Did competition for resources within the overlying water-column dominate interactions between Avalonian organisms? This has often been promoted as a significant factor in Ediacaran ecology, but has never been quantitatively tested. Our analytical approach to this and other questions of Ediacaran ecology promise to shed significant new light on the origin of the modern biosphere.