第一批复杂的生态系统

Encyclopédie environnement - écosystèmes complexes - first complex ecosystems

  今天的生态系统在组成它们的有机体之间相互依存关系的复杂性方面令人叹为观止。贯穿食物网的能量和生物量循环将细菌、单细胞生物、植物/藻类和各种大小、秉性极为不同的动物关联在一起。这些生物系统的稳定性是以动态平衡为基础的,而动态平衡对环境和人为因子非常敏感。在30多亿年的时间里,海洋生态系统的组成一直以微生物(细菌、古菌)或单细胞真核生物为优势。尽管这些有机体在碳、氮的生物地球化学循环和提高地球氧含量方面发挥了关键作用,但它们从未形成如我们现今所知的自然界中的复杂食物网。5亿多年前,前寒武纪末出现的多细胞生物和宏体生物以及前寒武纪—古生代过渡期中动物界的出现彻底改变了海洋世界及其运作模式。

1.埃迪卡拉纪神秘的海洋生态系统

环境百科全书-生命-生态系统
图1. 前寒武纪末期(埃迪卡拉纪)海洋生态系统的典型生物。 (a) 由长柄固着在海底的弹弓形叶状体生物群体;来自加拿大纽芬兰省米斯塔肯角(Mistaken Point)沉积的岩滩区。(b)对一个叶状体生物的重建,显示弹弓具有分形结构。(c-f) 俄罗斯白海动物群;分别为未确定型、狄更逊水母(Dickinsonia)、金伯拉虫(Kimberella)、三叶虫(Archaeaspis)。比例尺:10 cm (a),1cm (c-e)和5 mm(f)。[来源: M. Laflamme (a),Narbonne et al. 2009 [参见文献[1]] (b) ,J. Vannier (c-f)]
  在前寒武纪末,海洋环境中出现了全新的生物。它们以体型大(从几厘米到一米左右,之前从未到达过的体型)和前所未有的复杂构造而著称(图1)。

  这些神秘的生物出现在加拿大(如米斯塔肯角)、澳大利亚(如埃迪卡拉)、纳米比亚和俄罗斯北部(如白海)等许多化石遗址点,它们在5.75亿年至5.42亿年前大量出现,并生活在不同深度的海底,该地质层段与埃迪卡拉纪末期相对应。由于沙质沉积物和火山灰的瞬间沉积,它们柔软、轻逸和灵活的躯体的印迹得以存留,而在当今自然界中没有与之对应的生物。

  其中最为典型的叶状体生物[1]的特征是其波浪状的叶状体由柄牢牢固着在海底。然而,虽然这些固着生物有独一无二的构件和分形结构,然而的确达不到寒武纪的第一批动物解剖结构的复杂程度。它们显然没有口、消化系统和复杂的内部器官,因为它们具有与环境交换的巨大表面,被认为是通过直接吸收溶解的有机碳来获取食物(渗透营养)。

  在埃迪卡拉纪,海底的水-沉积物界面也被海绵动物和许多身体扁平的生物所占据,后者有时会让人联想到某些软体动物(mollusc)和现今节肢动物(arthropod)的两侧对称(图1)。它们中的一些动物,如金伯拉虫、狄更逊水母和一种三叶虫纲动物(Yorgia)产生的一些痕迹化石表明,它们(这些扁平化的生物)能移动并以覆盖了整个海床的细菌膜(bacterial film)为食,正如扁盘动物(placozoaire)一样,它们可能沿着腹面进行体外消化。

  当时的海洋生态系统以微生物垫渗透性营养的多细胞生物(如叶状体生物)占优势地位,依靠滤食微型食物(microphage在术语中的意思为“小噬细胞”,在此显然不是该意——校订者注)(如海绵动物通过它们的过滤系统和鞭毛细胞)或利用外部接触消化为主要的方式。这些取食策略完全适应于前寒武纪末海洋中可获取的资源,即水-沉积物界面上随处可见的大量溶解的有机物和普遍存在的微生物垫。在前寒武纪—寒武纪过渡期,埃迪卡拉纪生物的灭绝并不是由像大多数重大生物危机那样的全球范围内动荡导致的,而是由于最早的掘穴动物(生物扰动作用)破坏了埃迪卡拉纪生物的生境。埃迪卡拉纪生物面对第一批捕食者完全没有防御能力,无法存活。

2. 第一个动物群落:现代生态系统的原型

  顾名思义,寒武纪大爆发是指在化石记录中相对突然地出现了新的和解剖学上复杂的生物体,其中,我们可以肯定地识别出现今主要动物群的远祖(例如:节肢动物、蠕虫、软体动物、脊索动物,图2)。一些保存特别完好的沉积层(称为化石库),例如澄江(中国;约520 Ma)、伯吉斯页岩(加拿大;约505 Ma)、西里斯帕西特(格陵兰岛)和鸸鹋湾(澳大利亚)都向我们揭示了最早的海洋动物群落的存在。由于寒武纪早期的动物已经发展出运动和感觉能力(化石表明一些动物有头部神经系统),这些动物在环境中主动移动,并首次争相利用了多维生态位。这种动态标志着与埃迪卡拉纪非常重要的固着生活的海洋生物根本的区别,也是生态系统演化的一个不可逆转的转折点。

环境百科全书-生命-生态系统
图2. 来自保存特别完好的沉积层的寒武纪动物。(a) 威瓦西亚虫(Wiwaxia),一种由骨片保护原始的软体动物。(b) 环饰蠕虫(Cricocosmia),一种接近鳃曳动物(小阴茎形状的蠕虫)的蠕虫,由一系列的骨板保护。(c) 奥特瓦虫(Ottoia),一种鳃曳动物门蠕虫,显示其咽部外翻(左)和肠道内的食物残渣(右)。(d) 贫腿虫(Paucipodia), 叶足动物, 具无关节附肢的初级节肢动物。(e) 怪诞虫(Hallucigenia lobopodian)被认为是现代有爪类的祖先,背部有刺。(f) 奇虾(幼体)显示出强大的抓握附肢。(g)-(i)西德尼虫(Sidneyia),伯吉斯页岩的标志性节肢动物;全貌,肠道内容物(三叶虫)和咀嚼附肢的细节。(j) 瓦普塔虾(Waptia),腹部灵活的节肢动物。图a, c, e, e, g-j来自伯吉斯页岩(加拿大不列颠哥伦比亚省);b, d, f来自澄江(中国云南)。比例尺:图a、c、d、f-h为1cm;b、e、i为5mm;h为1mm。 [图片来源:让-范尼尔(J. Vannier),图e 自J.-B.卡农(J.-B. Caron)]
环境百科全书-生命-生态系统
图3. 在埃迪卡拉纪-寒武纪过渡时期,由于沉积物内生物扰动的增加和微生物垫的减少,海底发生了变化。理查德-布里格尼(Richard Bligny)的水彩画,灵感来自Fedonkin et al. 2007(见参考文献[2]
图中文字:EDIACARAN 埃迪卡拉纪,CAMBRIAN 寒武纪
  从寒武纪开始,许多与鳃曳动物高度相似的蠕虫定居在如今沉积物的内部。由于微生物垫的存在,海底处于长期封闭和层积的状态,但由于这些蠕虫日益激烈和深入的掘穴活动对海底造成了干扰(图3)[2] 。这一事件,有时被称作是“农艺革命”,因为它开辟了新的栖息地和资源,并改变了贯穿沉积物氧化还原梯度。正如对肠道内容物的研究所显示的那样,一些鳃曳类蠕虫(如奥特瓦虫)由于具有可外翻的带齿咽部,可以捕获生活在海底的各种小动物(如软舌螺、其他蠕虫、小型节肢动物)。捕食行为逐渐出现在寒武纪海洋生物群落中,这在前寒武纪是完全未知的。

环境百科全书-生命-生态系统
图4. 寒武纪群落的示意图。1,海绵;2,奥特瓦虫(捕食性鳃曳动物蠕虫);3,阴茎蠕虫(Spartobranchus)(管状肠虫);4,乌海蛭(Odontogriphus)(原始放牧软体动物);5,小软舌螺;6,卤虫类节肢动物;7,叶尾强钳虫(Fortiforceps)(有前额具附肢的节肢动物)。8,内克虾(Nectocaris)(原始自游软体动物));9,等刺虫(Isoxys)(自游节肢动物);10,乌葵虾(Hurdia)(捕食性奇虾);11,毛颚类动物(捕食性浮游生物)12,筛状奇虾(Tamisiocaris)(具滤食性附器的奇虾)。 [[图片来源:J. Vannier; simplified drawings from Briggs et al. 1994 [3] ; Caron et al. 2006 [4]; Caron et al. 2013 [5]; Daley et al. 2009 [6]; Hou et al. 2004 [7]; Smith and Caron 2010 [8]; Vannier 2012 [9]; Vannier et al. 2007 [10], 2009 [11]; Vinther et al. 2014 [12]; http://www.burgess-shale.rom.on.ca/]
  同时,位于沉积物上方的水体中居住着许多自游动物,如:

—刺胞动物,
—栉板动物,
—毛颚动物,
—原始软体动物和原始节肢动物(例如,等刺虫)

  对这些化石及其现生后代的比较研究表明(图4),这些物种在原始食物链中相互作用[3] [4] [5] [6] [7] [8] [9] [10] [11] 。格陵兰岛下寒武纪的Timisiocaris物种就是这些新营养关系中的一个很好的例子。它是标志性的捕食者奇虾(Anomalocaris)的近亲,它的大型附肢上生有梳和过滤刚毛,使它能够捕获在水体中滤食生活的浮游动物。微化石证明了这种以真核藻类和细菌为食的这类浮游动物的存在。

  然而,寒武纪海洋生物主要集中在水-沉积物界面,而海绵是固着动物群的主要组成部分。在所有保存完好的沉积层中,节肢动物是迄今为止最丰富和最多样化的附表底栖生物(图2)。它们的外部(分节,附肢)和内部(神经系统)结构图表明,其中一些与现今的甲壳动物和螯肢动物有亲缘关系。

  另一些属于现已灭绝的类群。它们有着关节和多节外骨骼,这可能有利于其拥有许多功能和特化。寒武纪节肢动物拥有抓握足和咀嚼附肢使其能够捕获猎物和减少产生食物颗粒。滤食巨型食物的取食方式(macrophagia)出现在寒武纪时期的许多食肉动物或以尸体为食的动物(食腐动物)中。例如,伯吉斯页岩(Burgess Shale)的节肢动物西德尼虫(Sidneyia)捕获、粉碎并食用小型三叶虫,其附肢和胃内容物如图所示(图2)。

  其他新特征的出现也促成了食物链发生重大变化。消化腺可提高酶降解食物的效率,从而促进滤食巨型食物的取食方式出现在许多寒武纪的节肢动物中。视觉也彻底改变了早寒武纪海洋生物之间的相互作用。例如,由数千个小眼组成的大型复眼使超级捕食者奇虾(Anomalocaris)能够发现并追踪它的猎物。毫无疑问,在寒武纪节肢动物中普遍存在的视觉已经显著改变了猎物-捕食者的关系,并在生态系统中引入了新的选择压力,导致了多种适应性反应。

环境百科全书-生命-生态系统
图5. 前寒武纪-寒武纪过渡时期海洋生态系统的复杂化,由理论上的三轴生态空间(生物的分层、食物模式和流动性)来呈现,该空间包含216个立方体,代表可能的生活方式(其中只有118个是生态上可持续的)。右图是根据古生物学数据对埃迪卡拉纪和寒武纪生态空间进行的比较填充。绿色和蓝色:在现存生态系统中确定的生活方式(对应118个立方体中的92个)。紫色:在埃迪卡拉纪和寒武纪确定的生活方式,它们占可能生活方式的10%左右;它们的数量在埃迪卡拉纪和寒武纪之间明显增加。[图片来源:简化自Bambach et al. 2007 [12] 及 Erwin & Valentine 2013 [13]]
图中文字: TIERING分层;Pelagic栖居上层的,浮游的;Erect (底栖)挺水的;Surficial(底栖)表栖的;Semi-inf (底栖)半内栖的;Shallow (底栖)浅内栖的;Deep (底栖)深内栖的;FEEDING 取食;Suspension 滤食;Surf Deposit沉积物表面取食;Mining 掘食; Grazing 牧食;Predatory 捕食;Other其他;MOTILITY活动水平;Non. Attached 非游动的,固着的;Non. Unattached 非游动的,未固着的;Facult. Attached 兼性固着的;Facult. Unattached 兼性非固着的;Fully Slow自由,缓慢运动;Fully Fast自由,快速运动;Ediacaran 埃迪卡拉纪;Cambrien寒武纪
  显而易见,虽然早在下寒武纪就已经存在复杂的食物网原型,但它与目前的生态系统不同,要量化生物体之间的相互作用在当时仍然非常困难。很少有定量的方法可以比较前寒武纪、寒武纪和目前的生态系统。然而,建造一个理论生态空间的三维模型[12] 就可以解决这个问题,它是根据三个坐标轴建立的(分层数、食物模式和生物体的移动),这使得我们可以更好地观察到生态位在地质时间的不同时期的占领情况(图5)[12],[13] 。由此,我们观察到在前寒武纪——寒武纪过渡时期的生态空间被填满,这将在奥陶纪生物大爆发事件中加强,最终呈现出我们所知道的海洋世界和运作方式。

 


参考资料与注释

封面图片。埃迪卡拉海洋的生命 [图片来源:© Ryan Somma (CC BY-SA 2.0),,经由维基共享资源]

[1] Narbonne G.M., Laflamme M., Greentre C. Trusler P. (2009) Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard’s Bay, Newfoundland. Journal of Paleontology 83, 503-528.

[2] Fedonkin M.A., Gehling J.G., Grey K., Narbonne G. Vickers-Rich P. (2007) The Rise of Animals. The Johns Hopkins University Press, Baltimore. 325 pp.

[3] Briggs D.E.G. (2015) The Cambrian Explosion. Current Biology 25, R864-R868

[4] Caron J.-B., Scheltema A., Schander C. & Rudkin D. (2006) A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale. Nature 442, 159-163.

[5] Caron J.-B., Conway Morris S. & Cameron C.B. (2013) Tubicolous enteropneusts from the Cambrian period. Nature 495, 503-506.

[6] Daley et al (2009) The Burgess Shale anomalocarididid Hurdia and its significance for early euarthropod evolution. Science 323, 1597-1600.

[7] Hou X.-G., Aldridge R.J., Bergström J., Siveter David J., Siveter Derek J. & Feng X.-H. (2004) The Cambrian Fossils of Chengjiang, China. Blackwell Publishing. 233 pp.

[8] Smith M.R. & Caron J.-B. (2010) Primitive soft-bodied cephalopods from the Cambrian. Nature 465, 469-472.

[9] Vannier J., Steiner M., Renvoisé E., Hu S.-X. Casanova J.-P. (2007) Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal Society London B 274, 627-633.

[10] Vannier J., Garcia-Bellido D.C., Hu S.-X. Chen A.L. (2009) Arthropod visual predators in the early pelagic ecosystems: evidence from the Burgess Shale and Chengjiang biota. Proceedings of the Royal Society London B 276, 2567-2574.

[11] Vinther J., Stein M., Longrich N.R. & Harper D.A.T. (2014) A suspension-feeding anomalocarid from the Early Cambrian. Nature 507, 496-500.

[12] Bambach R.K, Bush A.M. & Erwin D.H. (2007) Autecology and the filling of ecospace: key metazoan radiations. Palaeontology 50, 1-22.

[13] Erwin D.H. & Valentine J.W. (2013) The Cambrian Explosion: the construction of animal biodiversity. Roberts & Company Publishers. 406 pp.


环境百科全书由环境和能源百科全书协会出版 (www.a3e.fr),该协会与格勒诺布尔阿尔卑斯大学和格勒诺布尔INP有合同关系,并由法国科学院赞助。

引用这篇文章: VANNIER Jean (2024年3月13日), 第一批复杂的生态系统, 环境百科全书,咨询于 2024年11月1日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/zh/vivant-zh/first-complex-ecosystems/.

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The first complex ecosystems

Encyclopédie environnement - écosystèmes complexes - first complex ecosystems

Today’s ecosystems are amazing in the complexity of the interdependent relationships between the organisms that make up them. Energy and biomass circulate through food webs that unite bacteria, unicellular organisms, plants/algae and animals of extremely varied nature and size. The stability of these biological systems is based on dynamic balances that are nevertheless very sensitive to environmental and anthropogenic factors. For more than three billion years, marine ecosystems have been dominated by microbial organisms (bacteria, archaea) or unicellular eukaryotes. While these organisms have played a key role in the biogeochemical cycles of carbon or nitrogen and in raising the level of oxygen on our planet, they have never formed complex food webs as we know them in nature today. A little over 500 million years ago, the appearance of multicellular and macroscopic organisms at the end of the Precambrian and the advent of the animal kingdom during the Precambrian-Paleozoic transition revolutionized the marine world and its mode of operation.

1. The enigmatic marine ecosystem of the Ediacaran

Encyclopedie environnement - ecosystemes complexes - Organismes typiques ecosysteme marin du Precambrien - complex ecosystems
Figure 1. Typical organisms of the Terminal Precambrian (Ediacaran) marine ecosystem (a) Sling-shaped rangeomorph colony anchored to the bottom by a long stem; bench area of the Mistaken Point deposit, Newfoundland, Canada. (b) Reconstruction of a rangeomorph showing the fractal structures of the slingshot. (c-f) White Sea Fauna, Russia; respectively, undetermined form, Dickinsonia, Kimberella, Archaeaspis. Scales: 10 cm in (a), 1 cm in (c-e) and 5 mm in (f). [Source: M. Laflamme (a), Narbonne et al. 2009 [see ref. 1] (b) and J. Vannier (c-f)]
Completely new organisms appear in the marine environment at the end of the Precambrian. They are distinguished by their large size (from several centimetres to about a metre; never achieved before) and by their unprecedented architectural complexity (Figure 1).

Present in many fossil sites in Canada (e.g. Mistaken Point), Australia (e.g. Ediacara), Namibia and northern Russia (e.g. White Sea), these enigmatic organisms have colonized the seabed in abundance at variable depths between 575 and 542 million years, a geological interval corresponding to the end of the Ediacaran. The footprints of their soft, fleecy and flexible bodies, without equivalent in today’s nature, have been preserved thanks to the instantaneous deposition of sandy sediments or volcanic ash.

Among the most typical, rangeomorphs [1] are characterized by a wavy frondoStructure of a flattened, relatively large, leaf-like living organ or organism. with a stem firmly anchored to the bottom. These fixed organisms, unique in their modular and fractal structure, do not, however, reach the anatomical complexity of the first Cambrian animals. Apparently devoid of mouth, digestive system and complex internal organs, they were thought to have extracted their food by direct absorption of dissolved organic carbon (osmotrophyA mode of feeding that consists of feeding from dissolved substances. The osmotrophic organisms are nourished by transmembrane exchange, i.e. by diffusion of ions or small molecules through the cytoplasmic membrane. This type of nutrition, which is very common among microorganisms, is also provided by a number of animals, both free and parasitic. It is only possible in liquid environments (aquatic environments, internal fluids of animals or plants) or by the synthesis of enzymes that “digest” their solid environment.) thanks to their very large surface of exchange with the environment.

At the Ediacaran, the water-sediment interface of the ocean floor was also occupied by sponges and many flattened organisms sometimes evoking the bilateral symmetry of some molluscsEmbranchement of invertebrate animals, not segmented, with bilateral symmetry sometimes altered. They have a soft body (hence the name mollusk) usually composed of a head, a visceral mass, and a foot. They may have a calcareous shell produced by a mantle covering the visceral mass. and current arthropodsBranch of invertebrate animals whose organisational plane is characterised by a body segmented with articulated appendages and covered by a rigid cuticle or shell, which constitutes their exoskeleton, in most cases composed of chitin. The arthropod branch appeared 543 million years ago and is by far the one with the most species and individuals in the entire animal kingdom (80% of known species). (Figure 1). Traces produced by some of them such as Kimberella, Dickinsonia and Yorgia indicate that they moved and consumed the bacterial filmsAlso called biofilms; microbial community marked by the secretion of an adhesive and protective matrix. It is usually formed in water or in an aqueous medium. Biofilms were probably the first colonies of living organisms more than 3.5 billion years ago. With stromatolites, they seem to be the origin of the first biogenic rocks and reef structures, which then covered the entire seabed, probably by external digestion along their ventral surface as in the placozoairesMetazoans (animals) with the simplest organizational plan. These tiny (between 1 and 3 mm) flattened animals have no symmetry, no mouth, no digestive tract, no nervous system, no basal blade. They have no organs and only four different types of somatic cells. The vast majority of Ediacaran organisms appear to belong to evolutionary lines that appeared before those of animals in the strict sense ({tooltip}Eumetazoaires{end-text}Higher (animal) metazoans including all major animal groups except sponges and placozoa.).

The marine ecosystem of that time was essentially dominated by microbial mats and osmotrophic multicellular organisms (e. g. rangeomorphs), microphagesAnimals consuming very small amounts of solid food (particles) that must be absorbed in large quantities. The particles ingested range from organic debris a few nanometres in size to shellfish and shrimp. This is an important part of the krill on which whales feed (e.g. sponges via their filter system and their flagellatedcharacteristics of unicellular cells or organisms equipped with one or more flagella, structure ensuring their mobility. cells) or using external contact digestion. These feeding strategies were perfectly adapted to the resources available in the marine environment at the end of the Precambrian, namely an abundant flow of dissolved organic matter and microbial mats ubiquitous at the water-sediment interface. The extinction of Ediacaran organisms at the Precambrian-Cambrian transition would not be due to a global environmental upheaval like most major biological crises, but rather to the destruction of their biotopeLocation with relatively uniform determined physical and chemical characteristics. This environment is home to a set of life forms that make up biocenosis: flora, fauna, micro-organisms. A biotope and the biocenosis it supports form an ecosystem. by the very first burrowing animals (bioturbationActive mixing of soil or water layers by living species, mainly animals.). Completely defenceless, these organisms would not have survived the first predators.

2. The first animal communities: prototype of modern ecosystems

By definition, Cambrian Explosion refers to the relatively sudden appearance in the fossil record of both new and anatomically complex organisms, among which we can certainly recognize the distant ancestors of the main current animal groups (e. g. arthropods, worms, molluscs, chordates; Figure 2). Several exceptionally preserved deposits (called LagerstättenGerman word, plural, literally meaning “storage area”. Corresponds to geological sites of extreme fossil richness that have been remarkably preserved () such as Chengjiang (China; approx. 520 Ma), Burgess Shales (Canada; approx. 505 Ma), Sirius Passet (Greenland) and Emu Bay (Australia) reveal the existence of the first marine animal communities. Thanks to already developed motor and sensory capacities (e.g., sometimes fossilized cephalic nervous system), these early Cambrian animals were able to move actively in their environment and exploit for the first time a multitude of ecological niches. This dynamic marks a fundamental difference with the essentially fixed marine life of the Ediacaran and an irreversible turning point in the evolution of ecosystems.

Encyclopedie environnement - ecosystemes complexes - Animaux du Cambrien provenant de gisements - complex ecosystems
Figure 2. Cambrian animals from exceptionally preserved deposits. (a) Wiwaxia, a primitive mollusc protected by sclerites. (b) Cricocosmia, a worm close to priapulans (worms in the shape of a small penis) protected by a series of plates. (c) Ottoia, a priapulan worm showing its evaginated pharynx (left) and food remains in its intestine (right). (d) Paucipodia, lobopodian, primary arthropod with non-articulated appendages (e) Hallucigenia lobopodian considered as an ancestor of the current onychophores, carrying thorns on its back. (f) Anomalocaridae (juvenile) showing powerful prehensil appendages. (g)-(i) Sidneyia, emblematic arthropod of the Burgess Shale; general view, intestinal contents (trilobites) and details of the chewing appendages. (j) Waptia, an arthropod with a flexible abdomen. a, c, e, e, g-j come from the Burgess Shales (British Columbia, Canada); b, d, f from Chengjiang (Yunnan, China). Scales: 1 cm in a, c, d, f-h; 5 mm in b, e, i; 1 mm in h. [Source: © J. Vannier, except e (J.-B. Caron)].
Figure 3. Transformation of the seabed at the Ediacaran-Cambrian transition due to increased bioturbation within the sediment and decline of microbial mats. Watercolour by Richard Bligny, inspired by Fedonkin et al. 2007 (see ref. [2]).
From the beginning of the Cambrian, many worms close to the priapulansin the shape of a small penis. [2] colonize the interior of current sediments. Long sealed and laminated by microbial mats, the seabed is disturbed by the increasingly intense and deep burrowing activity of these worms (Figure 3) [2]. This event, sometimes referred to as the “Agronomic Revolution”, creates new habitats and resources and modifies the redox gradientChemical gradient between oxidized and reduced molecules. through the sediment. Some priapulan worms (e.g. Ottoia) captured a wide variety of small animals living on the bottom (e.g. hyoliths, other worms, small arthropods) thanks to their evaginableWho protrudes externally. pharynx with teeth, as shown in the study of their intestinal contents. Totally unknown in the Precambrian, predation thus appears within Cambrian marine communities.

Encyclopedie environnement - ecosystemes complexes - Representation schematique des communautes cambriennes
Figure 4. Schematic representation of Cambrian communities. 1, sponges; 2, Ottoia (predatory priapulan worm); 3, Spartobranchus (tubular enteropneuste); 4, Odontogriphus (primitive grazing mollusc); 5, hyolite; 6, bradoriid arthropod; 7, Fortiforceps (arthropod with prehensil frontal appendages; 8, Nectocaris (primitive swimming mollusc); 9, Isoxys (swimming arthropod); 10, Hurdia (predatory anomalocaridae); 11, chaetognathe (predatory planktonic organism); 12, Tamisiocaris (filter appendages anomalocaridae). [Source : J. Vannier; simplified drawings from Briggs et al. 1994 [3] ; Caron et al. 2006 [4]; Caron et al. 2013 [5]; Daley et al. 2009 [6]; Hou et al. 2004 [7]; Smith and Caron 2010 [8]; Vannier 2012 [9]; Vannier et al. 2007 [10], 2009 [11]; Vinther et al. 2014 [12]; http://www.burgess-shale.rom.on.ca/]
In parallel, the water columnVolume of water above the bottom. above the sediments is populated by a multitude of swimming animals such as:
cnidariansA branch (phylum) of aquatic animals (mainly marine) that is found in two forms: polyps, when fixed (as in the case of coral or sea anemones), and jellyfish when they are swimmers.,
ctenophoresSmall, hermaphroditic, predatory marine organisms. They have a vague similarity to jellyfish and are a very important part of the plankton, – {tooltip}chaetognates{ind-text}A branch (phylum) of arrow-shaped marine predators named after the mobile hooks that capture their prey. They play a major role in the planktonic ecosystem as the main direct predators of copepods and represent up to 10% of the zooplankton biomass.,
– primary molluscs and arthropods (e.g. IsoxysExtinct type of small primary arthropods that lived in the Lower Cambrian. Their main characteristic is the existence of a pointed bivalve shell.).

The comparative study of these fossils and their current descendants (Figure 4) suggests that the species interacted within a primitive food chain [3],[4],[5],[6],[7],[8],[9],[10],[11]. The Lower Cambrian Timisiocari species of Greenland is a good example of these new trophic relationships. Close cousin of the emblematic predator Anomalocaris, its large appendages were equipped with combs and filter bristles allowing it to catch zooplanktonAnimal plankton. It feeds on living matter, some species being herbivores and others carnivores. living in suspension in the water column. Microfossils attest to the presence of this zooplankton that consumes eukaryotic algae and bacteria.

However, Cambrian marine life is concentrated at the water-sediment interface, with sponges representing a major component of sessile faunaOrganizations living alone or in colonies and permanently fixed directly to the substratum. They are most often aquatic. This is the case, for example, for sponges, corals, hydrozoa, tunicates, bryozoans, etc.. Arthropods are by far the most abundant and diverse epibenthic organismsOrganisms living on the surface of the bedrock in the seabed area. in all deposits with exceptional preservation (Figure 2). Their external (segments, appendages) and internal (nervous system) organization plan suggests, for some of them, kinship relationships with current crustaceans and cheliceratesGroup of arthropods carrying chelicera, a pair of appendages close to the mouth, corresponding to the second pair of antennas in mandibules (crustaceans, insects…). This group includes merostomes (limules) and arachnids (spiders, scorpions, etc.). Only the horseshoe crabs are marine animals and live on the bottom..

Others belong to groups that are now extinct. Their articulated and multi-segmented exoskeleton has probably favoured the acquisition of many functionalities and specializations. Prehensil and chewing appendages allow Cambrian arthropods to capture prey and reduce food particles. Macrophagia appears in Cambrian times in many predatory animals or animals that feed on corpses (scavengers). For example, the Sidneyia arthropod of the Burgess Shale captured, crushed and consumed small trilobitesA class of fossil marine arthropods that existed during the Paleozoic (primary era) from Cambrian to Permian. The last trilobites disappeared during the mass extinction at the end of the Permian, there are 250 Ma. as indicated by its appendages and stomach contents (Figure 2).

Other innovations contribute to the major changes in the food chain. Digestive glands increase the efficiency of enzymatic degradation of food, thus promoting macrophagiaMode of nutrition of a living organism that feeds on large prey compared to it. in many arthropods in Cambodia. The vision also revolutionized the interactions between marine organisms from the early Cambrian period. Thus, large compound eyes made up of thousands of facets allowed the super-predator Anomalocaris to spot and track its prey. There is no doubt that vision, widespread among Cambrian arthropods, has significantly altered prey-predator relationships and introduced new selection pressures within the ecosystem, leading to multiple adaptive responses.

Encyclopedie environnement - ecosystemes complexes - Complexification de ecosysteme marin - complex ecosystems
Figure 5. Complexification of the marine ecosystem at the Precambrian-Cambrian transition visualized by a theoretical three-axis eco-space (tiering, food pattern and mobility of organisms) containing 216 cubes that represent possible lifestyles (only 118 of which are ecologically sustainable). On the right, the comparative filling of the Ediacaran and Cambrian eco-space according to palaeontological data. In green and blue: lifestyles identified in current ecosystems (corresponding to 92 out of 118 cubes). In purple: lifestyles identified in the Ediacaran and Cambrian, they represent about 10% of potential lifestyles; their number increases significantly between the Ediacaran and Cambrian. [Source: Simplified from Bambach et al. 2007 [12] and Erwin & Valentine 2013 [13].]
Although it is clear that a complex prototype trophic webDescribes some species relationships (predator-prey relationships in particular), energy and nutrient cycles and flows within ecosystems between producers, consumers and decomposers. The basic level of this network is that of autotrophic primary production, above this level, each link in a food chain corresponds to a complex trophic level. is established from the Lower Cambrian, it remains very difficult to quantify interactions between organisms as it is possible to do for current ecosystems. Few quantitative approaches compare Precambrian, Cambrian and current ecosystems. However, the construction of a three-dimensional representation of a theoretical ecosystem according to three axes (tiering, food mode and mobility of organisms) [12].Theoretical three-axis representation of a theoretical ecosystem according to three axes (tiering, feeding mode and mobility of organisms) allows us to visualize the occupation of ecological niches at different periods of geological time (Figure 5) [12],[13]. We can thus see a filling of the eco-space at the Precambrian-Cambrian transition, which will intensify in the Ordovician during the great Ordovician biodiversification eventPeriod in the history of the Earth during which the biodiversity of oceanic life has increased the most. It occurs about 40 to 80 million years after the Cambrian explosion. Its duration is of the order of 25 million years (a relatively short interval on the geological time scale), and is located during the lower and middle part of the Ordovician system, dated between 485 and 460 million years., to finally give the marine world the appearance and operating principles we know it.

 


References and notes

Cover image. Life in the Ediacara Sea [© Ryan Somma (CC BY-SA 2.0), via Wikimedia Commons]

[1] Narbonne G.M., Laflamme M., Greentre C. Trusler P. (2009) Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard’s Bay, Newfoundland. Journal of Paleontology 83, 503-528.

[2] Fedonkin M.A., Gehling J.G., Grey K., Narbonne G. Vickers-Rich P. (2007) The Rise of Animals. The Johns Hopkins University Press, Baltimore. 325 pp.

[3] Briggs D.E.G. (2015) The Cambrian Explosion. Current Biology 25, R864-R868

[4] Caron J.-B., Scheltema A., Schander C. & Rudkin D. (2006) A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale. Nature 442, 159-163.

[5] Caron J.-B., Conway Morris S. & Cameron C.B. (2013) Tubicolous enteropneusts from the Cambrian period. Nature 495, 503-506.

[6] Daley et al (2009) The Burgess Shale anomalocarididid Hurdia and its significance for early euarthropod evolution. Science 323, 1597-1600.

[7] Hou X.-G., Aldridge R.J., Bergström J., Siveter David J., Siveter Derek J. & Feng X.-H. (2004) The Cambrian Fossils of Chengjiang, China. Blackwell Publishing. 233 pp.

[8] Smith M.R. & Caron J.-B. (2010) Primitive soft-bodied cephalopods from the Cambrian. Nature 465, 469-472.

[9] Vannier J., Steiner M., Renvoisé E., Hu S.-X. Casanova J.-P. (2007) Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal Society London B 274, 627-633.

[10] Vannier J., Garcia-Bellido D.C., Hu S.-X. Chen A.L. (2009) Arthropod visual predators in the early pelagic ecosystems: evidence from the Burgess Shale and Chengjiang biota. Proceedings of the Royal Society London B 276, 2567-2574.

[11] Vinther J., Stein M., Longrich N.R. & Harper D.A.T. (2014) A suspension-feeding anomalocarid from the Early Cambrian. Nature 507, 496-500.

[12] Bambach R.K, Bush A.M. & Erwin D.H. (2007) Autecology and the filling of ecospace: key metazoan radiations. Palaeontology 50, 1-22.

[13] Erwin D.H. & Valentine J.W. (2013) The Cambrian Explosion: the construction of animal biodiversity. Roberts & Company Publishers. 406 pp.


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引用这篇文章: VANNIER Jean (2019年4月1日), The first complex ecosystems, 环境百科全书,咨询于 2024年11月1日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/en/life/first-complex-ecosystems/.

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