地学前缘 ›› 2020, Vol. 27 ›› Issue (6): 28-46.DOI: 10.13745/j.esf.sf.2020.6.2

• 早期生命演化 • 上一篇    下一篇

新元古代末期高家山生物群的生态多样性

华洪,蔡耀平,闵筱,柴姝,代乔坤,崔再航   

  1. 1. 西北大学 地质学系 早期生命与环境陕西省重点实验室与大陆动力学国家重点实验室, 陕西 西安 710069
    2. 中国地质调查局 成都地质调查中心, 四川 成都 610081
  • 收稿日期:2020-03-27 修回日期:2020-05-29 出版日期:2020-11-02 发布日期:2020-11-02
  • 作者简介:华洪(1966—),男,教授,博士生导师,主要从事前寒武纪古生物学与地层学的研究。E-mail:huahong@nwu.edu.cn
  • 基金资助:

    国家自然科学基金项目(41672025,41572012,41621003,41890844);中国科学院先导专项项目(XDB26000000);国家重点研发计划项目 (2017YFC0603101)

Ecological diversity in the terminal Ediacaran Gaojiashan biota

HUA Hong, CAI Yaoping, MIN Xiao, CHAI Shu, DAI Qiaokun, CUI Zaihang   

  1. 1. Shaanxi Key Laboratory of Early Life and Environments and State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
    2. Chengdu Center of China Geological Survey, Chengdu 610081, China
  • Received:2020-03-27 Revised:2020-05-29 Online:2020-11-02 Published:2020-11-02

摘要:

新元古代末期是生命演化的关键转折期,也是以微生物占主导的生态系统向显生宙以后生动物占主导的生态系统的转变期,埃迪卡拉纪大型软躯体生物以固着、底栖、食悬浮为特色,普遍缺乏运动能力。作为这一时期特殊代表的高家山生物群,是目前新元古代唯一一个以黄铁矿化三维保存的管状和锥管状化石为主导,兼有骨骼生物、原生动物、钙化蓝细菌类及遗迹化石的多门类生物组合,是研究埃迪卡拉纪末期生命演化和生态系统演变十分重要的载体。本文通过对高家山生物群古生态学的初步研究,揭示出在前寒武纪—寒武纪之交,生态系统已显示一定的多样性。为适应平底面上(level-bottom)微生物席的发育,高家山生物群的许多生物采取了适应性的生存策略,通过黏附或插入微生物席中,营底栖固着食悬浮(如Cloudina、Conotubus)或化学共生(可能的Shaanxilithes)或平躺(如Gaojiashania和Sinotubulites)食碎屑生活。底内遗迹化石表明存在可能的表栖和半内栖、可自由运动、食碎屑的造迹生物。Conotubus中常见的“回春”或“复苏”现象,Gaojiashania和Sinotubulites的身体扭转或生活姿态调整则是对频繁风暴事件的被动适应。

关键词: 高家山生物群, 新元古代末期, 生态多样性, 生命创新事件, 微生物席底, 风暴事件沉积

Abstract:

Ediacaran fossils mark a pivotal position in the evolution of life as it transitions between the predominantly microbial ecosystems of the Precambrian and the animal ecosystems of the Phanerozoic. Ediacaran communities were dominated by sessile epibenthos and their ecological structures were relatively simple. Many modern feeding modes such as macropredation, epibiosis, and interspecific competition or mutualism were absent or poorly represented. It is generally accepted that the soft-bodied Ediacara biota is part of a “failed” evolutionary experiment and it has no clear genetic relationship with Phanerozoic organisms. However, corresponding to the evolution of typical Ediacaran soft-bodied biota, there was a great proliferation of tubular animals at the end of the late Neoproterozoic, represented by the Gaojiashan biota, where Lagerst-tte fossil deposit hosts a variety of soft-bodied or lightly biomineralized tubular fossils (Shaanxilithes, Gaojiashania, Sinotubulites, Conotubus, and Cloudina), calcareous microfossils (Protolagena), and calcareous cyanobacteria.
Unlike other Ediacara fossil assemblages, the Gaojiashan biota is dominated by benthic sessile suspension feeders or detritus feeders. Of them, Cloudina is a millimetre-scale conical tube tapering adapically from an aperture to a rounded apex. The bulk of the tube is constructed by a series of successively stacked, repetitive, unevenly spaced, and funnel-shaped tube wall units. Cloudina occupied an epibenthic suspension-feeding life-mode, with the apex anchoring to microbially bound muddy substrate and the aperture extending upwards into the water column. Conotubus is a centimetre-sized conical tube consisting of a series of nested cylindrical-to-funnel-shaped tube walls (cylinders hereafter). This conotubular construction of nested cylinders is similar to that of the late Ediacaran fossil Cloudina. Integrated morphological, taphonomic and palaeoecological data suggest that it likely employed an epibenthic life-mode, with the apex anchoring to the muddy substrate and the aperture extending upwards into the water column and it was probably a suspension-feeding organism. Gaojiashania is a centimetre-size tube that consists of a series of repeating units. Each unit consists of a rigid ring and a flexible bucket-shaped tube wall. Each tube maintains a constant diameter and the rigid ring elements typically range from 1 to 2 mm in length, whereas the bucket components are more varied at 0.5-5 mm in length and are ornamented with transverse annuli. Gaojiashania, however, may have been a procumbent epibenthos, possibly with the rings anchoring to microbially bound muddy substrate. Sinotubulites are characterized by millimetre-to centimetre-size multilayer tubes open at both ends. The tube consists of two morphologically different walls: a multilayer inner wall with weak ornamentation, and a multilayer outer wall with transverse or oblique corrugations and sometimes longitudinal ridges. Surface ornamentation and polygonal shape of the cross-section suggest that Sinotubulites probably lived as an epibenthos lying on the sea floor. Shaanxilithes is a ribbon-shaped impression with constant widths (16 mm or more) and is characterized by a series of closely spaced transverse annulations. Shaanxilithes is tentatively interpreted as a sessile epibenthos, with one end anchoring to the sandy or muddy substrate. Therefore, the Gaojiashan biota is characterized by complex morphological types and diverse body plans of variable size and lifestyle, representing a major biological innovation in Earth’s evolution.
Flourishing microbial mats on the Precambrian level-bottom forced the Gaojiashan biota to evolve some adaptive strategies, either through fixation to (such as Cloudina, Conotubus, and possibly Shaanxilithes) or free laying on (such as Gaojiashania and Sinotubilites) the substrate by adhering to or inserting into microbial mats. To cope with the frequent disturbance by storm events, ecological adaptations were manifested in many Gaojiashan organisms. The polygonal cross-section and longitudinal ridges in Sinotubulites may stabilize the tube lying on the substrate, preventing it from rolling in strong currents. After being subjected to sediment obrution, Cloudina and Conotubus employed specialised palaeoecological strategies to rejuvenate and self-right the tubes, while Gaojiashania and Sinotibulites could partially self-right through curving, extension and constriction, indicating strong burial-resistant capabilities.
Abundant meandering traces and possible burrows in the Gaojiashan biota and in the comparable Shibanta biota imply that bilaterians were an essential part of later Ediacaran ecosystems and indicate the possible presence of epibenthic and semibenthic free-moving and detritus-feeding organisms. The complexity of predator-prey dynamics at the beginning of metazoan diversification is evidenced by borings on Cloudina tubes.

Key words: Gaojiashan biota, terminal Neoproterozoic, ecological diversification, biological innovation, microbial-mat substrate, storm event deposit

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