Large spinose microfossils in Ediacaran rocks as resting stages of early animals
Large spinose microfossils in Ediacaran rocks as resting stages of early animals
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DOI：
10.1073/pnas.0902322106
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发表时间：
2009-04
期刊：
影响因子：
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通讯作者：
Phoebe A. Cohen;A. Knoll;R. Kodner
中科院分区：
文献类型：
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作者：
Phoebe A. Cohen;A. Knoll;R. Kodner
大的（>100 μm），丰富的微体化石构成了埃迪卡拉纪（6.35 - 5.42亿年前）沉积岩中独特的古生物成分。古生代岩石中较小的棘状化石通常被解释为藻囊或藻纲，但埃迪卡拉纪种群在大小，形状，超微结构和内部内容物方面与现代藻类类似物不同。与此相反，许多后生动物滞育卵休眠阶段形成的包囊与大的埃迪卡拉纪微化石（LOEMs）具有相同的大小、分布和内部内容物特征。此外，透射电子显微镜观察动物休息的包囊揭示了3层壁的超微结构相媲美的LOEM类群。这些独特的埃迪卡拉纪微体化石的解释为早期后生动物生命周期的休息阶段提供了额外的观点，他们的功能形态和地层分布。基于与现代海洋无脊椎动物的比较，由LOEMs代表的无脊椎动物的生命阶段被解释为埃迪卡拉大陆架和平台环境中的底层水缺氧的长期事件的进化反应。正如这一假说所预测的，后期埃迪卡拉纪LOEM类群的消失与地球化学证据相吻合，表明缺氧沃茨冲击大陆架和平台的程度明显下降。因此，LOEM的形式，多样性和地层范围阐明了早期动物的生命周期演化受海洋氧化还原状态的影响。
Large (>100 μm), profusely ornamented microfossils comprise a distinctive paleontological component of sedimentary rocks deposited during the Ediacaran Period (635–542 million years ago). Smaller spinose fossils in Paleozoic rocks have commonly been interpreted as algal cysts or phycomata, but the Ediacaran populations differ from modern algal analogs in size, shape, ultrastructure, and internal contents. In contrast, cysts formed during the diapause egg-resting stages of many metazoans share features of size, ornamentation, and internal contents with large ornamented Ediacaran microfossils (LOEMs). Moreover, transmission electron microscopic observations of animal-resting cysts reveal a 3-layer wall ultrastructure comparable to that of LOEM taxa. Interpretation of these distinctive Ediacaran microfossils as resting stages in early metazoan life cycles offers additional perspectives on their functional morphology and stratigraphic distribution. Based on comparisons with modern marine invertebrates, the recalcitrant life stage represented by LOEMs is interpreted as an evolutionary response to prolonged episodes of bottom water anoxia in Ediacaran shelf and platform environments. As predicted by this hypothesis, the later Ediacaran disappearance of LOEM taxa coincides with geochemical evidence for a marked decline in the extent of oxygen-depleted waters impinging on continental shelves and platforms. Thus, the form, diversity, and stratigraphic range of LOEMs illuminate life cycle evolution in early animals as influenced by the evolving redox state of the oceans.