胚胎化石的磷酸盐化模拟实验研究

【作者】 张剑；

【导师】 华洪；

【作者基本信息】 西北大学 ， 古生物学与地层学， 2013， 博士

【摘要】 华南埃迪卡拉系陡山沱组及寒武纪早期的含磷地层中产出大量保存精美的磷酸盐化球状化石(phosphatized spheroidal fossils),由于其特殊的磷酸盐化保存方式,许多超微细结构(细胞级及亚细胞级)得以保存,并且以立体的方式展现出来,这些化石为研究后生动物的起源及早期后生动物的发育生物学提供了重要素材。对于磷酸盐化球状化石来说,其大小、纹饰和形态是判断化石属性的重要标准。研究者通常基于球状化石的与现代生物的一些特征比较(如大小、分裂方式、整体形态等)做出其生物属性的解释,将磷酸盐化球状化石的起源解释为多种来源。由于化石都经历了死亡、腐败和成岩等变化,化石的现有特征是一系列不同作用的结果,因此化石的大小特征是否真实的反应了生物本身的属性特点尚待确定。因为这些化石特征与生物结构对应关系理解存在分歧,故其特征直接与生物体对比常常引起争议。如果不把化石的不同形态表征和观察的化石特征相互区分的话,那么同一种个体的两种不同状态很可能会被描述成不同的属种,进而影响到更复杂的化石组合解释,尤其是建立个体之间的亲缘关系基础之上的分析。在目前的矿化实验中,虽然现有研究证实很多极易腐败的组织可以通过早期磷酸盐或硅质交代而得到精细保存,但对于有可能发生的初始形态变化,矿化实验提供的信息非常有限。目前没有足够的证据来揭示生物形态结构在不同环境下的变化趋势,这些直接的证据经常被后期成岩作用所掩盖。目前的实验结果表明环境化学梯度(比如氧气、酸碱度等)的变化可能是造成生物卵早期形态变化的主要原因。然而化石胚胎形态大小的变化显然不能通过化学分析测量。而针对早期不同环境的形态统计分析可以帮助了解这个趋势。由于形态变化是多因素造成的结果,单一条件下的实验有可能提供更好的论证依据,并可以揭示造成该形态变化的主要因素。现代人工模拟磷酸盐化动物卵实验可以为上述问题的解决提供科学实证。本实验通过模拟不同埋藏条件,发现武昌鱼(Carnis megalobramae)鱼卵可以在不同环境中保存。实验证实在不同保存条件下鱼卵的个体大小、纹饰和形态存在明显的差异,而且在不同环境下,这些鱼卵明显表现出不同的大小、纹饰和形态保存趋势以及巨大的形态差异,因而仅仅根据上述特征无法将其判定为同一物种。由于实验条件可以在自然环境条件下的实现,实验中重要表征消失的现象有可能在现实环境中重现,那么在地质历史中一些重要的生物属性缺失有可能广泛存在。实验结果期望能为磷酸盐化球状化石的鉴别提供参考,并为其成因探讨提供更多依据。更多还原

【Abstract】 A considerable number of phosphatized spheroidal fossils were found in the Ediacaran Doushantuo Formation and the early Cambrian phosphorus strata in South China. Because of the special mineral that is preserved, many ultra-fine structures (cellular and sub-cellular levels) of the biological specimens can be three-dimensionally preserved. These fossils provide important material for the study of origins and early ontogeny of metazoan.For the Doushantuo spherical microfossils, ornamentation and morphology are important criteria for judging the biological properties of the fossils. Researchers usually study the fossils’ biological properties by a comparative analysis between the characteristics of the spherical fossils (including the size, the cleavage pattern, and the overall split mode) and the modern biological characteristics. According to the complexity of the structure of these fossil cell structures and their similar characteristics with the morphology features of modern animal tissues, the Doushantuo spherical fossils have been widely interpreted as originating from a variety of sources. Such hypotheses are based on the remarkable similarities between modern eukaryotic embryos and the Doushantuo spherical fossils in size and cell division boundaries. However, recognition of the same types of fossils relied on comparisons between living organisms and fossilized remains. As fossils have all experienced death, corruption, decay and digenetic alteration, few specific characteristics can be used as standards to identify the same biological species. The relationship between the corruption and the remnant morphology of soft tissues has yet to be explored. Because there is a different understanding between fossil characteristics and the biological structure correspondence of the fossil, the understanding of these characteristics is at odds with the identification of fossils, which usually causes controversy. A more complex explanation for the fossil assemblage appears to contain more difficulties (especially with regard to establishing the genetic relationship between individuals).Although many mineralization experiments confirmed that vulnerable tissues can be exceptionally preserved, it provided only limited information on the initial biological decomposition. There are insufficient data to separate the primary structures and the corruption changes. The direct evidence may be concealed by subsequent diagenesis, and the remaining traces of the primary structure were not recognized. Clearly, the spatial pattern will not be measured by chemical analysis. Steep chemical gradients that originally existed in the environment (e.g.,O2, nitrate, pH, and redox) may be the main factors that cause morphological changes. The ability to measure morphological changes over time is vital as the decomposition process itself is dynamic. The mineralization experiments, which focus on mineralization, provide an indication of understanding the processes. Providing the possibility of measuring single factors at fine spatial resolutions with minimum disturbance can reveal this process. Whether the native structure changed in a dynamic environment should be explored by the researcher.Modern artificial simulation experiments of phosphate animal eggs can provide scientific evidence to settle the aforementioned disputes. This experiment simulated environmental changes to observe the early morphological changes of the Bluntnose black bream eggs(Megalobrama amblycephala). This study in a laboratory confirmed the occurrence of morphological variations under different conditions and that the eggs decomposed in different ways. It was found that the eggs can show a different morphology in the phosphate environment and that these eggs exhibited considerable morphological variations under different conditions. By comparing the perservational state in this experiment with those of other states, the results showed diversified morphological changes that may also have occurred in the real environment. These results may indicate that diversified morphological changes may be widely occurred in geological history. Thus, those eggs cannot be ascribed to the same species based only on the features. The experimental results are expected to provide a useful reference for the identification of phosphatized microfossils and provide a more empirical basis for their genesis.更多还原