10:15 〜 10:30
[BCG05-06] エディアカラ紀から初期カンブリア紀の微化石の顕微断層撮影と元素マッピングによる後生動物進化の原因解明
キーワード：カンブリア爆発、エディアカラ紀、元素マッピング
The Ediacaran to the Early Cambrian period is a very important age in biological evolution because metazoans became distributed all over the world during the period, which was the starting point of the metazoan evolution in the Phanerozoic era. The Weng'an biota is one of main Ediacaran biotas and comprises phosphatic microfossils in the ca. 580 Ma Doushantuo Formation, South China. The fossils retain their soft tissue structures because they are replaced by phosphate minerals. The fossils include spherical microfossils with ornamental membranes, which are several hundred µm in diameter and consist of one to several hundred small spheroids. They are commonly interpreted as animal embryos during cleavage. Small shelly fossils (SSF), which are phosphatic microfossils and include spheroidal fossils similar to those in the Weng’an, occur in the early Cambrian strata in the Kuanchuanpu region of South China. These spherical microfossils are generally interpreted as animal eggs, embryos and polyps, but their species are still being debated.
The purpose of this study is to establish a new methodology in order to estimate the original phyla of Weng'an and Kuanchuanpu microfossils and ecosystems of the Weng’an and Kuanchuanpu biotas. The methodology comprises combination of in-situ chemical analysis of major and trace elements and organic matter contents using SEM-EDS, LA-ICP-MS and LA-Raman spectroscopy, and three-dimensional morphological analysis using Synchrotron X-ray Micro-CT analysis, respectively.
The Synchrotron X-ray Micro-CT analysis allows us to select fossil samples that preserve primary biological structures and to estimate the original phyla of the fossils. The Laser Raman spectroscopy and SEM-EDS measurements enable us to determine the distribution of organic matter within the fossils and its host minerals. Actually, some fossils are composed of phosphate minerals with ubiquitous distribution of organic matter and carbonates, calcite and dolomite, with little organic matter. Therefore, it is considered that the biological tissues of the organism were replaced with phosphate for the fossils whereas the void portion was replaced with dolomite or calcite, respectively.
In addition, organic matter contents of the Kuanchuanpu fossils were higher than those of the Weng'an fossils, suggesting that the former preserves the primary biological information better than the latter. Furthermore, the homogeneous part of the fossil is composed of dolomite in Weng'an and calcite in Kuanchuanpu.
The distribution of Fe, Cu, Zn, Zn, As, Se, Sr, and I within the Kuanchuanpu microfossils and Cr, Ni, As, and I within the Weng'an fossils is well correlated with the distribution of organic matter, indicating that the organic matter hosts the elements so that the elemental distribution possibly still preserves the biological information. In addition, Se, Br, and U contents of the Weng’an microfossils are lower than their detection limits and those of the Kuanchuanpu microfossils, which is considered to reflect the secular changes of the dissolved elements in seawater. The changes in their contents probably influenced the evolution of life from the Ediacaran to the Cambrian. On the other hand, we could not find elemental distribution related with the phylum.
This method can be applied to fossils in which soft tissues are replaced with minerals, including Ornstein-type fossils. The combination of chemical and morphological information may provide new insights into the Ediacaran and early Cambrian organisms.
The purpose of this study is to establish a new methodology in order to estimate the original phyla of Weng'an and Kuanchuanpu microfossils and ecosystems of the Weng’an and Kuanchuanpu biotas. The methodology comprises combination of in-situ chemical analysis of major and trace elements and organic matter contents using SEM-EDS, LA-ICP-MS and LA-Raman spectroscopy, and three-dimensional morphological analysis using Synchrotron X-ray Micro-CT analysis, respectively.
The Synchrotron X-ray Micro-CT analysis allows us to select fossil samples that preserve primary biological structures and to estimate the original phyla of the fossils. The Laser Raman spectroscopy and SEM-EDS measurements enable us to determine the distribution of organic matter within the fossils and its host minerals. Actually, some fossils are composed of phosphate minerals with ubiquitous distribution of organic matter and carbonates, calcite and dolomite, with little organic matter. Therefore, it is considered that the biological tissues of the organism were replaced with phosphate for the fossils whereas the void portion was replaced with dolomite or calcite, respectively.
In addition, organic matter contents of the Kuanchuanpu fossils were higher than those of the Weng'an fossils, suggesting that the former preserves the primary biological information better than the latter. Furthermore, the homogeneous part of the fossil is composed of dolomite in Weng'an and calcite in Kuanchuanpu.
The distribution of Fe, Cu, Zn, Zn, As, Se, Sr, and I within the Kuanchuanpu microfossils and Cr, Ni, As, and I within the Weng'an fossils is well correlated with the distribution of organic matter, indicating that the organic matter hosts the elements so that the elemental distribution possibly still preserves the biological information. In addition, Se, Br, and U contents of the Weng’an microfossils are lower than their detection limits and those of the Kuanchuanpu microfossils, which is considered to reflect the secular changes of the dissolved elements in seawater. The changes in their contents probably influenced the evolution of life from the Ediacaran to the Cambrian. On the other hand, we could not find elemental distribution related with the phylum.
This method can be applied to fossils in which soft tissues are replaced with minerals, including Ornstein-type fossils. The combination of chemical and morphological information may provide new insights into the Ediacaran and early Cambrian organisms.