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The fossils of the Burgess Shale, like the Burgess Shale itself, formed around in the Mid Cambrian period. They were discovered in Canada in 1886, and Charles Doolittle Walcott collected over 60,000 specimens in a series of field trips up from 1909 to 1924. After a period of neglect from the 1930s to the early 1960s, new excavations and re-examinations of Walcott's collection continue to discover new species, and statistical analysis suggests discoveries will continue for the foreseeable future. Stephen Jay Gould's book ''Wonderful Life'' describes the history of discovery up to the early 1980s, although his analysis of the implications for evolution is largely superseded. The fossil beds are in a series of shale layers, averaging and totalling about in thickness. These layers were deposited against the face of a high undersea limestone cliff. All these features were later raised up above current sea level during the creation of the Rocky Mountains. These fossils have been preserved in a distinctive style known as Burgess shale type preservation, which preserves fairly tough tissues such as cuticle as thin films, and soft tissues as solid shapes, quickly enough that decay has not destroyed them. Moderately soft tissues, such as muscles, are lost. Scientists are still unsure about the processes that created these fossils. While there is little doubt that the animals were buried under catastrophic flows of sediment, it is uncertain whether they were transported by the flows from other locations, or lived in the area where they were buried, or were a mixture of local and transported specimens. This issue is closely related to whether conditions around the burial sites were anoxic or had a moderate supply of oxygen. Anoxic conditions are generally thought the most favourable for fossilization, but imply that the animals could not have lived where they were buried. In the 1970s and early 1980s the Burgess fossils were largely regarded as evidence that the familiar phyla of animals appeared very rapidly in the Early Cambrian, in what is often called the Cambrian explosion. This view was already known to Charles Darwin, who regarded it as one of the greatest difficulties for the theory of evolution he presented in ''The Origin of Species'' in 1859. However, from the early 1980s the cladistics method of analysing "evolutionary family trees" has persuaded most researchers that many of the Burgess Shale's "weird wonders", such as ''Opabinia'' and ''Hallucigenia'', were evolutionary "aunts and cousins" of present-day types of animal rather than a rapid proliferation of separate phyla, some of which were short-lived. Nevertheless, there is still debate, sometimes vigorous, about the relationships between some groups of animals. ==Discovery, collection, and re-examinations== (詳細はMount Stephen in Canada's Rocky Mountains by a construction worker, whose reports of them reached Richard McConnell of the Geological Survey of Canada. McConnell found trilobite beds there in 1886, and some unusual fossils that he reported to his superior. These were misdiagnosed as headless shrimps with unjointed appendages, and were named ''Anomalocaris'' because of their unusual appendages – but turned out to be pieces of a puzzle that took 90 years to solve. Similar fossils were reported in 1902 from nearby Mount Field, another part of the Stephen formation. These may have been why Charles Doolittle Walcott visited Mount Field in 1909. While taking photographs there Walcott found a slab of fossils that he described as "Phyllopod crustaceans". From late August to early September 1909, his team, including his family, collected fossils there, and in 1910 Walcott opened a quarry that he and his colleagues re-visited in 1911, 1912, 1913, 1917 and 1924, bringing back over 60,000 specimens in total. Walcott was Secretary of the Smithsonian Institution from 1907 to his death in 1927, and this kept him so busy that he was still trying to make time for analyzing his finds two years before his death. Although he drew attention to the exceptional detail of the specimens, which were the first known fossils of soft-bodied animals from the Cambrian period, he also had other research interests: the Early Paleozoic stratigraphy of the Canadian Rockies, which took up the great majority of his time there; and Precambrian fossils of algae and bacteria,〔 to which he assigned as much importance as to the fossils of animals.〔 He managed to publish four "preliminary" papers on the fossil animals in 1911 and 1912, and further articles in 1918, 1919 and 1920. Four years after Walcott's death his associate Charles Resser produced a package of additional descriptions from Walcott's notes. Walcott's classifications of most of the fossils are now rejected,〔 but were supported at the time, and he accepted a change for one of the few where his conclusion was disputed.〔 Many of the later comments were made with the benefits of hindsight, and of techniques and concepts unknown in Walcott's time.〔〔 Although in 1931 Percy Raymond opened and briefly excavated another quarry about above Walcott's "Phyllopod bed", there was very little interest in the Burgess Shale fossils from the 1930s to the early 1960s, and most of those collected by Walcott were stored on high shelving in back rooms at the Smithsonian Institution. Between 1962 and the mid-1970s Alberto Simonetta re-examined some of Walcott's collection and suggested some new interpretations.〔 Beginning in the early 1970s Harry Whittington, his associates David Bruton and Christopher Hughes, and his graduate students Derek Briggs and Simon Conway Morris began a thorough re-examination of Walcott's collection. Although they assigned groups of fossils to each member of the team, they all decided for themselves which fossils to investigate and in what order. Their publications and Stephen Jay Goulds' popularization of their work in his book ''Wonderful Life'' aroused enduring scientific interest and some public interest in the Cambrian explosion, the apparently rapid appearance of moderately complex bilaterian animals in the Early Cambrian. The continuing search for Burgess Shale fossils since the mid-1970s has led to the description in the 1980s of an arthropod ''Sanctacaris'' and in 2007 of ''Orthrozanclus'', which looked like a slug with a small shell at the front, chain mail over the back and long, curved spines round the edges. Recent digs have discovered species yet to be formally described and named.〔 They have also unearthed more and sometimes better fossils of animals that were discovered earlier, for example ''Odontogriphus'' was for many years known from just one poorly preserved specimen, but the discovery of a further 189 formed the basis for a detailed description and analysis in 2006.〔 A full pre-publication draft, free but without pictures, may be available at (【引用サイトリンク】format=PDF )〕 Re-examination of Walcott's collection also continues, and has led to the reconstruction of the large marine animal ''Hurdia'' in 2009. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fossils of the Burgess Shale」の詳細全文を読む スポンサード リンク
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