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WikiPedia: Kỷ_Ordovic http://www.abc.net.au/science/news/stories/s109775... http://adsabs.harvard.edu/abs/2008Sci...322...64H //www.ncbi.nlm.nih.gov/pubmed/18832639 http://arxiv.org/abs/astro-ph/0309415 //dx.doi.org/10.1111%2Fj.1502-3931.2009.00169.x //dx.doi.org/10.1126%2Fscience.1161648 http://ordovician.stratigraphy.org/uploads/OrdChar... http://www.stratigraphy.org/ICSchart/ChronostratCh... http://www.stratigraphy.org/gssp.htm http://news.bbc.co.uk/2/hi/science/nature/4433963....