美国西部干燥的山脉包含了距今4.4亿至5.1亿年前的奥陶纪岩石，其中充斥着热带海洋生物生存的迹象。
The dry mountain ranges of the western United States contain rocks dating back 440 to 510 million years, to the Ordovician period, and teeming with evidence of tropical marine life.
然而，有一些研究者认为，大约4.5亿年前的奥陶纪大灭绝是伽马射线爆引起的。
However, some researchers believe that a GRB was responsible for the Ordovician mass extinction approximately 450 million years ago.
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