from or referring to the period of time between about 543 and 490 million years ago, in which many different types of invertebrates (= animals with no back bone) first appeared寒武纪的(寒武纪,距今约5.43亿至4.9亿年前的地质时期,无脊椎动物开始大量出现。)
the Cambrian period寒武纪
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