The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes.
在这一系列周期性物种灭绝事件里，白垩纪晚期恐龙和菊石的灭绝仅仅是其中一个比较严重的事件。
Ammonoids, with their tightly coiled, spiral shells, are also widespread in the Permian fossil record.
鹦鹉螺化石，这些螺旋状的贝壳，经常分布在二叠纪的化石群中。
Ammonoids, with their tightly coiled, spiral shells, are also widespread in the Permian fossil record.
鹦鹉螺化石，这些螺旋状的贝壳，经常分布在二叠纪的化石群中。
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