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。
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