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。

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An extraterrestrial impact is most closely linked to the Cretaceous extinction event.

天外来客对地球的撞击与白垩纪的物种灭绝事件有着最为密切的联系。

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Changes in climate before the Cretaceous period caused severe fluctuation in sea level, resulting in the extinction of the dinosaurs.

白垩纪以前的气候变化导致了海平面的剧烈波动，引发了恐龙的灭绝。

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Its detection at unearthly levels in rocks 65 million years old was the first evidence in the case for an asteroid causing the mass extinction and global environmental mayhem that ended the Cretaceous period and began the Tertiary period.

BBC: Leaving our mark: Fossils of the future

The question is one of the most hotly contested in palaeontology and revolves around an apparent gap in the fossil record immediately prior to the K-T boundary - the distinct layer of geological sediments separating the Cretaceous and Tertiary periods, and associated with a massive asteroid impact and global extinction event.

BBC: Closing the 'three metre gap'

Besides bringing new species to the textbooks - and relegating them immediately to the "extinct" category - the study sheds new light on the extinction event, widely thought to have been caused by an enormous asteroid impact at the end of the Cretaceous period, about 65.5 million years ago.

BBC: Obama-named lizard was wiped out with the dinosaurs