这就造成了一个难题：数百万年前消失的特征是如何突然重现的？
And this poses a puzzle: how can characteristics that disappeared millions of years ago suddenly reappear?
现在的彼尔姆地区在数百万年前还属于彼尔姆海。
Many million years ago the territory of the present Perm region was occupied by the Perm Sea.
基于最近的研究，论文探讨了数百万年前全球气温与大气中高浓度二氧化碳之间的关系。
Building on recent research, the study examines the relationship between global temperatures and high levels of carbon dioxide in the atmosphere tens of millions of years ago.
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