Some people do say that if there are two past (actions)(events), the earlier one should be expressed with the past perfect. But if the meaning is clear without using the past perfect, as it often is, the past perfect is in fact not necessary. That is the case in your sentence.
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pokhan age in which great ice sheets existed in what are now temperate areas.The comparison is between the ice ages and 'now', so a past perfect doesn't seem appropriate. You use the past perfect to show that something happened before a reference point in the past, not that it happened before 'now'.